NASA News National Aeronautics and Space Administration Washington. D C 20546 AC 20 2 755-8370 Press Ki t RELE ASE NO: 78-181 Contents Fo r Release IMMEDIATE Project Pioneer Venus Encounter G E N E R A L RE LE AS E. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1 -1 5 E N C O U N T E R T I M E L I N E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6- 2 2 M I S S I ON PRO FIL E.. .. ... ... ... ... .. ... ... ... ... .. ... .. 2 3 O R B I T E R OP ER AT IO NS .. .. .. .. .. .. .. .. .... .. .. .. .. .. .... 23-34 M U L T I P R O B E OP ER AT IO NS .. .. .. .. .. .. .. .. .. . .. .. .. .. ... 35-43 T H E P L A N E T VE NU S. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 4 4 - 6 0 M A J OR Q U E S T I O N S A B O U T V E N U S . . . . . . . . . . . . . . . . . . . . . . . . . 6 1 - 6 2 H I S T O R I C A L D I S C O V E R I E S A B O U T V E N U S . . . . . . . . . . . . . . . . . . 6 3- 6 5 EXPLORATION OF VENUS BY SPACECRAFT ................. . 6 6 - 6 7 Mailed: November 2 8 , 1978
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( A l l t i m e s a r e P a c i f i c S t a n d a r d )
D e c . 2
Midday
D e c . 3
Maneuver t o o r i e n t O r b i t e r f o r i n s e r t i o n . B i t
r a t e lowered (1,024 t o 64 b p s ) ; s w i t c h t o omnia n t e n n a ; s p in -u p h i g h- g a in a n t e n n a ; i n c r e a s e s p i nr a t e t o 30 rpm; d es p i n h igh -ga in an t en na ; r a i s eb i t r a t e (64 t o 1,024 b p s ) .
11:OO p.m. Load b o th O r b i t e r command m e m o r i e s wi th s equencet o f i r e o r b i t i n s e r t i o n motor.
D e c . 4
1:OO a . m . S t a r t command m e m o r i e s t o f i r e o r b i t i n s e r t i o n
m o t o r ( O r b i t e r ) .7:51 a . m . O r b i t e r p a s s e s b e hi n d t h e p l a n e t n i n e m i n u te s
b e f o r e Venus o r b i t i n s e r t i o n ( V O I ) .
8:OO a . m . V O I o c c u r s b e hi nd V enus. S o l i d r o c k e t o r b i t i n s e r -t i o n m oto r b u rn s o u t i n a b o u t 30 seconds and changesv e l o c i t y a b o ut 3,780 km/hr (2,349 mph).
8:lO a . m . O r b i t e r e x i t s f r o m b e h i n d V e n u s .
8:15 a . m . C onfirm O r b i t e r r e t r o f i r e .
8:30 a . m . ( V O I + 30 m in ut es ) I n i t i a l O r b i t e r de sp in t o 1 5 rpm.
9:00 a.m. D e sp in O r b i t e r ' s h i g h - g a in a n t e n n a .
9:00 a . m . t o3:OO p.m. D e te rm i ne o r b i t o f O r b i t e r .
1:OO p.m. ( V O I + 5 ho ur s) Load r a d a r mapper memory f o r f i r s to r b i t .
3:OO p.m. ( V O I + 7 hour s ) Desp in O r b i t e r t o 6 rpm.
4:OO p.m. ( V O I + 8 h o ur s) R e o r i e n ta t i o n o f O r b i t e r s p i n a x i s
t o s o u t h c e l e s t i a l p o l e .
6:OO p.m. ( V O I + 1 0 h o u rs ) O r b i t e r hi g h- g ai n a n t en n a o r i e n t e dt o E ar th and t e l em e t r y swi t ched f rom omni an t enn at o h i g h- g a in a n t e n n a .
sequence starts. Remaining Orbiter instrumentswill be turned on at various times as requested
by the principal investigators during the firstorbit.
First orbital scientific
Dec. 5
12:OO Noon Start receiving IR weather map of planetatmosphere.
Dec. 6
a.m. Quick looks at solar wind ionosphere interaction
data from Electric Field Detector (EFD); thermalstructure of ionosphere data from ETP; Magnetometerreadings of solar wind and ionosphere; IR weathermaps; IR data on heat balance and distribution;Ultraviolet Spectrometer (WS) maps (Orbiter)
First Orbiter imaging scan by Cloud Photopolarimeter
(CPP)
1:OO a.m.
7 : O O p.m. First Venus image (black and white) received at
Ames from Orbiter CPP.
hours later.*Color image about five
*Experiment result dates and times are estimates and are
The t w o P i on e er f l i g h t s t o Venus w i l l e x p l o r e t h ea tm o sp h er e o f t h e p l a n e t , s t u d y i t s s u r f a c e u s i n g r a d a r
and de t e rmine i t s g l o b a l s ha pe and d e n s i t y d e s t r i b u t i o n .The f i r s t s p a c e c r a f t , P i o n e e r Venus 1, t h e O r b i t e r , w i l lmake e i g h t mon ths o r more o f r emote - sens ing and d i r e c tmeasurement. Pi on ee r Venus 2 , t h e M u l ti p ro b e , s e p a r a t e di n t o f i v e a t mo sp he ric e n t r y c r a f t , e i g h t m i l l i o n m i l e so u t from t h e p l a n e t , w i l l measure t he a tmosphe re f rom to p t ob ott om i n a b o u t two h o ur s a t p o i n t s s p r e a d o v e r t h e e n t i r eE a r t h - f a c i n g h e m i s p h e r e of Venus.
ORBITER OPERATIONS
On D e c . 2 , 1978, t w o d ay s b e f o r e a r r i v a l a t Venus, t h eO r b i t e r s p a c e c r a f t s p i n r a t e w i l l b e i n c r e a s e d t o 30 rpmand t h e O r b i t e r w i l l b e o r i e n t e d w i t h i t s 18,000-N (4,000-lb.)t h r u s t , s o l i d - f u e l e d r o c k e t e n gi ne p o i n t i n g f or wa rd , i n t h ed i r e c t i o n of t r a v e l .
On Dec. 4 , 1 97 8 a t 8:OO a.m. P S T, t h e sp ac ec ra f t commandw i l l command t h e r o c k e t t o i g n i t e f o r t h e 3 0-se co nd
o r b i t - i n s e r t i o n b ur n. T h i s w i l l r ed uc e s p a c e c r a f t v e l o c i t yby 3, 78 1 km/hr ( 2 , 3 4 9 m p h ), p l a c i n g P i o n e e r i n a 2 4-h ou ro r b i t a r ou n d V enus. The p l a n n e d o r b i t w i l l be i n c l i n e d 75d e g r e e s t o V en us ' e q u a t o r , w i t h i t s low p o i n t ( p e r i a D s i s )n e a r 1 8 . 5 d e g re e s n o r t h l a t i t u d e . The o r b i t ' s h i g h p o i n t( a p o a p s i s ) i s e x p e ct e d t o be a t an a l t i t u d e of 6 6 , 0 0 0 km( 4 1 , 0 0 0 m i . ) . P e r i a p s i s i n i t i a l l y w i l l be a t 300 km( 1 8 0 m i . ) , l a t e r r e d uc e d t o a s low a s 1 5 0 km ( 9 0 m i . ) .
W ith in h o ur s a f t e r t h e o r b i t i n s e r t i o n r o c k e t b u rn ,m e m b e r s of t h e O r b i t e r n a v i g a t i o n t eam will have measuredt h e a c t u a l s i z e and s ha pe o f t h e o r b i t . A f t e r s lo w in g t h es p i n r a t e and a d j u s t i n g o r i e n t a t i o n , t h e y w i l l commandf i r i n g o f t h r u s t e r s t o t r i m up t h e o r b i t t o a c c e p ta b l ed imens ions .
D u ri ng t h e 2 43-day p r im a ry O r b i t e r m i s s i o n , t h e o r b i tw i l l have a p e r i o d c l o s e t o 2 4 h o u r s . T h i s means t h a t m os ta c t i v i t i e s w i l l o c c u r o v e r t h e same D S N s t a t i o n e v e ry d ay .
The 24 -h ou r o r b i t h a s b ee n d i v i d . e d i n t o two p e r i o d s ,r e f l e c t i n g t h e ki nd of measuremen ts be in g t ak en . The pe r i -a p s i s ( o r b i t a l low p o i n t ) p e r i o d i s a b o u t f o u r h o u r s l o n g.The a p o a p s is ( o r b i t a l h i g h p o i n t ) p e r i o d i s 2 0 h o u r s l o n g .
M i ss io n o p e r a t i o n s w i l l u s e f i v e d a t a f o r m a t s d u r i n gt h e 4-hour p e r i a p s i s p e r i o d . T he sz f o r m a t s a r e d e s ig n e d t op e r m i t s em ph as is i n c e r t a i n i n s t r u m e n ts when d e s i r a b l e ; f o re x a m pl e , o n e p r o v i d e s i n t e n s i v e ae ro no my c o v e r a g e a t p e r i a p -
s i s , a n o t h e r s t r e s s e s o p t i c a l c o v er ag e.
The mapping format g i v e s 4 4 p e r c e n t o f t h e d a t a f l owt o t h e r a d a r m ap pe r f o r Venus s u r f a c e s t u d y , a n d d i v i d e s t h er e s t between t h e u l t r a v i o l e t s p e ct r om e t er and t h e i n f r a r e dr a d i o m e t e r .
N o r m a l l y , c o n t r o l l e r s w i l l u s e o n l y t wo d a t a f o r m a t si n t h e 20 -h ou r a p o a p s i s s eg m en t. The f i r s t of t h e s e w i l lb e f o r t a k in g p i c t u r e s o f t h e whole p l a n e t i n u l t r a v i o l e tl i g h t , which w i l l show t h e f o u r - da y r o t a t i o n o f V e nu s'c l o u d s i n s e q u en c e. Known as t h e i ma g in g f o r m a t , it a l l o -c a t e s 6 7 p e r c e n t o f t h e d a t a s t r e a m t o t h e i ma gi ng a n d
c l o u d p h o t o p o l a r i m e t e r i n s t r u m e n t , and d i v i d e s t h e r e s t amonqt h r e e s o l a r w in d- pl an et i n s t r u m e n t s an d t h e a s t r o n o m i c a l ex-
p e r i m e n t ' s gamma b u r s t d e t e c t o r . The o t h e r f o r m a t , knowna s t h e g e n e r a l f or m at , a l l o c a t e s d a t a r e t u r n among a l l O r -
b i t e r e x pe ri m en ts e x c e p t t h e p i c t u r e - t a k i n g c l ou d p ho to -p o l a r i m e t e r a nd t h e i n f r a r e d r a di o m et e r. A s much a s t h r e e -q u a r t e r s o f t h e t o t a l a po a ps is p e r i od w i l l b e d e vo t ed t oi m ag i ng , w h i ch h a s v e r y l a r g e d a t a r e q u i r e m e n t s .
D uring t h e f i r s t 80 d a y s i n o r b i t , Dec. 5 , 1 97 8 t oFeb. 23, 1979 , t h e O r b i t e r w i l l ? a s s beh ind V enus f o r oc-c u l t a t i o n p e r i o d s o f up t o 23 m i nu te s. T h i s a l lo w s t h er a d i o s c i e n c e team t o m ea su re e f f e c t s of Venus ' a tmosphereo n t h e s p a c e c r a f t r a d i o s i g n a l down t o a p p ro x i m at e ly 50 km( 3 1 m i . ) a l t i t u d e . S i n c e t h e n ar ro w beam s i g n a l i s b e n t
by t h e p l a n e t ' s a t mo sp he re , t h e a n t en n a ' s d i s h r e f l e c t o rw i l l be commanded a s much as 1 7 de gr ee s away from t h e
E a r t h - l i n e t o e x t e n d t h e t i m e o f r e c or d in g t h e s i g n a l as it
i s r e f r a c t e d a ro un d t h e s o l i d p l a n e t .
A l s o , d u r i n g o c c u l t a t i o n s , when c om m u ni ca ti o ns a r e c u to f f , t h e Venus O r b i t e r w i l l s t o r e d a t a i n i t s m i l l i o n - b i tmemory. A f t e r emergence of t h e O r b i t e r , t h e d a t a memoryr e a d o u t w i l l be u se d f o r r e t u r n o f s t o r e d d a t a . A secondo c c u l t a t i o n p e r i o d o c c u r s f ro m May 7-16 , 1 9 7 9 , a l l o w i n ga d d i t i o n a l r a d i o m ea su re me nt s o f t h e u p p er a tm o sp h er e.
D urin g t h e e i g h t months o n o r b i t , h e a l t h o f t h e s pa ce -
c r a f t w i l l be m o n i t o r e d th r o u g h t h e c o n t i n u o u s f l o w o f e n-g i n e e r i n g d a t a . R ed un dan t s y s te m s f o r t h e m os t c r i t i c a l
f u n c t i o n s ( s u ch as command and d a t a r e t u r n ) w i l l be u se d i f
Mission Opera t ions e n g i n e e r s a l s o w i l l t r i m t h e o r b i ta b o u t every 1 0 d ay s e i t h e r t o l o w e r p e r i a p s i s a l t i t u d ewhich i s g r a d u al l y r a i s e d by s o l a r g r a v i t y p e r t u r b a t i o n so r t o a d j u s t t h e o r b i t a l p e r io d when it d r i f t s f rom th e
d e s i r e d v a l u e .
The pr i m a ry m i s si o n e n d s a f t e r 2 4 3 d a ys . S h o r t l y a f t e r -w a r d s , t h e O r b i t e r a n d Venus w i l l be b e h i n d t h e Sun andcommunicat ions w i l l b e g ar b l e d o r c u t o f f for s e v e r a l d a y s.A f t e r e me rg en ce f ro m s o l a r b l a c k o u t , t h e o p p o r t u n i t y w i l lb e a v a i l a b l e f o r e x te n d ed m i ss i on o p e r a t i o n s .
On D e c . 9 , 1978, a t about 10:45 a . m . PST, t h e f o u r p r o b e sw i l l a r r i v e a t Venus and en t e r t h e a tmosphe re . The La rgeProbe w i l l descend t o V en us' s u r f a c e i n 5 5 m i n u t e s an d t h e
Nor th , Day and Nigh t p rob es i n abo u t 57 minu t e s .
La rge P robe En t ry Even t s
A t 2.5 h o u rs b e f o r e e n t r y ( t i m e o f e n t r y i s d e f i n e d a st i m e a t 200 km ( 1 2 0 m i . ) a l t i t u d e ) , t h e L a rg e P r o b e ' s commandu n i t w i l l o r d e r warmup o f t h e b a t t e r y a nd r a d i o r e c e i v e r .Twenty-two m inu t e s be fo re en t r y , t h e p robe w i l l b e g i n t r a n s m i s s i o no f r a d i o s i g n a l s t o E a r t h . A t e n t r y m i n u s 1 7 m i n u t e s , t h ep ro be b e g in s t r a n s m i t t i n g d a t a a t 256 b ps . The command u n i ti n i t i a t e s warmup of t h e s ev en s c i e n t i f i c i n s t r u m en t s a b oa rd ,p l u s i n s t r u m e n t c a l i b r a t i o n . F i v e m i nu t es b e f o r e t h e p eake n t r y d e c e l e r a t i o n p u l s e o f 320 G , t h e p r o b e w i l l be t r a v e l i n g
4 1 , 6 0 0 km/hr ( 2 6 , 0 0 0 mph).
The t i m e r w i l l command d a t a s t o r a g e f o r t h e a t m o s p h e r i cs t r u c t u r e e x p e r i m en t d u r i n g e n t r y c om mu ni ca ti on s b l a c k o u t .
T h i r t y - e i g h t s e c o n d s a f t e r e n t r y , t h e 316-kg ( 7 0 0- l b .)p r o b e b e g i n s t h e d e s c e n t p h a s e , d e p l o y s i t s p a r a c h u t e a n dj e t t i s o n s i t s f o rw a rd a e r o s h e l l - h e a t s h i e l d . F o r t y - t h r e es ec on ds a f t e r e n t r y , a t a n a l t i t u d e of 66 km ( 4 0 m i . ) , a l li n s t r u m e n t s s h o u l d b e o p e r a t i n g . S e v e nt e en m i n u te s l a t e r ,
a t a n a l t i t u d e of 4 7 km ( 2 8 m i . ) , t h e p a r ac h u te i s j e t t i s o n e d ,and t h e a e r o dy n am i ca ll y s t a b l e p r e s s u r e vesse l d e s ce n d s t ot h e s u r f a c e i n 39 m i n u te s , i m pa c ti ng 5 5 m i nu t es a f t e r e n t r y .
T h e p r o b e j e t t i s o n s i t s p a r a c h u t e t o s p e ed i t s d e s c e n t t h r o u g hVenus' ve r y dense a tmosphe re , so t h a t i t r e a ch e s t h e s u r f a c eb e f o r e h e a t d e s t r o y s it.
D u r in g d e s c e n t , t h e L ar g e P r o b e ' s s e v e n i n s t r u m e n t s w i l lh av e o b t a i n e d d a t a t o d e t er m i ne a l t i t u d e a n d c o m po s it io n o fc l ou d l a y e r s , a tm o sp he re c o n s t i t u e n t s , t e m p e r a t u re , p r e s s u r e ,d e n s i t y , w ind f l o w and v a r i a t i o n s of h e a t f l o w i n t h e a t mo s ph e re .
Large Probe w i l l i m p a c t t h e s u r f a c e a t a b o u t 36 km/hr( 2 2 mph). (None of t h e pr ob es i s r e q u i r e d t o s u r v i v e i m p a c t,b u t t h e y may r e t u r n some s u r f a c e d a t a , e s p e c i a l l y t h e t h r e es m a l l e r p r o be s , which r e t a i n t h e i r h e a t s h i e l d s . )
The 91 kg (200 lb.) North, Day and Niqht probes also will
enter the planet's atmosphere at about 4 1 , 6 0 0 km/hr (26,000 mph).However, the angles of their flight paths into the atmospherevary greatly. This means that entry heating and durations
of maximum deceleration pulses vary widely. Peak decelera-tion forces vary from 200 G to 5 6 5 G . Entry times also
differ by 11 minutes. A s with the Large Probe, entry isdefined as occurring at an altitude of 200 km (120 mi.).
Three hours before entry, the stable oscillator in the
radio transmitter for one-way Doppler tracking and the batteryon each smaller probe are warmed up by commands from theonboard command units. Twenty-two minutes before entry, each
probe begins transmission of radio signals to Earth.
Seventeen minutes before entry, the smaller probes begintransmitting data at 64 bps. The command unit initiates
warmup and calibration for the three instruments on each of
the probes.
Five minutes before entry, two cables and weights ofthe yo-yo despin system are deployed to reduce the spin ratesof each probe from 48 to 15 rpm. The high spin rates imparted
by the Bus earlier were needed to disperse the probes to entrypoints widely spaced over the planet. However, this widedispersion a l s o means that the smaller probes enter Venus'
high upper atmosphere somewhat tilted o f f their flight paths.The "spindown" of the probes is needed to make it easier for
aerodynamic forces to line up the axes of the probes with the
flight paths. This must occur quickly before heating at the
edges of the probes' conical heat shields becomes serious.Cables and weights are jettisoned immediately after spindown.
Five minutes before the peak deceleration pulse ofatmospheric entry, the command unit orders the "blackout"
format for storage of spacecraft data, plus heat shieldtemperature and accelerometer measurements for the atmos-pheric structure experiment. This is to assure no l o s s ofdata during the 10-to-15-second communications blackout atentry.
Within the first minute (18 to 4 6 seconds) after entry,
the nephelometer window on each Probe is opened, and the
atmospheric structure and net flux radiometer housing doorsare opened and instrument booms deployed.
At this time, the upper descent phase begins, with thethree probes in the altitude range of 72 to 65 km ( 4 3 to
39 mi.) and all instruments operating. The instrumentcompartment doors on each side of the smaller probe after-
A small vane on the pressure sensor inlet prevents thespin rate from falling to zero rpm, enabling instruments to
make observations over a full rotation of the probe.
At entry plus 16.4 minutes, as the thickening atmosphere
interferes with radio transmission, the data rate is reducedto 16 bps. This occurs at an altitude of 30 km (18 mi.).
From this point, the North, Day and Night probes descendinto Venus' increasingly dense lower atmosphere, impactingon the surface at 36 km/hr (22 mph) 57 minutes after their
entry times. Unlike the Large Probe, the smaller probesretain their heat shields to the surface. The density ofthe atmosphere is so great that the drag of these aero-
dynamic surfaces slows the probes to the desired descentspeed.
Bus Events
Eighty-eight minutes after all probes have entered theVenus atmosphere, the Bus will enter on the day side of the
planet at a high latitude in the southern hemisphere. TheBus has no heat shield for high-speed entry, and is expected
to burn up two minutes after entry. However, the Bus carriestwo experiments on the composition of the atmosphere, an ionand a neutral mass spectrometer. These instruments measureconstituents of the ionosphere and upper atmosphere from
200 km (120 mi.) down to 115 km (71 mi.). These are the
missions' only atmospheric composition measurements between
150 and 115 km (93 and 71 mi.), a region that contains themaximum density of the Venusian ionosphere. The Bus, with
i t s more powerful transmitter, returns this data to Earthat 1,024 bps before its destruction.
Entry Data Return--Probes, Bus and Orbiter
The four probes and the Bus will return their data
during a critical 1-hour, 38-minute-period on Dec. 9.
This will extend from Large Probe entry at 10:45,a.m. PSTto Bus burnup at 12:23 p.m. PST.
The probes and Bus will return data almost simultaneously.
All the spacecraft radio data directly to Earth. This means
that Deep Space Network stations must find, receive and recordfive entry craft data signals plus Orbiter data, all at thesame time. To make sure of getting this critical data, the
two D S N 64-m (210-ft.) antennas at Canberra, Australia, andGoldstone, Calif., will receive and record all incomingsignals simultaneously.
F r e q u e n c i e s of t h e p ro be s i g n a l s w i l l s h i f t w i d e l yb e c a u s e of t h e a b r u p t s p a c e c r a f t v e l o c i t y c ha ng es a t e n t r y - -from 4 1 , 6 0 0 km/hr ( 2 6 , 0 0 0 m p h ) down t o a b o u t 6 5 km/hr
( 4 0 mph)-- a nd s i g n a l d i s t o r t i o n s by V en us ' d e n s e a t m o s ph e r e .To f u r t h e r co m p l ic a te s i g n a l r e c e p t i o n s , t h e r e i s a 10- to -15
second communica t ions b lackout a t peak e n t r y d e c e l e r a t i o n .For a s h o r t t i m e t h e r e i s n o s i g n a l a t a l l , and when t h es i g n a l d oe s resume i t s f r e q u e n c y h as s h i f t e d . T o a s s u r eg e t t i n g d a t a streams from t h e f i v e e n t r y c r a f t , g ro un dr e c e i v e r s w i t h b a n d w i d t h s s u f f i c i e n t t o c ov er a l l p o s s i b l e
f re qu en cy s h i f t s w i l l r e c e i v e and r e c o r d t h e i n c o m in gs i g n a l s .
The DSN's rea l - t ime rece iver s y st em s a t b o t h G o ld st on eand Canber ra w i l l s e r v e a s b a ck u ps t o t h e wide-band record inga r r a y s . F or t h i s r e a l - t i m e r e c e p t i o n of t h e s i g n a l s , t h e
DSN h a s a c q u i r e d s p e c i a l a u t o m a t i c t u n i n g e qu ip me nt t o
e n a b l e o p e r a t o r s t o s t a y l o ck e d o n p ro be s i g n a l s d u r i n g a s
much of t h e p ro b e e n t r y an d d e s c e n t p e r i o d s a s p o s s i b l e ,d e s p i t e s h i f t s i n s i g n a l f r e qu e n c ie s .
S c i e n t i s t s w i l l c a l c u l a t e a tm o s ph e ri c wind v e l o c i t i e sa n d d i r e c t i o n s f r o m measurements of c h a n ge s i n p r ob et r a j e c t o r i e s ( v e l o c i t i e s ) , t h r ou g h t r i a n g u l a t i o n m e as ur em en tsi n v o l v i n g f o u r s t a t i o n s . Two S T D N s t a t i o n s a t Guam a n dS a n t i a g o , C h i l e , w i l l r e c o r d Bus an d p r o b e d a t a , a l o n g w i t ht h e D S N s t a t i o n s a t Canber ra and Golds tone .
Venus is the planet most similar to Earth in size, mass
and distance from the Sun. But its surface is much hotter,
its atmosphere much denser, and its rotation much slower than
that of Earth.
The diameter of Venus is 1 2 , 1 0 0 km ( 7 5 1 9 mi.), compared
with Earth's 1 2 , 7 4 5 km ( 7 9 2 0 mi.). The mass of Venus is
0.81 times that of the Earth. The mean density of Venus is
5 . 2 6 grams per cubic cm compared with Earth's 5 . 5 grams per
cubic cm.
Because Venus is closer to the Sun, it receives abouttwice as much energy as Earth. However, it is more reflective
than Earth because of its cloudy atmosphere. As a result of
these two competing factors, Venus absorbs about the sameamount of solar energy as Earth. Thus Venus would be expected
to have a temperature very similar to Earth's. In fact, thesurface of Venus is very hot, about 480 degrees 2 ( 9 0 0 degrees F).
This theory for the high temperature of Venus assumes that
the atmosphere allows the passage of the incoming solar radiationto the lower atmosphere and the surface. However, the atmosphere
restricts the passage of heat radiation from the surface and thelower atmosphere back into space. The heat is trapped. Earthhas a modest greenhouse effect that raises its surface temperature
by about 35 degrees C (95 degrees F.),but in some parts of the
infrared spectrum heat can escape by direct radiation from theEarth's surface to space. Because of its density, compositionand clouds, the Venus atmosphere is very thick, and because it
is mostly carbon dioxide, it is essentially opaque to outgoingheat radiation at all important wavelengths.
One of the most puzzling aspects of Venus is its lack ofwater. If Venus is as dry as it seems, where did the oceans ofVenus go, if any ever existed? One speculation is that the
water rose into the upper atmosphere and was dissociated by
solar ultraviolet radiation into hydrogen and oxygen. Thehydrogen escaped into space from the top of the Venus atmosphere,
and the heavier oxygen diffused down to the oxidized crust.Detailed analysis shows that it might not be Practical for Venus
to have lost an ocean of water by such a route.formed close enough to the Sun so that the temperature prevented
water from being incorporated into the solid material that formedthe planet. If so, Venus would never have had enough water
within its rocks to form early deep oceans like those of Earth.
Direct measurements of gases within the Venus atmosphere maypoint toward one of two alternatives: Either that water was
not incorporated into Venus as much as on Earth, or that wateroutgassed and was subsequently lost.
T h e r o t a t i o n of Venus i s v e r y slow a nd i n a r e t r o g r a d ed i r e c t i o n , t h a t i s , o p p o s i t e t o t h e d i r e c t i o n o f t h e p l a n e t ' sr e v o l u t i o n a b o u t t h e Sun and t o t h e r o t a t i o n o f m o s t o t h e rp l a n e t s .
a round the Sun a r e i n o p p os i t e d i r e c t i o n s , t h e l e n g th of a s o l a rday on Venus i s 1 1 7 E a r t h d a y s ( 5 8 . 5 E a r t h d a y s of " d a y l i g h t "58 .5 E a r t h d ay s o f n i g h t ) .
V enus tu rn s on i t s a x i s o nc e i n 2 43 .1 E a r t h d ay s .
S i n c e V en us' r o t a t i o n o n i t s a x i s a nd r e v o l u t io n i n o r b i t
The o r b i t s of Ear th and V enus a r e t i l t e d t o e ac h o t h e ra b o u t 3 .5 d e g r e e s . V en us ' a x i s i s t i l t e d a b ou t 6 d e g r e e sf r o m p e r p e n d i c u l a r t o t h e p l a n e of t h e p l a n e t ' s o r b i t .co mpares w i t h E a r t h ' s a x i a l tilt of 23.5 degrees which produceso u r s e a s o n s . T hu s, s e a s o n a l e f f e c t s o n Venus a re smal l .
T h i s
Some s c i e n t i s t s b e l i e v e t h a t Venus' p e r io d o f r o t a t i o ni s t i e d
t ot h e r e v o l u t i o n o f t h e E a r t h an d Venus a r ou n d t h e
Sun. Venus p r e s e n t s t h e same hem isphere toward E a r t h a t each
c l o s e s t a p p r o a c h ; t h a t i s , e a c h t i m e t h e p l a n e t p a s s e s b e tw ee nSun and E a r t h . I f t h e r o t a t i o n of Venus i s l o c k e d t o t h e c lose
a p p r o a c h e s o f E a r t h a nd Venus, t h e n t h e i n t e r n a l d i s t r i b u t i o n ofmass w i t h i n Venus s h o u l d b e s l i g h t l y asymmetric.
Why does V e n u s r o t a t e s o slowly when m o s t o t h e r p l a n e t sr o t a t e i n p e r io d s of h o u r s r a t h e r t h a n m on th s? One s p e c u l a t i o ni s t h a t a l a r g e b ody h i t Venus and s topped i t s r o t a t i o n . T h isl a r g e bo dy m ig h t ha v e be e n c a p t u r e d a s a s a t e l l i t e i n t o ar e t r o g r a d e o r b i t a nd l a t e r impacted wi th Venus t o s t o p i t sn or ma l r o t a t i o n a nd r o t a t e it s lo w ly i n a n o p p o s i t e d i r e c t i o n .
It c o u l d be t h a t Venus was formed f r o m l a r g e f r a g m e n t s ,and a s a r e s u l t of t h e combined im p ac t s o f t h e s e f r a g m e n t s n e v e rhad much ro ta t i o n . A ccord ing t o a n o t he r s u g g e st i o n , s o l a r t i d a le f f e c t s i n V en us ' d e n s e a t m o s ph e r e may have slowed r o t a t i o nand t h e n " t u rn e d t h e p l a n e t o v e r " , a c c o u nt i n g f o r i t s backwardr o t a t i o n .
Radar as tro no me rs have mapped a n area o n t h e E a r t h - f a c i n g
s i d e of t h e p l a n e t a s l a r g e a s A s i a and have found w ha t app ea r st o be a r u gg e d s u r f a c e . A c c or d in g t o t h e r a d a r r e s u l t s , t h e re
a re huge sha l low c r a t e r s a s w e l l a s an enormous volcano whichmay be as l a r g e i n a r e a , t h o u g h n o t a s h i g h , a s Olympus Monson Mars ( t h e s o l a r s y s te m ' s l a r g e s t d i sc o v e re d so f a r ) . Radara s t r o n o m e r s a l s o d e t e c t e d wh at a p p e a r s t o be an enormous canyon.T h i s chasm i s 1 4 0 0 k m ( 8 7 0 m i . ) l o n g , 1 5 0 k m ( 9 5 m i . ) w i d e , andsevera l k i l o m e t e r s d e e p .
Venus' I n t e r i o r and Absence of Magnet ic F i e l d
U n li ke t h e E a r t h , V enus h a s no s i g n i f i c a n t m a gn e ti c f i e l d .
C on ve ct io n c u r r e n t s i nT h e g e n e r a t i o n of E a r t h ' s f i e l d i s a t t r i b u t e d t o a s e l f - s u s t a i n i n gdynamo i n t h e f l u i d core o f t h e p l a n e t .t h e co re g i v e r i s e t o e l e c t r i c c u r r e n t s t h a t p ro du ce t h e e x t e r n a l
m a g n e t i c f i e l d . T h i s t h e o r y , w hic h a l s o seems t o a pp ly t oJ u p i t e r , p r e d i c t s t h a t s lo w -s p in n in g p l a n e t s l i k e Venus shouldno t have magne t i c f i e l d s .
Venus i s a p la n e t whose shape cou ld be v e ry c l o s e t o asph e re acc o rd ing t o r a d a r measu remen t s. They show i t s e q u a t o rt o be a l m o s t a p e r f e c t c i r c l e . B ec au se t h e p o l e s d o n o t r o t a t ei n t o view a s d o p o i n t s on t h e e q u a t o r , c i r c u l a r i t y aro un d t h ep o l e s c a n n ot b e m ea su re d. The l a c k o f i r r e g u l a r i t i e s i nshape , and of a s a t e l l i t e makes it d i f f i c u l t t o d e t er mi ne t h ei n t e r n a l d e n s i t y d i s t r i b u t i o n of t h e p l a n e t . Most models oft h e i n t e r i o r a r e based on i t s s i m i l a r i t y t o E a r t h , c o n s i s t i n gof a l i q u i d c o r e , a s o l i d m a n tl e and a s o l i d c r u s t . B ut t h e
t r u e n a t u re o f t h e i n t e r i o r of t h e p l a n e t i s ve ry much i nd o u b t b e c a u s e s c i e n t i s t s d o n o t k n o w V e n u s ' t h e r m a l s t r u c t u r eo r t h e n a t u r e of t h e m a t e r i a l s which make up i t s mass.
The Atmosphere of Venus
Carbon d iox ide i s t h e d o m in an t g a s i n t h e V e nu s ia na tmosphe re . T h e r e a re a l s o t r a c e s of w a t e r , carb on monoxide ,h y d r o c h l o r i c a c i d and hy dr og en f l u o r i d e . Free oxygen has neverbeen found.
T h e c l o u d s w hi ch o b s c u r e t h e s u r f a c e of V e n u s c o n s i s t of
t h i c k h a ze s of d r o p l e t s b e l i e v e d t o be made o f s u l f u r i c ac i d .Venus ' c louds a re p a l e y e ll o w a nd v e r y r e f l e c t i v e , r e t u r n i n gi n t o s p a c e s o m e 75 p e r c e n t o f t h e s u n l i g h t f a l l i n g on them.
Space prob e measurements have shown t h a t t h e r e a r e d i s t i n c tc l o u d l a y e r s much h i g h e r t h a n t e r r e s t r i a l c l o u d s . P h o t o g r a p h st a ke n i n u l t r a v i o l e t l i g h t r e ve a l a f o u r - d a y r o t a t i o n of t h e
m ar kin gs i n t h e s e c l o u d s . T h i s r o t a t i o n i s l i k e t h a t of t h ep l a n e t , i n a r e t r o g r a d e d i r ec t i o n . Unusua l dynamics o f t h ea tmosphe re a r e r e q u i r e d t o a c co u nt f o r t h i s high-speed c loudmot ion .
The g e n e r a l l y a c c e p t e d f i g u r e f o r a tm o sp h er ic c a r bo nd i o x i d e on Venus i s 9 7 p e r c e n t . However , measurements made
by e a r l y Vene ra s p a c e c r a f t (USSR) d i f f e r from r a d i o o c c u l a t i o nmeasuremen t s sugges t i ng t h e p r e s e n c e o f a b o u t 7 0 p e r c e n tc a rb o n d i o x i d e i n t h e Venusian atmo sphe re. And, i f t h e r ei s much a rgon i n t h e a tmosphe re , t h e amount of carb on d i ox id ec o u l d be as l o w a s 2 5 p e r c e n t .
Adding to the uncertainty is the fact that the percentagesdetermined by the Veneras were obtained by sampling the
atmosphere in regions where there are sulfuric acid droplets.
The presence of the acid may have contaminated these measurements.
It is therefore possible to argue that the carbon dioxide is
considerably less than 9 7 per cent, with the remainder being
made up by some combination of nitrogen and argon.
The amount of carbon dioxide is important because it plays
a major role in the interpretation of the microwave spectrum
of the planet. If the atmosphere is 9 7 per cent carbon dioxide,the microwave observations permit the presence of as much as
0.1 per cent water below the clouds. Some instruments on the
most recent Veneras 9 and 10 indicated that water vapor
constituted about 0.1 per cent of the atmosphere below the
main clouds. At the cloud tops it is only 0.0001 per cent,
however. But, if there is another gas in the atmosphere of
Venus that is not a good microwave absorber, the planet's
atmosphere might contain more water than is now believed.
Carbon dioxide is also important to theories about theevolution of the atmosphere of Venus, and to the radiative
properties of the present atmosphere and its dynamic
characteristics.
The atmospheres of both Venus and Earth are assumed to
have originated from gases that were released from the interiors
of the planets which were hot when the planets first formed. In
the case of Earth, most of the outgassing may have occurred soon
after formation, from the heat of formation. Venus may neverhave had much water to outgas in the first place if it was
formed from parts of the solar nebula that were poor in water.Or it may be that Venus formed with as much water as the Earth,but this water has now been lost.
The Earth holds its water in its oceans because it is muchcooler than Venus and there is a "lid" on its atmosphere. This
lid is the very cold tropopause where the temperature riseswith altitude. This prevents heated water vapor from rising
by convection to cooler heights where it could be,dissociated
by solar ultraviolet radiation. But if Earth were moved to
the same distance from the Sun as Venus, conditions could
change drastically. The additional solar energy would be
If Venus had been formed from the same mix of materialsas Earth and then outgassed its volatiles, we would expect it
to have an atmosphere about 350 times as massive as Earth's.Carbon dioxide would account for a surface pressure of about100 atmospheres, and water vapor would account for about 150
atmospheres. On Earth most of the 100 atmospheres of carbon
dioxide is tied up in carbonate rocks which are chemicallystable at terrestrial temperatures, but unstable at Venus
temperatures. Earth's oceans, if vaporized, would result in
an atmospheric pressure of about 250 atmospheres. Venusdoes indeed have nearly 100 atmospheres of carbon dioxide,
but the water is apparently absent. There are no oceans,
and the atmospheric water vapor is a minor constituent.
One of the major questions to be answered by Pioneer Venusis just how much water vapor is present. Water vapor would
be broken down by solar ultraviolet radiation into oxygenand hydrogen. The hydrogen would escape into space leavingthe oxygen behind. Effectively the oceans would be leaking
into space.
This could have happened to Venus. If the primitive
atmosphere of Venus consisted mostly of steam (because theplanet is closer to the Sun than Earth), the resulting
convective atmosphere could not have had a barrier to
convection. The water vapor would have dissociated into
hydrogen and oxygen. Calculations suggest that withinabout 30 million years perhaps 90 per cent of the water
could have been lost to the planet, but all could not be
lost in this way.
Furthermore, there is no easy way to explain what
happened to the leftover oxygen other than that it reacted
with the surface rocks. Yet without running water tocontinually expose fresh rocks for oxidation, the process
might be insufficient to remove all the oxygen.drift might be a possible mechanism to expose fresh rocks.
There is a question, too, of what happens to the oxygen nowreleased in the upper atmosphere by photodissociation ofcarbon dioxide to produce the carbon monoxide observed
spectroscopically. The incorporation of oxygen with sulfurto form the sulfuric acid droplets does not seem to account
On Venus, because of the high surface temperatures,
reactions between rocks, minerals and the atmosphere are
expected to be much faster than on Earth. However, on Earth
the action of running water constantly exposes new rocks to
the action of the atmosphere and aids oxidation and otherreactions between the rocks and the atmosphere. This is not
happening on Venus. If fresh rocks are not being exposed bysome other mechanism, the atmosphere of Venus may not haveachieved equilibrium with surface materials.
The Venus atmosphere can be divided into three distinctregions:
the ionosphere and the exosphere; a region of clouds; and a
region from the base of the clouds to the surface.
a region above the visible cloud tops which includes
Upper Atmosphere
The upper atmosphere of Venus has an ionosphere which is
different from that of Earth. Because Venus does not have asignificant magnetic field, the solar wind interacts directly
with the upper atmosphere and the ionosphere of the planet.
Among the atmospheric regions of Venus, the upper atmosphereabove the cloud tops is best understood. It has been investigated
from Earth and from flyby and orbiting spacecraft. Above 150 km
( 9 0 mi.) it is more rarefied than the atmosphere of Earth at the
same height. Like Earth's atmosphere, it is ionized by incoming
solar radiation to produce positively-charged ions and free
electrons of an ionosphere, which is thinner and closer to the
surface of the planet than Earth's ionosphere. Like Earth's
ionosphere, the ionosphere of Venus has layers at which the
number of electrons per cubic centimeter (electron density)peaks. In Earth's ionospheric layers, the peak electron density
is about 100,000 to 1,000,000 electrons per cubic centimeter, and
occurs at an altitude of about 250 to 300 km (150 to 180 mi.).
The major ion is singly-charged carbon dioxide.
Mariner 10 found two clearly defined layers in the nighttime
ionosphere: a main layer at 142 km ( 8 7 mi.) altitude and a
lesser layer at 124 km ( 7 6 mi.). The peak intensity of the latter
was about 7 8 per cent of the higher layer. On the dayside there
was one main layer at 142 km ( 8 7 mi.) and several minor layers,
including one at 128 km ( 7 8 mi.) and another at about 1 8 0 km
(110 mi.). The Venera 9 and 10 orbiters obtained similar results,
From a practical standpoint, Venus has no intrinsic
magnetic field. The field of Venus is less than 1/10,000 ofEarth's field. There is a region of rarefaction (lessened
density) of the solar wind flow at Venus, and the characteristics
of the plasma there indicate that Venus absorbs part of the flux
of the solar wind. On the dayside of Venus, there is a sharpboundary to the ionosphere at 350 to 500 km ( 2 1 0 to 305 mi.).
This is believed to be caused by the interaction of the solar
wind with Venus' atmosphere. On the night side of the planet,
the ionosphere extends high into space and probably into a
plasma tail stretching away from the Sun.
Temperatures have been measured in regions above the visiblecloud layers by radio occultation. The temperature of theexosphere (region where particles escape the planet) was derivedfrom density variation with altitude found by the ultraviolet
experiments of spacecraft. From observations of the ultraviolet
radiation from hydrogen and helium atoms, it is calculated that
the temperature of the exosphere of Venus when Mariner10
flewpast the planet was about 1 2 7 degrees C ( 2 6 0 degrees F). At such
a temperature, the thermal escape of helium gas would be negligibleAccordingly it is thought that if helium outgassed from the rocks
of Venus as it did on Earth the gas might have accumulated in
the upper atmosphere of Venus.about 800 km (480 mi.) and contains up to 10,000 atoms per
cubic centimeter.
A corona of hydrogen begins at
Haze Layers
At least two tenuous layers of haze can be seen in high
resolution pictures of the limb (edge of the disc) of Venus.They extend from equatorial regions to higher latitudes. They
may be associated with temperature inversions in the highatmosphere, and may result from processes similar to those in
Earth's atmosphere which produce layers of aerosols in thestratosphere. Aerosols are solid or liquid particles suspended
in an atmosphere. The stratified layers of haze are in the
region 8 0 to 90 km (50 to 5 6 mi.) above the surface of Venuswhere the atmospheric pressure is between 50 and 0.5 millibars.
(Pressure at Earth's surface is 1000 millibars). These haze
layers are extremely tenuous. At the topmost haze layer, if
the atmosphere is mainly carbon dioxide, the temperature shouldbe - 7 5 degrees C. However, temperatures determined from
occultations differ appreciably above 60 km ( 3 7 mi.)t suggestingtemperature inversions that separate the haze layers from thetopmost convective cloud deck as well as the upper from thelower haze layers. In the region above 50 km (30 mi.), the
daytime atmosphere is about 15 degrees C ( 5 9 degrees F)warmer than the temperature at night.
Below the upper atmosphere is the 18-km (11-mi.)-thick
region containing the clouds of Venus visible from Earth. While
the clouds of Venus look extremely opaque, they are in fact very
tenuous. Veneras 9 and 10 determined that visibility within the
clouds is between 1 and 3 km (0.6 to 1.8 mi.). They are morelike thin hazes than terrestrial clouds. The particles makingup the clouds of Venus are spherical and about one to two
microns in diameter. These droplets apparently consist ofsulfuric acid, with concentrations varying from 50 to 500
per cubic centimeter.
The presence of sulfuric acid clouds explain the extremedryness of the Venus upper atmosphere. Nearly all the water
has chemically bound up in the sulfuric acid droplets. The
density of Venus' atmosphere at this level is about one-tenththe density of Earth's atmosphere at sea level. Sulfuric acid
clouds remain as clouds over a wider range of temperature than
water clouds, although high temperatures cause some of thewater to evaporate from the droplets. There is evidence of
the presence of fluorine in the Venus atmosphere. This element
probably combines with water into the extremely stable and
corrosive fluorosulfonic acid. But none of these acids can
account for the absorption of ultraviolet radiation by the
clouds. There must be an unknown ultraviolet absorber in theclouds which gives rise to the dark markings seen in ultravioletpictures of Venus.
One speculation is that the dark regions seen in ultraviolet
light are oxygen-depleted regions where a significant amount ofultraviolet-absorbing sulfur is being produced. There appears
to be a whole series of compounds of sulfur, oxygen and halo-qens that enter into the chemistry of the atmosphere of Venus.
The Pioneer Venus measurements of the constituents of theatmosphere of Venus with a mass spectrometer and gas chromatographshould contribute greatly to our understanding of these chemicalprocesses that are responsible for the Venusian clouds and theirmarkings.
The dark markings of the clouds, seen in ultraviolet light,
have characteristic forms that have been studied from Earth.
There are horizontal Y-shaped features which sometimes have atail. There are features that look like a reversed letter C.
The features in the form of a reverse letter C appear moreoften on the evening terminator than on the morning terminator.
Other features are like a reversed C with a bisecting bar.Sometimes there are two parallel equatorial bands. The
patterns are a l s o almost always symmetrical about the equator
of Venus. The arms of these features are always open in the
direction of their retrograde motion which varies between
In the upper atmosphere the effects of solar heating are
ignificant, and the C-bar, C- and Y-shaped features are all.ssociated ith the sub-solar point, which is the point where
the Sun shines down on the Venus atmosphere from directlyoverhead. However, the features move around the planet and
are not fixed with respect to the sub-solar point.
A big question about Venus' atmosphere is whether the
apparent motions of the ultraviolet markings are a result ofactual movement or merely a wave motion. The evidence today
points to an actual movement of mass: i.e., winds. But there
is some evidence of wave motions, diurnal tides and parallelequatorial belts.
The division between the high wind velocities of the
stratosphere, and the near calm of the dense surface atmosphere
seems to come at about the 56 krn (36 mi.) level. The big change
in wind velocity thus appears to take place at the bottom of the
clouds where there must be a shear zone. Thus, the cloud bottomsare expected to be extremely ragged.
The Soviet probes measured the amount of solar radiationdown to the surface. Above 50 km (31 mi.), scattering appears
to be by the cloud particles. Below about 25 km (15 mi.), thescattering is Rayleigh scattering; i.e., by much smaller air
molecules. At the surface, with the Sun's position about 30
degrees from overhead, the integrated flux was measured as
being about equal to that on an overcast day on the Earth at
sea level in mid-latitudes.
The high velocity winds in the Venus atmosphere might arise
because the planet has such a massive and deep atmosphere. Large-scale eddies containing a lot of energy could transport momentum
from low to high altitudes with a high amplification. The ionwind speeds in the dense lower atmosphere produced by the heat
from the Sun and the rotation of the planet are amplified intothe thin upper atmosphere.
Lower Atmosphere
The penetration of Veneras 9 and 10 into the lower atmosphereproduced new information about this region. At about 50 km (30 mi.)altitude, the wind velocity appears to be about 130 kph (80 mph).
At the landing site of Venera 9, the local wind velocity variedfrom 1.2 to 2.5 kph (.9 to 1.4 mph); at the Venera 10 site, itvaried from 2.9 to 4.7 kph (1.8 to 9.2 mph). The two landers
thus confirmed a low wind velocity c lose to the surface, as well
T h e r e a re s t i l l many u n r e s o l v e d q u e s t i o n s a b o u t t h ea tm osphere of Venus t h a t need t o be answ ered , such a s :
0 H o w does t he V enus weather m a c h i n e r e a l l y work?
0 I t i s r e a l l y a greenhouse e f f e c t t h a t makes Venusso h o t compared w i t h t h e E a r t h ? O r i s t h e re adynam ic cause?
Did Venus once have a more moderate s u r f a c et e m p e r a t u r e ?
0 What c a u s e s t h e d a r k m a rk in g s i n t h e Venus c louds?
0 What are t h e c o n s t i t u e n t s of t h e Venus a tm osphere?
Therm al em iss ion from t h e u pp er a t mo sp h er e d i f f e r s v e r y
l i t t l e between ni g h t and day and between l o w and h i g h l a t i t u d e .
T h i s i n d i c a t e s a dynam ic a c t i v i t y w i t h i n t h e a tm osphere , ands u g g e s t s t h a t h e a t i n s u b s t a n t i a l amounts i s b e in g t r a n s f e r r e da ro un d t h e p l a n e t h o r i z o n t a l l y . T here a r e dynamic a c t i v i t i e sa t all l e v e l s b e c a u s e s p a c e c r a f t h a ve d e te r m in e d t h a t t h e s o l a rr a d i a t i o n p e n e t r a t e s t h ro ug h t h e c l o u d s a nd , t h e r e f o r e , a f f e c t st h e atmosphere down t o t h e s u r f a c e . D i r e c t s o l a r h e a t in g i smost i m p o r t a n t a bo ve 56 km ( 3 4 m i . ) ; dynamic e f f e c t s b e l o w t h a t .
Over t h e w hol e of t h e p l a n e t t h e r e i s a l s o t h e e f f e c tof t h e a t m o s p h e r e a t t h e e q u a t o r r i s i n g a s it i s warmed by
s u n l i g h t , a n d s i n k i n g n e a r t h e P o l e s , as it c o o l s .
T h e S u r f a c e o f Venus
Radar h a s revealed l a r g e - s c a l e f e a t u r e s t h a t su g g es tt e c t o n i c s a nd i m p a c t m o ld in g of V e n u s ' t o p o g r a p h y . D e t a i l so f t h e s u r f a c e have b e en p r o v i d e d b y t h e t w o S o v i e t l a n d e rs p a c e c r a f t .
The r a d a r o b s e r v a t i o n s r e v e a l a l a r g e - s c a l e g r a n u l a rs t r u c t u r e , s u g g e s t i v e of a r o ck - st r ew n d e s e r t . L ar ge b u ts ha l l ow c i r c u l a r f e a t u r e s , m o s t l i k e l y c r a t e r s , a r e f o un d i ne q u a t o r i a l re g i o n s . Some a re a s o f h i g h r a d a r r e f l e c t i v i t y a r ei n t e r p r e t e d a s e x t e n s i v e l a v a f l o w s a n d m o u n t a i n o u s a r e a s . A
major chasm s t r e t c h e s 1 4 0 0 k m ( 8 7 0 m i . ) n o r t h a nd s o u t h a c r o s s
t h e e q u a t o r.
A t f i v e d e g r e es s o u t h l a t i t u d e and 320 d e g r e e s l o n g i t u d ei s t h e h i g h mounta in B e t a w i t h a c r a t e r e d t o p l i k e t h e l a r g e
M a r t i a n v o l c a n o e s . T h e r e a r e a l s o a r c u a t e r i d g e s . One is a tl e a s t 800 k m ( 4 8 0 m i . ) l o n g . T h e r e a re mounta inous a r e a s whichmay be v o l c a n i c o r a r e s u l t o f c r u s t a l p l a t e m ovements.
Pho tographs from one S o v i e t l a n d e r s p a c e c r a f t c on fi rm ad r y ro ck y s u r f a c e t h a t h a s been f r a c t u re d and moved abo u t byunknown p r oc es se s . The second l a nd e r p roduced a p i c t u r e ofr o c k s w i t h r ou nd ed ed g e s a nd p i t t e d s u r f a c e s . The fo rm s o ft h e s e rocks may be e x p la i ne d by v o l c a n i c a c t i v i t i e s ha vi ng
t a k en p l a c e on the s u r f a c e .
T h e e x i s t e n c e o f c r a t e r s o n Venus s u g g e s t s t h a t i t s s u r f a c eh a s n o t b ee n s u b j e c t e d t o t h e m a jo r t e c t o n i c c h an g es e x p e r i e n ce don E a r t h , b u t t h a t it has p r o b a b l y e v o l v e d s o m e w h a t a l o n g t h esame l i n e s a s Mars. Some o l d c r a t e r e d t e r r a i n i s p r e s e r v e dw h i l e o t h e r p a r t s h av e be en m o d i f i e d by t e c t o n i c s and vo l can i sm.
Venus migh t , i ndee d , have evo lved t o a s t a g e b etw ee n t h a t ofMars and t h a t o f t h e E a r t h .
Venera 9 l a n d e d a t 33 degrees n o r th l a t i t u d e . I t s p i c t u r eshows heaps o f ro ck s , m os t l y abou t 30 c m ( 1 2 i n . ) o r more i n
s i z e , and w i t h r a t h e r s h a rp edges. The f o r m a t io n o f t h e s e r oc ksi s b e l i e v e d t o be a s s o c i a t e d w i t h t e c t o n i c p r o c e s s es . T h e l a n d e ri s b e l i e v e d t o b e on t h e s i d e of a h i l l i n which t h e r e i s somedownward movement of t h e rocks . The s ha rp edges and l a c k ofround ing of t h e r oc ks a t t h i s s i t e s u g ge s t t h a t t h e y w e r eformed from breakage of h a r d , l a y e r e d rocks , p o s s i b l y a l a v af low.
Venera 1 0 l anded a t 1 5 d eg re es n o r t h l a t i t u d e , i n a n a reaw i t h a much smoother s u r f a c e . T h i s i s b e l i e v ed t o be a p l a t e a uo r p l a i n o f g r e a t e r r e l a t i v e a g e t ha n t h e s i t e of Venera 9 .
T here a r e s o m e r o c k y e l e v a t i o n s which a r e c o v e r e d w i t h ar e l a t i v e l y d ar k , f i n e - gr a in e d s o i l . T h i s i m p l i e s t h a t t h e
rocks have been weathered , p o s s i b l y by c h em i c a l a c t i o n w i t ht h e a t m o s p h e r e . It i s u n l i k e ly t h a t t h e g e n t l e w in ds a t t h es u r f a c e c o u ld h av e be en r e s p o n s i b l e for t h e w e a th e r in g .G e n e r a l l y a t t h i s s i t e t h e m a t e r i a l of t h e V e nu si an s o i l i sd a r k , b u t t h e r e a re o u t c r o p s of l i g h t e r - c o l o r e d rock p e n e t r a t i n gt h e s o i l . Some o f t h e d a r k s o i l f i l l s d ep r e s s i on s o f t h eo u t c r o p s . T h i s s u r f a c e i s i n t e r p r e t e d a s being much o l d e r andmore weathered t ha n t h e s u r f a c e s e en a t t h e Venera 9 s i t e . T h e
w e a t h e r i n g p r o c e s s may be a c h e m ic a l i n t e r a c t i o n b et we en t h eh o t r o c k s an d t h e a t m os p h er e , p o s s i b l y by m i n e r a l a c i d s andwater v a p o r .
Measurements made by t h e s p a c e c ra f t in d i c a t e t h a t t h e
s u r f a c e rocks have a de n s i t y between 2 . 7 and 2 . 9 grams perc u b i c c e n t i m e t e r , w hich i s t y p i c a l of t e r r e s t r i a l b a s a l t i cr o c k s .
Surface temperatures appear to be high enough to makeportions of the surface glow a dull red. They are high enough
to melt zinc, but not most common rocks. The Venus rocks at
the two landing sites are about as radioactive as terrestrial
lavas and granites. This suggests that Venus, like Earth,has differentiated by heating to form a dense core and a
lighter crust.
Though it has dramatic major features, the surface issmoother than that of Earth and Mars. Radar-measured minimum
to maximum height differences are 10 km (6 mi.) -- the height
of Mt. Everest. This compares with 20 km (12.4 mi.) on theEarth, from the bottom of the Mariannas Trench to the top
of Everest. It compares with 30 km (18.6 mi.) on Mars, from
the floor of the Hellas basin to the peak of Olympus Mons.Craters on Venus seem to be shallower than on the other worldsof the inner solar system.
On the Moon and Mercury, and to a somewhat lesser extent
on Mars, the ratio of craters diameter to depth is about 10to 1. On Venus, according to the radar surveys, the ratiois more like 100 to 1. The craters on Venus seem to beextremely shallow; the reason is not known. It could result
from plastic deformation of the hot surface or from someweathering process.
0 What a r e t h e r o l e s of g l o b a l c i r c u l a t i o n and l o c a lt u r b u l e n c e i n s t a b i l i z i n g t h e u pp er a tm os ph er e?
0 What a r e t h e e f f e c t s of t h e n e u t r a l p a r t i c l e s o n io no -
s p h e r e c o m p o si t io n ?
0 How h i g h d oe s s u p e r r o t a t i o n ( fo u r- da y r o t a t i o n ) of t h ec l o u d t o p s e x t e n d ?
0 S i n c e V enus h a s no m a g n e t i c f i e l d , t h e s o l a r w i n d i n t e r -a c t s d i r e c t l y w i t h t h e uppe r a tmosphe re . What mechani smsd oe s t h i s c r e a t e , and do th e y a f f e c t t h e lower a tmosphe re?
0 Where di d Venus ' a tmo sphe re c o m e f rom and where i s i t
go ing?
The main sources of Venus ' a tmosphe re p robab ly a r e o u t -
g a s s i n g from t h e i n t e r i o r , g a s e s from t h e o r i g i n a l s o l a rn e b u l a an d some s o l a r wind p a r t i c l e s .
e Where i s t h e water t h a t may ha ve on ce been on Venus?
The o b v i o us an s we r s a r e t h a t it e i t h e r " l ea k ed " t o s p a c eb e c a u s e o f h i g h Venus h e a t i n g , o r it w a s n e v e r t h e r e .Bu t numerous q ue s t i o ns r ema in .
0 Why does Venus ' a tmosphere d i f f e r s o much f rom t h a t ofi t s ' 't w in " p l a n e t , E a r t h ?
0 I s a l l V enus t e r r a i n r e l a t i v e l y low co mp are d t o E a r t h a n dM a r s o r does V en us ' " i n v i s i b l e h e mi sp h er e " c o n t a i n h i g hmounta ins and deep canyons comparable t o t h o s e o n E a r t hand Mars?
0 I s V enus a s c l o s e t o a p e r f e c t s p he re a s t h e e q u a t o r ia lmeasurements sugges t?
0 Does V enus' i n t e r i o r c o n t a i n l a r g e c o n c e n t r a t i o n s o f h ig hd e n s i t y m a t e r i a l .
T he l o c k i n g o f 1 7e nu s' r o t a t i o n t o E a r t h ' s o r b i t s u g g es t ss u c h m as s c o n c e n t r a t i o n s .
0 What i s t h e s u r f a c e t o po gr ap hy ?
0 What i s t h e c o m p o s it i on of t h e s u r f a c e ?
Ninevah t a b i e t s r e c o r d o b s e r v a t i o n s o f V e nu s.
U si ng t h e n ew l y- in v en te d t e l e s c o p e , G a l i l e of i n d s t h a t V e n u s e x h i b i t s p h a s e s l i k e t h o s e of
t h e Moon.
M i k h a i l V . Lomonosov ( U . S .S . R ) i n t e r p r e t s o p t i c a l
e f f e c t s ob se rv e d d u ri n g t r a n s i t o f V enus a s duet o a n a t m os ph er e o n t h e p l a n e t .
Johann H . S c h ro te r (Germany) con clud es Venus h a s
a n a t m o sp h e re b e c a u s e t h e c u s p s a t t h e c r e s c e n tphase ex t e nd beyond t h e g e o m e t ri c a l c r e s c e n t .
Joh ann Wurm (Germany) de te rm in es t h e dia me te r oft h e v i s i b l e d i s c o f Venus a s 12 ,2 93 km (7 ,6 39 m i . ) .
S c h i a p a r e l l i c o nc lu d es from h i s o b s e rv a t io n s t h a t
V enus r o t a t e s i n 2 25 d a ys .
Edward S t . John ( U . S . ) a n d S e t h B . N i c h o l s o n( U . S . ) s u g g e s t t h a t Venus i s a d r y , d u s t y wo rl d
b e ca u se t h e y c a n n o t d e t e c t a ny water v a po r i n i t sa tmosphe re .
L y ot m ea su re s t h e p o l a r i z a t i o n o f s u n l i g h t re -
f l e c t e d fro m t h e c l o u d s o f Venus a nd i n t r o d u c e sa new m eth od o f i n v e s t i g a t i n g t h e s i z e a n d n a t u r eo f p a r t i c l e s i n i t s c l o u d s .
Walter S . Adams ( U . S . ) and Theodore Dunham ( U . S . )
d e t e c t c a r b o n d i o x i d e i n t h e a t m o sp h e re o f V en us .
R u p e r t W i l d t (U .S .) shows t h a t t h e h i g h s u r f a c et e m p e r a t u r e of Venus could a r i s e f r o m a g r e e n -h ou se e f f e c t i n a n a tm os ph er e w i t h a h i g h p r o p o r -t i o n of c a rb o n d i o x i d e .
F r e d H o yl e ( U n i t e d Kingdom) s u g g e s t s t h a t t h eVenus c louds a r e a photochemica l hydrocarbon smog.
R a d i o waves a t 3-cm wavelength a r e d e t e c t e d f r o m
V enus a n d show t h a t t h e s u r f a c e t e m p e r a t u r e isv e r y h i g h ; a b o u t 330 d e g r e e s C ( 62 5 d e g r e e s F . ) .
C h a r l e s B oy er ( F r a n c e ) d i s c o v e r s a fou r -day r o t a -t i o n p e r io d o f t h e u l t r a v i o l e t m ark ing s i n t h ec lo ud s o f Venus.
A douin D o l l f u s ( F r a n c e ) d e t e r m i n e s p r e s s u r e a tc l o u d t o p s a s 9 0 m i l l i b a r s , u s i n g p o l a r i m e t r y .
C a r l Sagan ( U . S .) c a l c u l a t e s h e a t i n g i n atmos-p h e r e w i t h l a r g e a mo un ts o f c a r b on d i o x i d e a nd
w a t e r v a p o r , c o n c l u d e s s u r f a c e t e m p e r a t u r e c a nb e r a i s e d by g re e nh o u se e f f e c t t o a bo ve t h e b o i l -i n g p o i n t of w a t e r , 1 0 0 d e g r e e s C ( 2 1 2 d e g r e e s F . ) .L o w r a d a r r e f l e c t i v i t y o f Venus r u l e s o u t anyp o s s i b i l i t y o f t h e r e b e i n g l a r g e b o d ie s of w a t e ron t h e p l a n e t ' s s u r f a c e .
R a d a r o b s e r v a t i o n of Venus e s t a b l i s h e s r o t a t i o na s r e t r o g r a d e i n a p e r i o d o f a p p r o x i m a t e l y 2 4 0d a y s .
M a r i n e r 2 f l y b y c o n f ir m s h i g h s u r f a c e p r e s s u r e( a t l e a s t 75 a t m o s p h e r e s ) a n d t e m p e r a t u r e ( a b o u t650 degrees K) a n d show s no s u b s t a n t i a l m a g n e t i cf i e l d .
M a r i n er 5 f l y b y u s e s r a d i o o c c u l t a t i o n t o measures t r u c t u r e o f u p p er a tm o sp he re a nd l o c a t e h e i g h to f c l o u d s ab o ve s u r f a c e ; d i s c o v e r s i o n o s p h e r e a ndf i n d s t h a t c a rb o n d i o x i d e i s major compound ofa tmosphe re .
James P o l l a c k ( U . S . ) a nd S ag an c a l c u l a t e g r e e n-h o u s e e f f e c t f o r m a s s i v e V enus a t m o s ph e r e , show-
i n g t h a t s o l a r e n erg y a lo n e c a n h e a t s u r f a c e t oabove 450 de gr ee s C ( 8 45 d e g r e e s F . ) .Radius of Venus s u r f a c e de t e rm ined f rom ra d a rt o be 6 ,050 km (3 ,750 m i . ) w i t h u n c e r t a i n t y ofl e s s t h a n 5 km ( 3 m i . ) .
S u r f a c e t e m p e r a t ur e s a n d p r e s s u r e s a r e e s t i m a t e dfrom r a d i o a n d r a d a r d a t a a s 477 d e g r e e s C (890d e g r e e s F . ) and 9 0 a t m o s p h e r e s .
U.S.S.R. probes ,Vene ra 5 and 6, s u c c e s s f u l l y l a ndo n s u r f a c e , d e t e r m i n e a c c u r a t e t e m p e r a t u r e ( 7 50
d e g r e e s K) a n d p r e s s u r e ( 9 0 a t m o s p h e r e s ) , a l s os t r u c t u r e o f l o w e r a tmosphe re .
A n a l y s is of p o l a r i z a t i o n d a t a by James Hansena nd A l b e r t A r k in g ( U . S .) shows t h a t t h e c l o u dp a r t i c l e s a r e s p h e r i c a l w i th a r e f r a c t i v e i nd exof 1 . 4 4 , r a d i u s of 1 . 0 5 pm and a l o c a t i o n a t a
p r e s s u r e l e v e l o f 50 m i l l i b a r s .
A . T . Young and G . S i l l (U.S.) i n d e p e n d e n t l yc o n c l u d e t h a t t h e p o l a r i z a t i o n d a t a im ply t h a tVenus c louds are composed o f s u l p h u r i c a c i dd r o p l e t s .
U . S . S . R . Venera 8 l a n d e r m ea su re s r a d i o a c t i v e
c o n t e n t of s u r f a c e r o c k s , c o n c l u d es Venus i sd i f f e r e n t i a t e d . A ls o d e te rm in e s t h a t s u n l i g h t( a few p e r c e n t ) p e n e t r a t e s t o s u r f a c e .
O b s e r v at i o n s o f c a rb o n d i o x i d e a b s o r p t i o n s i nVenus atmosphere show a 2 0 p e r c e n t f l u c t u a t i o no v e r a f ou r -d a y p e r i o d , i n t e r p r e t e d a s upwardand downward motions of cloud de c k p l a n e t w i d e .
R a da r s c a n s o f Venus r e v e a l hu ge s h a l l o w c r a t e r son t h e p l a n e t ' s s u r f a c e .
Po l l ac k m akes ob se rv a t io ns o f V enus f rdm h igh-
f l y i n g a i r c r a f t and c on c lu d es t h a t c lo u d s a r ed ee p h a ze s o f s u l f u r i c a c i d d r o p s .
R i c h a r d G o l d s t e i n (U S .) produces h i u k ? s o l u -t i o n r a d a r im ag es of s m a l l a re a s of t h e p l a n e t ' ss u r f a c e s ho win g many t o p o g r a p h i c f e a t u r e s .
M a r i n e r 1 0 ( f l yb y ) o b t a i n s d e t a i l e d L l t r a v i o l e tp h o to g r ap h s o f c l o u d s , d e te r m in e d c i r c u l a t i o np a t t e r n s i n up p er a t mo s ph e re .
U.S.S.R. Venera 9 and 1 0 l a n d e r s p h o t o g r a p hs u r f a c e a t two l o c a t i o n s , s ho wi ng e x p os e d r o c k s
a nd e v i d e n c e o f e r o s i o n p r o c e s s e s .
A r v y d a s K l i o r e ( U .S .) a nd c o l l e a g u e s c o n c l u d efrom r a d i o o c c u l t a t i o n d a t a t h a t a d d i t i o n a ld i s c r e t e c l ou d l a y e r s e x i s t below t h e main s u l -f u r i c a c i d c l o u d s .
R ad ar i m ag e s w i t h t h e u p g r ad e d A r e c i b o r a d a r
i n d i c a t e l a r g e v o lc a n oe s and c r a t e r s o n p l a n e t .
V enus ha s been exp lo re d by 1 3 s p a c e c r a f t of w h i c h t h r e ew e r e American and 1 0 w e r e R u ss ia n . F i v e o f t h e s e s p a c e c r a f tw e r e f l y b y s a nd e i g h t w e r e l a n d e r s . S e v e r a l o f t h e R u s s ia ns p a c e c r a f t c o n s i s t e d o f b o t h o r b i t e r s a n d l a n d e r s which s ep a-
r a t e d on a r r i v a l a t Venus. The re co rd i s a s f o l l o w s :
Venera 1 ( U . S . S . R . ) A f l y b y s p a c e c r a f t ; p a s s ed VenusMay 1 9 6 1 . No s c i en ce d a t a werer e t u r n e d , a c co r di n g t o r e p o r t s .
M a r i n e r 2 ( U . S . ) A f l y b y s p a c e c r a f t : p a s s e d VenusDecember 1 9 6 2 . Discovered t h a t t h et e m p e r a t u r e a v e r a g e s 426 d e g r e e s C
( 7 9 9 d e g r e e s F . ) o n b o t h n i g h t a n dd ay he m is p he r es , a nd t h a t t h e p l a n e th a s v i r t u a l l y n o m ag n et ic f i e l d andno r a d i a t i o n b e l t s .
Venera 2 (U.S.S.R.) A f l y b y s p a c e c r a f t ; p a s s ed VenusF e b r u a r y 1 9 6 6 . An a t t e m p t t o p h o to -g r a p h Ven us a p p a r e n t l y w a s n o ts u c c e s s f u l .
Venera 3 (U .S .S .R. ) A l a n d e r s p a c e c r a f t : e n t e r e d t h eatmosphere March 1 9 6 6 . N o r e p o r t so f any s c i e n t i f i c d a t a b e in g r e t u r n e d .
phere o f V enus O c tober 1 9 6 7 , andr e t u r n e d d a t a d u r i n g d e s c e n t t o a
few a tm osp heres . D e te rmined th ea tm osphere i s m a in l y c a r b o n d i o x i d e .
Venera 4 (U.S.S.R.) A l a n d e r s p a c e c r a f t ; e n t e r e d atmos-
M a r i n e r 5 ( U . S . ) A f l y b y s p a c e c r a f t ; p a ss e d October
1 9 6 7 . P r o v i d e d t e m p e r a t u r e a ndp r e s s u r e p r o f i l e s t o 527 degrees C
( 9 8 1 d e g r e e s F . ) and 1 0 0 a tm ospheresa t t h e s u r f a c e . D et er mi ne d t h e d e-t a i l e d s t r u c t u r e of t h e io n o sp h er e ,a n d d i s c o v e r e d t h e a t o m i c h y d r o g e nc o r o n a .
Venera 5 ( U . S . S . R . ) A l a n d e r s p a c e c r a f t : d e s c e n t c a p s u l e
e n t e r e d t h e a t mo s ph e re i n May 1 9 6 9 .M ea su re d t e m p e r a t u r e , p r e s s u r e a nda t m o s p h e r i c c o m p o s i t i o n .
Venera 6 ( U . S . S . R . ) A l a n d e r s p a c e c r a f t : c a p s u l e e n t e r e dt h e a tmosp here May 1 9 6 9 . Determined
low w a t e r v a p o r c o n t e n t : s u g g e s t e dp r e s en c e o f n i t r o g en . M easu red c a r -
b o n d i o x i d e a s 93 t o 9 7 p e r c e n t o fa tmosp here , and oxygen l e s s t h a n 0 . 4p e r c e n t : s u r f a c e p r e s s u r e o f n e a r l y1 0 0 a tmospheres .
Venera 7 ( U . S . S . R . ) A l a n d er s p a c e c r a f t ; e n t r y c a p s u lepe ne t r a t ed th e a tmosphere December1 9 7 1 ; d a t a were t r a n s m i t t e d f o r 23m i n u t e s f ro m t h e s u r f ac e . M easu reda s u r f a c e t e m p e r a t u r e o f 543 d e g r e e s C( 1 , 0 0 9 d e g r e e s F . ) and a p r e s s u r e o f9 0 a tmospheres .
Venera 8 ( U . S . S . R . ) A l a n d e r s p a c e c r a f t : c a p s u l e la nd edJ u l y 1 9 7 2 , a nd t r a n s m i t t e d s u r f a c ed a t a f o r 1 0 7 minu tes . De te rminedamounts of u ran ium, thor ium and po ta s-
sium i n s u r f a c e m a t e r i a l s and showedt h e y w e r e s i m i l a r t o amounts i n t e r -
r e s t r i a l rocks . Measured a s u r f a c et em p e r a t u r e o f 530 d eg r ee s C (986d e g r e e s F . ) .
Mar iner 1 0 (U.S.) Mercury-bound s p a c e c ra f t : pass ed VenusF eb r u a r y 1 9 7 4 . O bta ine d f i r s t p i c -t u r e s from s p a c e c r a f t . R ev ea le d t h es t r u c t u r a l d e t a i l s of t h e c l o u ds i nu l t r a v i o l e t l i g h t . C onfirmed t h e c - ,y- and ?s i - shaped c loud mark ing s ,and f o u r- d ay r o t a t i o n o f t h e s e mark-i n g s . Found s i g n i f i c a n t am ounts o fh e l iu m and co n f i rm ed t h e p r e s en c e o fh yd ro gen i n t h e u p p e r a t m o s p h e re .P ho to gr ap he d h i g h - a l t i t u d e h a z e l a y e r s .
Venera 9 (U.S.S.R.) A l a n d e r s p a c e c r a f t . C a p s ul e r e ac h eds u r f a c e O ct o be r 1 97 5 a t 33 degrees N .
l a t i t u d e , 293 d e g r ee s l o n g it u d e . R e -
t u r ne d f i r s t p i c t u r e from t h e s u r f a c eof Venus. Measured wind sp ee ds , p r es -s u r e , t e mp e ra t ur e and s o l a r r a d i a t i o n
f l u x t h r o u g h o u t t h e a t m o s ph e re t o t h e
s u r f a c e . O r b i t e r s u rv e ye d p l a n e t .
Venera 1 0 ( U . . . . ) A l a n d er s p a c e c r a f t ; c a p s u le reached
s u r f a c e O c t o b er 1 97 5 a t 1 5 d e g r e e s N .
l a t i t u d e , 295 d e g r ee s l o n gi t u d e . R e -
t u r ne d s ec on d s u r f a c e p i c t u r e . O r b i t e r
s u r v ey e d p l a n e t a nd l o ok e d a t s u r f a c ew i t h b i s t a t i c r a d a r . D eterm ined s u r f a c ee l e v a t i o n s d i f f e r e d b o n ly a f e w k i l o -meters a lo ng o r b i t e r T ra ck .
The Pi on ee r Venus mi ss ion w i l l be ac com plis hed by two
s e p a r a t e s p a c e c r a f t , t h e 9 r b i t e r and t h e M u lt ip ro be . T heO r b i t e r , c a r r y i n g 1 2 s c i e n t i f i c i ns t r u m en t s , w i l l g l o b a l l ysu rv ey Venus' a tmosphe re and su r r oun d ing env i ronmen t . I t
w i l l s t u d y t h e V e n us ia n s u r f a c e and p e r fo r m o ne a s t r o n o m i c a le x p e r i m e n t .
The Mul t iprobe w i l l d i v i d e i n t o f i v e a tm os ph ere e n t r yc r a f t as i t approa ches Venus f rom Ea r th . These a r e t h et r a n s p o r t e r B us , t h e L a r ge a nd t h r e e S m a ll P r o b e s . Thef o u r p r ob e s w i l l measure Venus ' a tmosp here f rom i t s t e n u o u sb e g i nn i n g s down t o t h e d e ns e s u p e r h e a t e d r e g i o n s a t t h e s u r -f a c e . A f t e r l a u n c hi n g t h e p r o b e s , t h e B us, t o o , w i l l e n t e rand measure com pos i t ion of Venus ' upper a tmosphere .
T o ge th er t h e f i v e a tm o s ph e ri c e n t r y c r a f t w i l l c a r r y1 8 s c i e n t i f i c i n s t r u m e n t s . The L ar ge P ro be c a r r i e s s e ve ni n s t r u m e n t s ; t h e S m a ll P r o b e s , t h r e e e a c h , and t h e B us, tw o.
T o m e e t t h e P i o n e e r Venus r e q u i r e m e n t f o r tw o r e l a t i v e l ys i m p l e an d low c o s t s p a c e c r a f t , d e s i g n e r s c ho s e s p i n n i n g ve -h i c l e s . S p in n in g c y l i n d r i c a l s p a c e c r a f t p ro v id e s t a b i l i t yw i t h minimum we igh t , good s o l a r c e l l deployment , v iewingf o r e xp er im e nt s i n a f u l l c i r c l e and s p i n s c a n f o r t h eimag ing sys t em.
The B a s i c Bus
The Venus O r b i t e r a n d Venus M u l t i p r o b e s p a c e c r a f t s h a r ea ' ' b a s i c b us'' d e s i g n . T h r ee q u a r t e r s of t h e s y s te m o n t h eb a s i c b u s e s a r e common t o b o t h s p a c e c r a f t . I n t h e M u l t i-p r ob e d e s i g n , t h e f o u r a t mo s ph er e e n t r y p r o b e s a r e mountedon t h e f l a t s u r f a c e w hich i s t h e t o p o r f o r w ar d e nd of t h eb u s c y l i n d e r .
The common sys t ems on t h e b a s i c bus f o r bo tf i s p a c ec ra f ti n c l u d e a t h e r m a l l y - c o n t r o l l e d e q u i p m e n t a n d e x p e r i m e n t s c o m -p a rt m en t ; s o l a r - e l e c t r i c p a n e l s , b a t t e r i e s and power d i s -t r i b u t i o n sys t em; fo rward and a f t "omni" an t enn as ; communi-
c a t i o n s sy s te m ; d a t a - p r o c e s s i n g s y st e m ; Sun a nd s t a r s e n s o r sf o r o r i e n t a t i o n r e f e r e n c e d u r i n g c r u i s e and ma ne uv ers ; hy-d r a z i n e p r o p e l l a n t t a nk s ; and t h r u s t e r s f o r o r i e n t a t i o n ,c o u r s e ch an g es an d s p i n - r a t e c o n t r o l .
T h e b a s i c bu s p o r t i o n s o f b o t h s p a c e c r a f t a re t h e i rmain b o d i es , f l a t c y l i n d e r s , 2 .5 m ( 8 . 3 f t ) i n d i am e te r and1 . 2 m (4 f t . ) h ig h.
The bus es p ro v id e a s p i n - s t a b i l i z e d p l a t f o r m f o r
s c i e n t i f i c i n st ru m e nt s , s p a c e c r h f t s ys te ms and i n t h e caseo f t h e M u l ti p ro b e , t h e f o u r p r ob e c r a f t . A c i r c u l a r e qu ip -ment s h e l f w i t h a n a r e a o f 4 . 3 7 sq . m ( 5 0 sq . f t . ) i s l o c a t e di n t h e u p p er o r f ow a rd e n d o f t h e b u s c y l i n d e r . The s h e l fi s mounted on t h e f o rw a r d e nd of t h e t h r u s t t u b e , t h e r i g i d
s t r u c t u r e w hich c o nn e ct s t h e s p a c e c r a f t t o t h e l a un c h ve-h i c l e . T w e l v e e q u a l l y s p ac ed s t r u t s s u p p o r t t h e e qu ip me nts h e l f p e r i m e t e r from t h e ba s e of t h e t h r u s t t ube . Thec y l i n d r i c a l s o l a r a r r a y i s , i n t u r n , a t t a ch e d t o t h e e q ui p-m en t s h e l f w i t h 24 b r a c k e t s .
T he rm al l o u v e r s ( f i f t e e n on t h e O r b i t e r an d e l e v e n ont h e M u lt ip ro b e) a t t a c h e d t o t h e l ow er s u r f a c e of t h e e q u i p -ment s h e l f , o pen and c l o s e ( w i t h h e a t - s e n s i t i v e - b i m e t a l l i cs p r i n g s ) t o c o n t r o l h e a t r a d i a t i o n from t h e e q uip me ntcompar tment . La rge h e a t p ro duc e r s , such as r a d i o a m p l i -f i e r s , a r e l o c a t e d o v e r s e v e r a l o f t h e s e l o u v e r s .
Maneuver System
The maneuvering sys tem of t h e b a s i c b us c o n t r o l s s p i nr a t e s , makes c o u r se a nd o r b i t c o r r e c t i o n s , and m a i n ta i n ss p i n a x i s po s i t io n -- u su a l l y p e r p en d i c ul a r t o t h e e c l i p t i cfor b o t h s p a c e c r a f t .
Benea th t he equ ipmen t compar tmen t , a l s o a t t a ch e d t o t h et h r u s t t ub e , a re t w o c o n i c al - h em i s p h er i c p r o p e l l a n t ta n k s ,3 3 c m . ( 1 2 . 8 i n . ) i n d i am e t er . The t a n k s s t o r e h y d ra z in ep r o p e l l a n t f o r two a x i a l and f o u r r a d i a l t h r u s t e r s . T hesecan chan ge s p a c e c r a f t o r i e n t i a t i o n , s p i n r a t e o r v e l o c i t y .
The maneuver sys te m ha s one mid- range Sun se ns or , twoe x t e nd e d - ra n g e Sun s e n s o r s , a nd a s t a r s e n s or t o s e n s es p a c e c r a f t o r i e n t a t i o n and p r o vi d e a r e f e r e n c e f o r f i n d i n gs p i n - a x i s a n g l e . The s t a r s e n s o r i s mounted on t h e equi p-m ent s h e l f a n d h a s a l o o k a n g l e of a b o u t 57 d e g r e e s t o t h es p i n a x i s . S u n s e n s o r s a r e a l l a t o ne p o i n t on t h e e q u ip -
m ent s h e l f p e r i m e t e r . They look r a d i a l l y t h ro u g h a n op en-i n g i n t h e s o l a r a r r a y and see t h e Sun on e a ch r o t a t i o n .
R ed un dant d a t a p r o c es s o r u n i t s f o r m a t t h e Sun an ds t a r s e n s or o u t p ut s f o r t e l em e t r y t r a n s m is s i o n t o t h e E a r t h ,t o c a l c u l a t e s p a c e c r a f t o r i e n t a t i o n . T hese d a t a p r o ce s s o rsa l s o p r o v i d e s e q ue nc ed f i r i n g commands t o t h e t h r u s t e r s t omake o r i e n t a t i o n , v e l o c i t y
w i t h t h e s p i n a x i s , and a r e l o c a t e d a t t op and bo t tom o ft h e b us c y l i n d e r , d i a g o n a l l y o p p o s i t e e ac h o t h e r . They p o i n ti n o p p o s i t e d i r e c t i o n s , and t o t u r n t h e b us s p i n a x i s ,b ot h f i r e i n p u l s e s i n o p po s i t e d i r e c t i o n s . T o speed up o rs lo w down t h e b us a l o n g t h e d i r e c t i o n o f i t s s p i n a x i s , o n l yo ne t h r u s t e r i s p u l se f i r e d a t t w o p o i n t s 1 8 0 d e g r e e s a p a r ta round the c i r c l e of b us r o t a t i o n . E i t h e r t h e t o p o rb o tt o m t h r u s t e r c an be p u l s e d d e p en d in g on d e s i r e d d i r e c -t i o n o f v e l o c i t y c ha ng e.
and s p i n r a t e c h a ng es .
T h e s y s t em ' s t w o a x i a l t h r u s t e r n oz z le s a r e a l i g n e d
The O r b i t e r h a s a t h i r d a x i a l t h r u s t e r . T hi s i s l o c a -t e d o n t h e b o t t om of t h e bu s c y l i n d e r an d a l l o w s c o n t i n u o us
f i r i n g o f two bo tt om t h r u s t e r s t o make t h e moves i n an ax i a ld i r e c t i o n n eeded f o r o r b i t c h an ge s.
The f o u r r a d i a l t h r u s t e r s a re a r r a ng e d i n two p a i r s ,w i t h t h e p a i r s p o i n t i n g i n o p p o si t e d i r e c t i o n s . They a re
mounted app ro x imate ly i n a p l an e p e r p e n di c u la r t o t h e s p i na x i s , and t h i s p l a ne p a s s e s t hr o ug h t h e c e n t e r o f g r a v i t y .The r a d i a l t h r u s t e r s c ha nge t h e v e l o c i t y i n a d i r e c t i o np e r pe n d i cu l a r t o t h e s p i n a x i s .
T he se r a d i a l t h r u s t e r s a l s o h av e b ee n p l a c e d a t f o u re q u i d i s t a n t p o i n t s a r ou nd t h e p e r i m e t e r o f t h e b us c y l i n d e r .T h i s h a s t h e e f f e c t of p o i n t i n g them a t o p p o s i te a c u t e a n-
g l e s t o t h e c i r c l e o f r o t a t i o n . The r e s u l t i s t h a t f i r i n gtwo of them 1 80 d e g r e e s a p a r t , t o g e t h e r , w i l l slow downt h e s p i n r a t e . The o t h e r two w i l l speed it up.
Power System
The bus s o l a r power s ys t em p ro v id es 28 -vo l t DC e l e c -t r i c power t o O r b i t e r and Mu lt ip ro be s c i e n t i f i c i n s tr u m e n tsa nd s p a c e c r a f t s ub sy st em s . Seven r e s i s t i v e s h u n t l i m i t e r sho ld th e maximum vo l t ag e a t 30 .8 v o l t s . When th e vo l t a ged ro ps below 27.8 v o l t s , t h e b a t t e r i e s s t a r t t o s h a r e t h el o a d t hr ou gh d i s c h a r g e c o n t r o l l e r s . S ma ll s o l a r a r r a y s re-
A s w i tc h p r o t e c t s t h e m a i n pow er bus f rom c u r re n to v e r l o a d s o r u n d e r v o l ta q e by a u t o m a t i c a l l y t u r n i n g
o f f i n s t r u m e n t s , s w i t ch e d l o a d s , and t r a n s m i t t e r b u se s .Th e s y s t e m 's a r r a y of s o l a r c e l l s i s s l i g h t l y s m al l e r f o rt h e M u l t ip r o be bu s t h a n f o r t h e O r b i t e r bu s b e ca u s e o f t h e
h i gh er pow er demands o f O rb i t e r ' s 1 2 exper im en t s . -The O r b i t e r
s o l a r a r r a y h a s 7 . 2 sq . m (77 . 8 sq . f t . ) of 2 x 2 cm ( . 8 x . 8 i nc e l l s . When a t r i q h t a n g l e s t o t h e S un l i n e , t h e s e p r o v i d e 226
w a t t s n e a r E a r t h and 3 1 2 w a t t s a t V enus . The M ul t i p rob e s o l a ra r r a y h a s 6 . 9 sq. m ( 6 5 . 7 sq. f t . ) of c e l l s a n d p r o v i d e s 2 1 4w a t t s n e a r E a r t h and 2 4 1 w a t t s a t Venus.
The power sy s t em 's two 7 .5 ampere-hour n ickel -cadmiumb a t t e r i e s p ro v id e a t o t a l o f 252 w a t t h ou rs of e l e c t r i c a le n e r g y . Power i s p r o v i d e d t o i n s t r u m e n t s fro m t h e s c i e n c epower b u s t h r o u g h r e d u n d a n t b u s e s i n t h e power i n t e r f a c eu n i t . O n-of f pow er sw i t c h i ng i s p e r fo r m e d i n t h e i n d i v i -d u a l i ns t r um e nt s f o r f l e x i b i l i t y i n s t e a d of c e n t r a l i z e ds w i t c hi n g i n t h e power i n t e r f a c e u n i t . The poweri n t e r f a c e u n i t p r o v i d e s o n- of f s w i t c h in g f o r p r o p ul s i o n
h e a t e r s .
- .
Communications System
The com m unica t ions sys t em fo r t h e t w o b u s e s c a n r e c e i v ecommands f r o m E a r t h i n any s p a c e c r a f t o r i e n t a t i o n t hr ou ght w o r e d u n d a n t S-band t r a n s p o n d e r s , c o n n ec t ed t o tw o omnid i r e c t i o n a l a n te n na s . (A t r a n s p o n d e r i s a r a d i o s y st e m
which receives i n c o m i n g s i g n a l s a n d t u n e s t h e o u t g o i n gt r a n s m i t t e r t o a f r equency w hich i s a t a c o n s t a n t r a t i ot o t h e in co min g s i g n a l . ) T h i s means t h a t D oD ple r s h i f ti n r a d i o f r e qu e n cy d ue t o s p a c e c r a f t m o ti on c a n be measuredp r e c i s e l y o n r a d i o t r a n s m is s i o n s f r o m b o t h E a r t h t o s pa ce -c r a f t and s p a c e c r a f t t o E ar th -- be ca us e f r e q u e n c i e s , b o t hl e a v i n g t h e E a r t h and a r r i v i n g a t t h e E a r t h a re known pre-c i s e l y . T h is a l lows s p a c e c r a f t v e l o c i t y m ea su re me nt s a c -c u r a t e t o .003 kph.
a d d r e s s a b l e ( re sp o nd s o n ly t o c e r t a i n f r e q u e n c i e s , andt h e r e c e i v e r s a re a u t o m a t i c a l l y r e v e r s e d by t h e command pro-c e s s o r l o g i c i f no command i s r e c e i v ed f o r 3 6 hours . H ence ,i f one f a i l s t h e o t h e r t a k e s over . T h e two r e c e i v e r o u t -p u t s a re c ro s s- co n ne c te d t o r e du n da n t e x c i t e r s , e i t h e r ofw h i c h c a n b e s e l e c t e d by ground command, T h e t r a n s p o n d e rp r o v i d e s e i t h e r a f i x e d - r a t i o in co min g t o ou t g oi n g c a r r i e rf r e q u e n c y , o r a f ix e d- fr eq u en c y c a r r i e r s i g n a l i n case off a i l u r e o f t h e two-way system.
The r e c e i v e r p o r t i o n of e a c h t r a n s p o n d e r i s f r equency-
The s p a c e c r a f t - t o - E a r t h r a d i o l i n k i s prov ided by anS-band t r a n s m i t t e r , w h ic h c a n r a d i a t e a t 10 o r 2 0 w a t t s , w i t hr e d u n c a n t power a m p l i f i e r s o p e r a t i n g t h r ou g h e i t h e r t h efo r e o r a f t I'omni" an t enn as . The omnis cove r a hemisphe rel o o k i n g f o r w a r d o r a f t . B o t h O r b i t e r and Mul t i p robe space -c r a f t h a ve , i n a d d i t i o n t o t h e two Bus om nis, s p e c i a l i z e dan t ennas wh ich w i l l be d e s c r i b ed i n s e c t i o n s o n t h i e r com-
munica t i ons . E i t he r omni an t en na can be s e l e c t e d by g roundcommand. One omni an t en n a i s c o n n e c t e d t o o n e o f t h e two
r e d u n d a n t r e c e i v e r s , and t h e o t h e r omni ( o r o t h e r s p a c e c r a f tan t en na d es ig na te d by command) i s c on ne ct ed t o t h e o t h e rre ce iv e r . Th is a r rangemen t can be rev er se d by commarid.
Command Sys e m
The b a s i c bu s command s y s t e m accepts incoming commandsf r o m t h e bus r a d i o r e ce iv e r s . Command demodu la to r s t u rn ont h e sy st em , c o n v e rt t h e s i g n a l t o a u s a b l e b i n a r y b i ts t r e a m , a n d p a s s i t on t o c ross -c onne c ted command pr oc es so rs .Commands a re e i t h e r s t o r e d f o r l a t e r ex e c ut i on , o r e x ec ut edimmedia t e ly . Sp ac ec ra f t u n i t s r e ce iv e commands f rom re -
dun da nt command o u t p u t modules . The command sy st em a c c e p tsa pulse-code-modulated/frequency-shift-keyed/phase-modulated(PCM/FSK/PM) d a t a s t r e a m a t f o u r b i t s p e r se co nd .
Each command word c o n s i s t s of 48 b i t s i nc lu d in g 1 3b i t s f o r s y n c h r o n i z a t i o n , w hich g i v e s a one - in - a -mi l l i onp ro b ab i l i t y o f a f a l s e command. The sys t em has a t o t a l o f1 9 2 pulse commands and 1 2 mag nit ude commands. The commandmemory can s t o r e up t o 1 2 8 commands ( r ed un da nt ly ) f o r l a t e re x e c u t i o n .
Data Handl ina Svstem
The t e l e m e t r y p r o c e s s o r f o r t h e b us d a t a h a n d l i n g s y st ems am p le s s c i e n t i f i c a nd e n g i n e e r i n g m ea su re men t s o u r c e s i nsequence . I t t r a n s m i t s an i n s t r u c t i o n word t o t h e P i o ne e rCommand Module (C M) encode r wh ich add res ses a data modulet o r ea d o u t t h e s e l e c t e d channe l .
The i n t e r r o g a t e d c h a nn e l can be e i t h e r a n a l o g , i e r i a ld i g i t a l o r b i n a r y o n e - b i t ( y es -n o ) i n f o r m a t i o n . The PC M
e n c o d e r s h i p s t h e e nc od ed m ea su re me nt t o t h e t e l e m e t r y p r o -c e s s o r , w h e r e it i s f r a m e - f o r m a t t e d , c o n v o l u t i o n a l l y coded
and used t o b i p h a s e m o d ul at e a s u b c a r r i e r . The s u b c a r r i e rt h e n p h a s e m o d u l a t e s t h e o u t g o i n g c a r r i e r s i g n a l .
T h e t e l e m e t r y p r o c e s s o r s and PCM e n c o de r s a r e c r o s s -c o n n ec t ed a nd f u l l y r e d un d a n t.
C r i t i c a l t e l e m e t r y m ea su re me nts a r e a s s i g n e d d a t a c han -n e l s o n t w o d i f f e r e n t d a t a m o d u l e s . The d a t a h a n d l i n gsys tem s c a n a c c e p t up t o 256 c h a n n e l s of d a t a .
A l l P i o n e e r Venus t e l e m e t r y d a t a a r e b i n a r y ( a s e r i e s
of ones and z e r o e s ) , and a l l d a t a " wo rd s" c o n s i s t o f e i g h to n e s and z e r o es a r r a ng e d i n t h e o r d e r d et e rm i n ed by t h e i n -f o r m a ti o n t h e y c a r r y . A nalog d a t a a r e c o n v e rt e d t o e i g h t -b i t w ord s. D at a i n p u t s a r e m u l t i p l e x e d and f o rm a t t ed i n t o
f r am es o f 6 4 e i g h t - b i t s w o rd s. Of t h e 6 4 w or ds , t h r e e a r e re -
q u i r e d f o r s y nc h r o n iz a t i on and i d e n t i f i c a t i o n , and t h r e ea r e su bco mm uta ted f o r s p a c e c r a f t h o u se k e ep i ng d a t a .
The ou tpu t of t h e d a t a s y st em i s an 8 t o 2 048 b i t p e rsecond PCM/PSK c o n v o l u t i o n a l l y co ded d a t a s t r e a m , b i p h a s emodulated on a 16384 Hz s u b c a r r i e r .
The O r b i t e r S p a c e c r a f t
The Venus O r b i t e r s p a c e c r a f t i n c o r p o r a t e s t h e b a s i c P i o n e e rB u s . I t a l s o c o n s i s t s of a d e sp u n, h i g h - g a i n d i s h a n t en n a on a3- m ( 1 0 - f t . ) m a s t t o r e t u r n t h e l a r g e volume of
O r b i t e r e x p e r im e n t s an d i ma gi ng d a t a t o E a r t h . The O r b i t e rc a r r i e s 1 2 s c i e n t i f i c i ns t ru m e nt s , a m i l l i o n - b i t d a t amemory t o s t o r e o b s e r v a t i o n s (when t h e s p a c e c r a f t i s b e h i n dVenus, o r t h e y c a nn ot be t r a n s m i t t e d t o E a r t h f o r o t h e rr e a s o n s ) , and a s o l i d - f u e l r o c k e t motor f o r i n s e r t i o n i n t o
o r b i t a t t h e p l a n e t .
The Q r b i t e r , i n c l u d i n g an t e n n a m a st , i s n e a r l y 4 . 5 m( 1 5 f t . ) h i g h . The b a s i c b u s c y l i n d e r m ak in g up i t s
main body i s abou t 2 . 5 m ( 8 . 3 f t . ) i n d i a m e t e r , a n d 1 . 2 m(4 f t . ) h i g h . Launch w e i g h t o f t h e O r b i t e r i s about 582 kg( 1 2 8 0 l b s . ) w i t h 45 kg ( 1 0 0 I b s . ) of s c i e n t i f i c i n st r um e n ts .
Weight a f t e r o r b i t a l i n s e r t i o n i s 368 kg ( 8 1 0 l b s . ) .
T h r e e i n s t r u m e n t s ( t h e m ag ne to me te r e l e c t r o n t e m p e r a t u r eprobe and e l e c t r i c f i e l d d e t e c t o r ) h ave s e n s o r e l emen t smounted on booms. The magnetometer s e n s o rs a r e mounted on
t h e t h r e e - s e c t i o n , d e pl o ya b le 4 . 7 m ( 1 5 .5 f t . ) boom. A
s i n g l e s e n so r i s m ounted abou t tw o- th i rds o f t h e way ou tfrom t h e b us c y l i n d e r , and a p e r p e n d i c u l a r p a i r a r e mounted
a t th e boom's end. The boom is d e p lo y e d a f t e r l a u n c h byf i r i n g p y r o t e c h n i c d e v i c e s , and e x te n d s r a d i a l l y from t h eupper r i m o f t h e c y l i n d e r . T h e b o o m p o s i t i o n s t h e sensors
a t a p o i n t o f m i n i m u m m a g n e t ic i n t e r f e r e n c e fro m t h e s p ac e -c r a f t .
The b a l l - l i k e s e n s o r s ( a n te n n a s) f o r t h e e l e c t r i c f i e l dd e t e c t o r s p r i n g o u t 0 . 6 m ( 2 6 i n . ) a f t e r j e t t i s o n of t h el au n ch f a i r i n g . The e l e c t r o n t e m p e r a tu r e p r ob e u s e s t w o
sensor e lements mounted a t r i g h t a n g le s t o on e a no t h er .The a x i a l s e n s o r i s m o u n t e d p a r a l l e l t o t h e s p i n a x i s a nde x t e n d s t h ro u g h t h e t h er m a l t o p co v e r. The r a d i a l s e n s o ri s on a 1 . 0 m ( 4 0 i n . ) boom, d e plo ye d a f t e r o r b i t i n s e r t i o n .
The gamma r ay b u r s t d e t ec to r u se s two de t e c t o r s mountedon t h e e q u i p m e n t s h e l f a b o u t 1 8 0 d e g r e e s a p a r t . T h i s a l -lows complete coverage of t h e c e l e s t i a l s p h er e f o r a l l po-s i t i o n s o f s p a c e c ra f t r o t a t i o n .
O r b i t e r S c i e n t i f i c I ns tr um en ts
A l l 1 2 s c i e n t i f i c i ns tr um en ts a r e mounted d i r e c t l y ont h e t o p s i d e o f t h e e qu ip men t s h e l f . E i g h t o f t h e i n s t r u -m en ts v ie w t h e p l a n e t t h r o u g h e i t h e r t h e s i d e o r t o p of
t h e b u s c y l i n d e r . Of t h e e i g h t , two ( t h e c l o u d P ho to -p o l a r i m e t e r an d t h e r a d a r ma pp er ) e mp loy s c a n n i n g se n so r swhich move th ro ug h a r a n g e o f 1 4 0 d e q re e s i n a p l a n e p e r -p e n d i c u l a r t o t h e bu s e x pe ri m en t s h e l f .
O rb i t e r An tenna Svst ems
A b a s i c p a r t o f t h e O r b i t e r sy stem , n o t p a r t of t h eb a s i c b u s, i s t h e d es pu n, h ig h -g a in p a r a b o l i c - r e f l e c t o ra n t e n n a , w h ic h f o c u s e s a 7 . 6 degree-wide r a d io beam on t h eE a r t h t h r o u g h o u t t h e m i s s i o n . The a n t e n n a d i s h i s 1 0 9 c m
(4 3 i n . ) i n d ia m e t e r , and a m p l i f i e s t h e O r b i t e r r a d i o s i g -
n a l 3 1 6 t i m e s . Venus and t h e O rb i t e r w i l l be 2 0 3 m i l l i o n k m( 1 2 6 m i l l i o n m i . ) f a r t h e r f ro m E a r t h a t t h e en d o f t h e243-day O r b i t e r p r im a r y m i s s i o n t h a n a t P l a n e t - a r r i v a l .The antenna i s n ee ded t o r e t u r n d a t a a t h i g h r a t e s o v e rt h e s e d i s t a n c e s . The h i g h- g a in a n t e n n a d i s h , a s l e e ved i p o l e a n t e n n a , an d t h e f o rw a r d r ro mn i" a n t e n n a a r e a l lmounted on t h e despun 2 . 9 - m ( 9 . 8 - f t. ) m a st F r o j e c t i n g upa l o n g t h e s p i n - a x i s f r o m t h e t o p of t h e O r b i t e r c y l i n d e r .The s leeve d i p o l e a n t e n n a b r o a d c a s t s a r a d io beam whichforms a p a n ca k e -l ik e p a t t e r n ar ou nd t h e s p a c e c r a f t , p e r-p e n d i c u l ar t o i t s s p i n a x i s . T h i s p r o v i d e s a b a c k u p f o r
t h e narrow -beam d i s h a n t e n n a i n c a s e of f a i l u r e o f t h e d e s p i ns y st em . The b u s a f t omni a n t e n n a p r o v i d e s t h e f o u r t h O r -
b i t e r a n te n n a . The o mn is b r o a d c a s t i n a h e m i s p h e r ic p a t -t e r n , f or wa rd o r a f t .
S i n c e t h e O r b i t e r d i s h a n t e nn a d oe s n o t s p i n , as doest h e s p a c e c r a f t be lo w i t , it c o n s t a n t l y f a c e s E a r t h , b o t h o nc r u i s e a nd o r b i t . The d es pu n c o n d i t i o n o f t h e an t enna andi t s m a s t i s m a i n t a i n e d by b e a r i n g , e l e c t r i c motor , ands l i p - r i n g a r r a n g e m e n t .
A q u a d r i p o d s t r u c t u r e , m o u n t e d o n t h e upper end of t h e
b us t h r u s t tu b e , s u p p o r t s t h e Bear ing and Power TransferAssembly ( B A P T A ) w h i c h m e c h a n i c a l l y d e s p i n s t h e a n t e n n a s .The m a s t i s a t t a c h e d t o t h e d es pu n f l a n g e o f t h e b e a r i n gassembly. The t h r e e a n t e n n a s o n t h e m a s t a r e c o n n ec te d t ot r a n s m i t t e r s a n d r e c e ive r s by a s e r i e s of t r a n s f e r s w i t c h e sth rough t h e d u a l f r e q u e n c y r o t a r y j o i n t . P u l s e commandsfro m E a r t h t o t h e s e s w i t c h e s a re p r o v i d e d t h r o u g h t h e B A P T A
s l i p r i n g s and b ru s he s .
The c o n t r o l s y s te m p r o v i d e s r e d u n d an t d e s p in c o n t r o le l e c t r o n i c s t o d r i v e o ne G f two redundant BAP TA m o t or s t od e s p i n a n d p o i n t t h e h igh-ga in an t en na t oward t h e E a r t h .The d e s p i n c o n t r o l s ys t em f u n c t i o n s as a c l o s e d l o o p ,
au tonomous ly ope ra t i ng t h e s y st em t o m a i n t a in a n t en n ap o i n t i n g .
Motor to rq ue commands a r e ge ne ra te d by t h e desp in con-t r o l e l e c t r o n i c s b a se d upon Sun o r s t a r s e n s o r and B A P T Am a s t e r i n d e x p u l s e s . An e l e v a t i o n d r i v e m a i n t a i n s a n t e n n ap o i n t i n g d u r i n g o c c u l t a t i o n s .
F o r t h e o c c u l t a t i o n e x p e r i m e n t s , t h e O r b i t e r c a r r i e s ane x t r a 7 5 0 m i l l i w a t t X-band t r a n s m i t t e r , w hose s i g n a l f r e -quency i s a lw ay s m a i nt a in e d a t a r a t i o of 1 1 . 3 t o t h a t oft h e main S-band t r a n s m i t t e r . B o t h S and X-Band s ig n a l s a r et r a n s m i t t e d by t h e d i s h a n t e n n a , w h ic h c a n b e moved 1 5 de-
grees from t h e E a r t h l i n e a s t h e O r b i t e r pa ss es behind. Venus.T h i s p e r m i t s k e ep i n g t h e r a d i o beam t o b e a i m e d ' a t V en us'
jupper a tmosphere f o r a l o n g e r t i m e . R e f r a c t i o n by t h e a t -mosphere bends t h e nar row-beam s i q n a l a round t h e p l a n e t soi t r e a ch e s E a r t h d e s p i t e t h e s e p o i n t i n g a n g l e s .
The X-band s i g n a l cannot be modula ted , and i s o n l y f o rs tu dy o f a tmosphe re e f f e c t s on r a d i o s i g n a l s a t two,wave-l e n g t h s . The X-band beam w id t h i s 2 . 2 degrees compared w i t h
the S-band 7 . 6 d e g r e e s .
Ground commands co n t r o l t h e an t en na po i n t i n g an g l e .The e l e v a t i o n d r i v e f o r t h e a n te n na d i s h c o n s i s t s of amoto r -d r iven j acksc rew. E l e c t ro n i c s con ve r t commands i n t od i s c r e t e p u l s e s t o c o n t r o l t h e m o t o r .
F o r p e r i o d s when t h e s p a c e c r a f t i s be hi nd Venus and ra -dio communicat ion i s c u t o f f , t h e d a t a memory c a n s t o r e up
t o a m i l l i o n d a t a b i t s . O c c ul t a t io n s l a s t up t o 2 6 m i n u t e s ,and a m i l l i o n - b i t memory a l l o w s d a t a t o be t a k e n a t a min-
i m u m r a t e o f ab o u t 700 b i t s p e r second i n t h i s t i m e . Thed a t a s t o r ag e c ap ac i t y can a l s o h e l p when D eep S p ace N etw ork(DSN) s t a t ions a r e n o t l i s t e n i n g t o t h e O r b i t e r f o r v a r i o usr e a s o ns . S t o r e d d a t a a r e p l ay ed b ack a t a minimum r a t e of1 7 0 b p s , and t h e O r b i t e r can p l a y b ac k d a t a w h i l e t a k i n g
and t r a n s m i t t i n g new d a t a .
O rb i t e r Da ta-Hand ling Sys tem
The O r b i t e r s p a c e c r a f t d a t a -h a n d li n g s ys te m u s e s t h e
bus da ta sys t em component s , p lu s i t s mil l ion -b i t memory . I t
a c c e p t s i n f o r m a t i o n fr om s p a c e c r a f t s ys t em s an d t h e 1 2s c i e n t i f i c i ns t r um e nt s i n s e r i a l d i g i t a l , a n a l og and one-b i t b i n a r y ( ye s- no ) form . I t co n v e r t s an a l o g an d ye s- noi n f o r m a t i o n t o s e r i a l d i g i t a l f or m, a nd a r r a n g e s a l l i n f o r -m at io n i n f o rm a ts f o r t r a n s m is s i on . T h i s c o n s i s t s of acon t inuo us sequence o f majo r t e l e me t r y f r a mes , eac h composedo f 6 4 minor f rame s. Each minor f rame co nt a in s 6 4 e i g h t -
b i t words ( 512 b i t s p e r m i no r f r a m e ) . The w ord s i n a minorf r a m e a r e a r r a ng e d i n t o on e of 1 3 preprogrammed formats,s e l e c t a b le by command. Each minor fr ame co n t a i n s w i t h i nit:
e H i gh - ra t e s c i e n c e o r e n g in e e r in g d a t a ( i n one
o f t h e 1 3 f o r m a t s ) ;
Sub-commutated d a t a f o r m a t s :0 S p a c e c r a f t d a t a ; a nd
e Frame s y n c h r o n i z a t i o n d a t a .
The t h r e e sub -commuta ted d a t a fo rmat s i n each mino rf ra me c a r r y d a t a whi ch c a n b e r e p o r t e d a t low r a t e s . Onei s f o r l o w - ra t e s c i e n c e a nd s c i e n c e h o us e ke e p in g d a t a , a ndt h e t wo o t h e r s a r e f o r l o w- ra te s p a c e c r a f t e n g in e e r in g d a t a .
The O r b i t e r ' s 1 3 h i g h - r a t e d a t a f o r m a ts i n c l u d e s e v ens c i e n c e f o r m at s f o r u s e on o r b i t . The o t h e r h i g h - r a t e f o r -m a t s a re D a t a memory pla ybac k ( c o nt a in in g some r e a l - t i m e
s c i e n c e ) , D at a memory r ead o u t ( s t o r e d d a t a o n l y ) , Launch-c r u i s e , E n gi n ee r in g -o n ly f o r m a t, A t t i t u d e c o n t r o l s y st e mfo rm at ( f o r ma ne uv ers ) , and Command memory r ea do ut for ma t .
The d a t a s y s te m o p e r a t e s i n r e a l - t i m e f o r t e l e m e tr ys to r age mode . I t s memory s t o r e s b o t h s c i e n c e an d e n g i n e e r i n gd a t a . T welv e t e l e m e t r y s t o r a g e p l a y b a c k an d r ea l - t im e d a t ar a t e s be tw ee n 8 an d 2048 b p s a r e a v a i l a b l e . A r a t e o f1 0 2 4 bps i s u se d d ur i n g i n t e r p l a n e t a r y c r u i s e .
Of t h e s e ve n s c i e n c e f o rm a t s u se d on o r b i t , f i v e a re f o rt h e c l o s e - i n p e r i a p s i s s e c t i o n of t h e o r b i t . Two a re f o rt h e f a r - o u t a p oa p si s p o r t i o n of t h e o r b i t .
Of t h e f i v e c l o s e - i n f o r m a t s , t w o e m p h a s i z e a c q u i s i -t i o n o f aeronomy da t a . A t h i r d g e n e r a l f or ma t a l l ow s d a t at a k i n g by v i r t u a l l y a l l e xp er im e nt s.
T h e f o u r t h c l o s e - i n f o r m at , t h e O p t i c a l , i s f o r j u s tt w o i n s t r u m e n t s . I t a l l o c a t e s 73 p e r c e n t o f t h e d a t astream t o t h e i n f r a r e d r ad i o m e te r , t h e r e s t of t h e p h o t o -p o l a r i m e t e r . T h e l a s t fo rm a t , t h e Mapping fo rma t , g iv es
4 4 p e r c e n t of t h e d a t a s t re a m t o t h e r a d a r m ap pe r, and t h er e s t i s d i v i d e d among f o u r o t h e r " ma pp in g" t y p e i n s t r u m e n t s .
Of t h e two s c i e n c e f o r m a t s f o r t h e f a r - o u t a p o a p s i so r b i t a l s eg m en t, t h e I ma gin g f o r m at p r o v i d e s 67 p e r c e n t oft h e d a t a s t ream f o r c l o u d p ho t o p ol a r im e t e r p i c t u r e s o fVenus ' c l o ud s , and t h e r e s t f o r f o u r s p a c e e nv i ro n m en t i n -s t r u m e n t s . The G e n er a l f o rm a t f o r a p o a p s i s c a r r i e s d a t a f o ra l l i n s t r u m e n t s e x c e p t t h e i n f r a f e d a nd im a gi ng i n s t r u m e n t s ,b u t makes b i g a l l o c a t i o n s t o t h e s p a c e e n v i r o n m en t m ea su re -m e nt s o f t h e m a g ne t om e te r , s o l a r w in d i n s t r u m e n t a nd t h egamma r a y b u r s t d e t e c t o r .
O r b i t a l I n s e r t i o n R ocket
The o r b i t a l i n s e r t i o n m otor r e du c es O r b i t e r v e l o c i t yby 3 ,816 kph (2 ,366 mph) f o r o r b i t a l cap tu re by Venus .I t i s a s o l i d p r o p e l l a n t e n gi n e, a t t a c h e d t o t h e bu s t h r u s tt u be be low th e equ ipment compar tmen t. The eng ine ha s1 8 , 0 0 0 Newtons ( 4 0 0 0 l b s . ) of t h r u s t , and t h e i n s e r t i o n r o-c k e t bu r n r e d u c e s O r b i t e r w e ig h t by 1 8 1 kg ( 3 9 8 l b . ) .
T h e M u l t i p r o b e S p a c e c r a f t
The f i r s t si m u l ta n e o u s m u l t i p l e - e n t r y c r a f t m ea su re -ments of t h e a tm o sp h er e o f a n o t h e r p l a n e t w i l l be accompl i shedby th e Venus Mu l t ipro be .
The f ou r p robes w i l l be l aunched f rom the Mul t ip ro heBus 1 3 m i l l i o n km ( 7 . 8 m i l l i o n m i . ) from t h e p l a n e t a ndw i l l t he n f l y t o t h e i r e n t r y p o i n t s , two on t h e d ay s i d eand two on the n i g h t s i d e of Venus.
The M u l t i p r o b e s p ac ec r a f t w e i gh s 9 0 4 kg ( 1 , 9 9 0 l b . )and c a r r i e s 5 1 kg ( 1 1 2 l b s . ) o f s c i e n t i f i c i n st ru m en t s. The
s p a c e c r a f t c o n s i s t s o f t h e P i o n e e r Venus b a s i c bu s m od i fi e dt o c a r r y t h e f o u r a tm os ph er e p r o b es . I t s d i a m e t e r i s t h a to f t he Bus, 2 .5 m ( 8 . 3 f t . ) . From t h e b o tt om o f t h e Bus t ot h e t i p o f t h e m ain p ro b e, it i s 2 . 9 m ( 9 . 5 f t . ) h iq h .
D ur in g t h e f l i g h t t o Venus, t h e f o u r p r ob e s a r e c a r r i e don t h e Bus by a l a r g e i n v e r t e d c on e s t r u c t u r e a nd t h r e eeq u a l l y - s p a ced c i r c u l a r c lamp s s u r r o u n d i n g t h e con e. T hesea t ta c hm e n t s t r u c t u r e s a r e b o l t e d t o t h e Bus t h r u s t t u b e ,t h e s t r u c t u r a l l i n k t o t h e lau n ch v e h i c l e . The L a rg e P r ob ei s c e n t e r e d o n t h e Bus s p i n a x i s , and i s launched towardVenus by a p y r o t ech n i c - s p r i n g s e p a r a t i o n s y s tem . The r i n gs u p p o r t c la mp s a t t a c h i n g t h e S m a l l Probes a re h inged . Fo rl au n ch o f t h e Sa r n l l P r o b e s , t h e c la mp s o pe n by t h e f i r i n go f ex p l o s i v e n u t s . When o p en , t h ey a l l o w t h e p r o b es t os p i n o f f t h e Bus i n a t a n g e n t i a l d i r e c t i o n due t o Bus ro-t a t i o n . C o n t r o l l e rs i n c r e a s e Bus s p in f rom 15 t o 4 8 rpmf o r S m a ll P r o b e l a u n c h .
The Mu l t i p r ob e ' s fo rward omni an tenna ex ten ds above t heto p o f t h e Bus c y l in de r , and an a f t omni ex t end s down belowit. B ot h omni an t en n as have h em i s p h e r i c r ad i a t i o i - p a t t e r n s .A t t a ch e d t o t h e e qu ip me nt s h e l f i s an a f t -po in t ing , med ium-g a i n ho rn a n t e n n a , f o r u se d u r i n g c r i t i c a l maneuvers whent h e a f t e n d o f t h e s p a c e c r a f t i s p o i n t e d t o w a r d t h e E a r t h ,
a s it i s when th e probe s a r e lau nched toward Venus.
T h e r em a i n i n g s y s t em s o n t h e M u l t i p ro b e s p a c e c r a f t a r et h o s e c a r r i e d o n b o t h O r b i t e r and M u l t i p r o b e b u s e s . T h ese c o m -mon bus systems a r e : The instr ume nt-e quip men t compartment andb a s i c b us s t r u c t u r e : t h e s o l a r a r r a y , b a t t e r i e s and power d i s -t r i b u t i o n s y st em ; t h e S un an d s t a r s e n s o r s , p r o p e l l a n t s t o r a g et a n k s an d t h r u s t e r s o f t h e b u s m an eu ve rin g an d s t a b i l i z i n g s y s -t e m . Ot he r Bus sys tems a r e t h e t r a n s m i t t e r s , r e k e i v e r s and p ro -c e ss o rs of t h e bus communicat ions , command and d a t a ha ndl ing s ys tem.
These s y st e ms a l l o w t h e Bus t o p r o v i d e f o r t h e M u l t i -p ro be s p a c e c r a f t , a s i t d oe s f o r t h e O r b i t e r , a s t a b l e , r o -
t a t i n g p l a t f o r m and a p r o t e c t i v e , t e m p e r a t u r e - c o n tr o l l e de nv ir on me nt f o r t h e s c i e n t i f i c i n s t r um e n t s and s p a c e c r a f tsys t ems .
They a l s o p r o v i d e e l e c t r i c power, make maneuvers, re-ce i v e commands, p r o c es s ex p e r i m en t d a t a , and t r a n s m i t d a t at o B a rt h.
Mul t ip ro be Data System
The d a t a s y st e m f o r t h e M u l t ip r ob e s p a c e c r a f t u s e s t h es t an da rd bus componen ts . However, d a ta fo rmat s a r e o r -g a n i ze d t o m e e t r e q u i r e m e n t s o f t h e M u l t i pr o b e m i s s i o n .T h e M u l t i p r o b e d a t a s y s t em h an d l e s d a t a fro m b o t h Bus andp r o be s b e f o r e p r ob e la u n c h. A f t e r p r ob e l a u n c h , i t h a n d l e sBus d a ta on l y . The p robes have t h e i r own da ta sys t ems .(See s e c t i o n s d e s c r i b i n g t h e s e . )
The M u l t i pr o b e d a t a s y s t em acce p t s en g i n ee r i n g andm i s s i o n o p e r a t i o n s i n f o r m a t i o n f ro m t h e f o u r p r o be s a b o a r dt h e s p a c e c r a f t , u n t i l pr ob e la u nc h , a s w e l l as f rom theM u lt ip ro be bu s i t s e l f . I t a l s o h a n d l e s d a t a f ro m t h e t wo
e x p e ri m e n ts c a r r i e d o n t h e M u l ti p r ob e b u s. A s on the O r -b i t e r , t h e sys tem a c ce p ts d a t a i n s e r i a l d i g i t a l , a na logand o n e - b i t b i n a r y s t a t u s ( ye s- no ) form. I t c o n v e r t s t h ea n a l o g d a t a t o s e r i a l d i g i t a l b in a r y from an d a r r an ge s a l li n f or m a t io n f o r t r a n s m i s s i on t o E a r t h i n t h e s t a n d a r d P i -
oneer Venus s e r i e s of majo r t e l em et ry f r ames , each composedof 6 4 minor f rames.
Each minor frame i s composed of a s e r i e s of 6 4 e i g h t -b i t w o rd s. The w or ds i n a m i no r fr am e a r e a r r an g ed i ns e v e r a l f o r m a t s . Each m in or f ra me c o n t a i n s h i g h - r a t es c i en ce o r en g i n ee r i n g d a t a , p l u s sub -com mu ta ted f o r m a t s ,s p a c e c r a f t d a t a , and f ra me s y n c h r o n i z a t io n d a t a . One sub-
commutated format c a r r i e s l o w - r a t e s c i e n c e a n d s c i e n c ehousekeep ing da ta ; two a re f o r lo w -r a te s p a c e c r a f t i n f o r -m ak io n. T welve r ea l - t i m e ( no d a t a s t o r ag eo n t h e M u l t i -p r ob e ) d a t a t r a n s m i s s i o n r a t e s b e t w e e n 8 and 2 0 4 8 b p s canb e u s ed . L i k e t h e O r b i t e r , t h e M u l t ip r o be a l s o h a s h i g h-d a t a - r a t e f o r m at s f o r A t t i t u d e c o n t r o l ( us ed d u r i n g maneu-v e r s ) , f o r E ng in eer i ng d a t a o n l y , and f o r command- memoryr e a d o u t . A s i n g l e f o rm a t f o r a tm os ph er e e n t r y t r a n s m i t sh i g h - r a t e s c i e n c e d a t a . Assuming t h e e x pe c t e d d a t a r a t eo f 1 0 2 4 bps a t e n t ry , d a t a r a t e f o r t h e t w o Mult iprobe Busex p e r i m en t s w i l l b e 256 b p s f o r t h e n e u t r a l mass s p e c t r o -m e t e r , and 1 1 2 b ps f o r t h e i o n mass s p e c t r o m e t e r .
After launch of its four probes 20 days out fromVenus, the Multiprobe Bus becomes a probe itself, providing
the mission's only high upper atmosphere composition
measurements. These operate as the Bus enters but beforeit starts to burn up at 115 km (71 mi.) altitude.
These two mass spectometer instruments are attached to
the equipment shelf with their inlets projecting abovethe flat top of the Bus cylinder.
VENUS ATMOSPHERIC PROBES
Because of its high pressures (nearly 100 times Earth's),high temperaturesatmosphere presents a difficult problem for flight designers.
The high entry speeds of about 41,600 kph (26,000 mph) addto the problem.
The Large and Small Probes are geometrically similar.The main component of each is a spherical pressure vessel,
which houses the scientific instruments and the following
spacecraft systems: communications, data, command and power.
The Large Probe weighs about 316 kg (698 lbs.); the SmallProbes, 93 kg (206 lbs.) each.
Conical aeroshells provide stable flight paths and heat
protection for all four probes during atmospheric entry.heat shield-carrying aeroshells are 45-degree cones with spheri-
cally blunted tips, whose radii are equal to half the base radii
of the cones.
All instruments within the pressure vessels of all fourprobes require either observing or direct sampling access tothe hostile Venusian atmosphere. This access is one of the
hardest problems of the mission.
sealed penetrations of several types. Each Small Probe has
seven. Pressure vessel penetrations for all probes include15 sapphire and one diamond window.
and corrosive constituents, Venus'
The
The Large Probe has 14
The Large Probe
The Large Probe weighs about 316 kg (695 lbs.) and isabout 1.5 meters (5 feet) in diameter. It returns data at
256 bps. Its seven scientific instruments weigh 28 kg
(62 lbs.). These include two instruments to identify atmos-
phere components. The other five instruments will measurethe clouds, atmospheric structure, energy distributioncirculation.
of the planet.
andThe probe enters at the equator on the day side
The Large Probe consists of the forward aeroshell-heat
shield, the pressure vessel and the aft cover. Both aero-shell and aft cover are jettisoned at main chute deployment.
The spherical pressure vessel is 7 3 . 2 cm ( 2 8 . 8 in.) indiameter, and is made of titanium for light weight and high
strength at high temperatures. Because it is jettisoned atrelatively cool, high altitudes, the aeroshell can be made
of less heat-resistant aluminum.
The weight limits on interplanetary spacecraft and the 14hull penetrations required that the pressure vessel be designedwith great care and machined with precision for both lightness
and strength. The flight vessel has been tested successfully
under Venus-like conditions of 100 Earth atmospheres of pressure
and 470 degrees C (900 degrees F) temperatures. Test vesselshave withstood higher pressures.
The vessel is made in three pieces, joined by flanges,
seals and bolts. Sections are the aft hemisphere, a forwardcap and a flat ring section between the two. The vessel has14 sealed penetrations (one for the antenna, four for electri-
cal cabling, two for access hatches and seven for scientific
instruments). Four instruments use nine observation windows
through four of the hull penetrations. Eight windows are of
sapphire, and one of diamond. These materials admit light or
heat at the wavelengths being measured, while withstanding
Venusian heat and pressure. The solar flux radiometer has
five windows through one hull penetration; the nephelometer,
two windows and the infrared and cloud particle instruments,
one window each.
Three vessel penetrations are inlets for direct atmosphere
sampling by three instruments--mass spectrometer, gas chroma-tograph and atmosphere structure experiment. At its aft pole
the spherical vessel has a hemisphere pattern antenna for
communication with Earth. Two four-inch arms on one side holdthe reflecting prism for the cloud particle instrument. Asingle arm on the other side has a temperature sensor at its tip.Three parachute-shroud towers are mounted above aerodynamic drag
plates, spaced equally around the equator of the sphere. The
vessel has an electronics access port for system checkout, anda cooling port used in ground tests.
The 4 . 9 meter ( 1 6 . 2 f t . ) - d i a m e t e r d a c r o n ma in p a r a c h u t e i s
of t h e c o n i c a l r i b b o n t y p e . L o ca te d i n a curved compar tmenton one s i d e o f t h e a e r o s h e l l , t h e m or ta r -dep loyed dacron p i l o t
c h u t e i s 7 6 crn ( 2 . 5 f e e t ) i n d i am et er . A f t e r s e p a r a t i o n oft h e a e r o s h e l l , a f t c o v er a nd main c h u t e h av e o c c u r r e d , t h e
p r e s s u r e vesse l d e s c e n ds t o t h e s u r f a c e . The m o ti on i s s t a -b i l i z e d by l o c a t i n g t h e c e n t e r o f g r a v i t y o f t h e p r e s s u r e
v e s s e l w e l l f o rw a rd a nd by a n a i r f l o w s e p a r a t i o n r i n g a ro un dt h e s p h e r e ' s e q u a t o r . Drag p l a t e s on t h e f lo w s e p a r a t i o n r i n gs l o w t h e d e s c e nt r a t e , and vanes a t t a c h e d t o t h e a i r f l o w r i n gm a i n t a i n s p i n f o r c o n t i n u o u s v ie w in g i n a f u l l c i r c l e by thee x p e r im e n t s d u r i n g d e s c e n t . A f a i r i n g c ov e rs t h e fo rw ardhem i sphe re of t h e p r e s s u r e v e s s e l , p r o v id i n g a smooth aerody-na mic s u r f a c e d u r i n g d e s c e n t .
H e a t P r o t e c t i o n
The L a rg e P ro b e p r e s s u r e v e s s e l i s made of t i tanium f o rh e a t r e s i s t a n c e . W it hi n t h e s p h e r i c a l v e s s e 1 , i n s t r u m e n t s a nd
s y s t e m s a re mounted on t w o p a r a l l e l s h e l v es made o f b e r y l l i u mt o se rve a s h e a t s i n k s . E quipm ent i n s i d e t h e v e s s e l i s f u r t h e rp r o t e c t e d f r o m h e a t by a 2 . 5 c m (1 i n . ) - t h i c k k a pt on b l a n k e t ,
which c o mp le te ly l i n e s t h e i n t e r i o r .
S c i e n t i f i c I n s t r um e n ts
The s ev en s c i e n t i f i c i n s t ru m e n t s on t h e L a r g e P r o b e i n c l u d et h e g a s c h r o m a t o g r a p h a n d mass s p e c t r o m e t e r , w hi ch measure t h e
c o m p o s i t i o n of V enu s' a tm o sp h er e d i r e c t l y . The o t h e r f i v e i n s t r u -ments e i t h e r "look o u t " w indows o r s e n s e v e h i c l e m o t io n s a nd /o r t e m p e r a t u r e w i t h a c c e l e r o m e t e r s an d a w i r e - c o n n e c t e d h e a t
s e n s o r , r e s p e c t i v e l y .
The i n f r a r e d r a d i o m e t e r r e q u i r e s a d ia mo nd window becausediamond i s t h e o n l y m a t e r i a l t r a n s p a r e n t t o t h e a p p r o p r i a t e
w a ve le ng th s an d a b l e t o w i t h s t a n d t h e h i g h t e m p e r a t u r e s a ndp r e s s u r e s of t h e atmos phere . T hi s window i s a b o u t t h r e e -
q u a r t e r s o f a n in c h i n diamete r a nd a n e i g h t h of a n i n c h t h i c k( ab o ut t h e s i z e o f a q u a r t e r ) . I t w ei ghs 1 3 . 5 c a r a t s and w a sshaped by diam ond c u t t e r s i n The N e t he r l and s f rom a 20 5-ca ra ti n d u s t r i a l g r a d e r o ug h dia mo nd . T h e n e p h e l o m e t e r ( c l o u d - s e n s o r )us es tw o s ap ph i r e window s. The c lo ud p a r t i c l e i n st ru m e nt d i r e c t s
a l a s e r beam t h r o u g h a s a p p h i r e window t o a n o u t s i d e r e f l e c t i n gpr i sm and back t o i t s s e n s o r . T h e s o l a r f l u x ra d io m e te r h a s f i v e
S c i e n t i f i c i n s tr u m e n t and s p a c e c r a f t s y s t e m s d a t a a r er e t u r n e d by t h e c om m un ic at io n s s y s t em . S p a c e c r a f t d a t a i n -c l u d e i n t e r n a l t e m p e r a t u r e a nd p r e s s u r e m e as u re m en ts , e l e c -
t r i c a l c u r r e n t f l o w a nd v o l t a g e a nd o n -o r -o f f s t a t u s o fs y s t em s a nd i n s t r u m e n t s .
The p r o b e’ s s o l i d s t a t e t r a n s m i t t e r and h e m i s p h e r i c a lc o v er a ge a n t e n n a r e t u r n a 256-bps d a t a s t re a m t o E a r t h . T h e
s y st e m u s e s f o u r 1 0 - w a t t s o l i d s t a t e a m p l i f i e r s p r ov id in g at r a n s m i t t e r power of 4 0 w a t t s .
A t r a n s p o n d e r r e c e ive s an S-band c a r r i e r wave a t 2 . 1 GHz,and s e t s t h e p r o b e t r a n s m i t t e r t o s e n d a t 2.3 GHz. The t r a n s -p o n d e r r e c e i v e r i s u s e d o n l y f o r two-way Doppler t r a c k in g .The i nc om in g s i g n a l c a r r i e s no i n f o r m a t i o n , a nd t h e p r o b e d o e snot rece ive commands .
Command System
Once t h e L a r ge Probe h a s s ep a r a t ed from t h e Bus, onboarde le c t r o n ic s p ro v i de a l l p robe commands. The command sys temc o n s i s t s o f a command u n i t , a p y r o te c h n i c c o n t r o l u n i t an dt h e s e n s o r s t o s e r v i c e t h e command u n i t .
The s y s t e m ca n p r o v i d e 6 4 s e p a r a t e commands f o r s p a c e c r a f ts ys te m s a nd s c i e n t i f i c i n s t ru m e n t s. I t c o n t a i n s t h e c r u i s et i m e r ( t h e on l y o p e r a t i n g u n i t d u r i n g t h e 24-day pe r iod be-
tween Bus s e p a r a t i o n an d e n t r y ) , a n e n t r y s e qu e n ce programmerand a command d e c o d e r . Commands a re i n i t i a t e d by a cl o ckg e n e r a t o r o r a G - s w i t c h t o s e n s e d e c e l e r a t i o n fo rce s . A t e m -p e r a t u r e s w i t c h p r o v i d e s b ac ku p f o r t h e t i m e r a t p a r a c h u t ej e t t i s o n .
T h e p y ro t ec h n ic c o n t r o l u n i t i s made up of 1 2 s q u i b d r i v e r s
which p r o v i d e c u r r e n t t o f i r e e x p l o s iv e n u t s f o r s e p a r a t i o nof t h e a e r o s h e l l , t h e a f t c o v e r an d m ain c h u t e ; a nd a c t u a t o r sf o r t h e cab le c u t t e r , p i l o t c h u t e mortar and m a s s s p e c t r o m e t e ri n l e t cover.
Data Handling System
The Large Probe d a t a h a n d l i n g u n i t ca n a c c e p t 36 a n a l o g ,1 2 s e r i a l d i g i t a l , and 2 4 o n e - b i t ( ye s- no ) s t a t u s c h a n n e ls ’ f ro m
s c i e n t i f i c i n s t r u m e n t s and p r ob e s y st em s . The u n i t c o n v e r t st h e a n a l o g an d y es -n o d a t a t o s e r i a l d i g i t a l form and a r r a n g e sa l l d a t a i n major t e l e m e t r y frames composed of 1 6 minor frames
f o r t im e - mu l ti p le x e d t r a n s m i s s i o n t o E a r t h . Each minor f ramei s composed of a s e r i e s of 6 4 e i g h t - b i t w o r d s ( 5 1 2 d a t a b i t sp e r m i n o r f rame) .
The data handling system provides for two data formats:
blackout and descent. A storage capacity of 3 0 7 2 bits is pro-
vided by a data memory, for use during entry blackout.the blackout period, the stored data will be read out of the
memory and telemetered in the descent format. Data are stored
at 1 2 8 bps. In the descent format, transmission will be at2 5 6 bps. Allocation of this bit rate among the seven Large
Probe experiments will range from 16 to 44 bps per experiment.Only the atmospheric structure and nephelometer experiments
will use the entry blackout storage format at 72 bps and 4 bps,respectively. Two subcommutated formats for lowrate phenomena
also provide housekeeping data, and additional data for theatmospheric structure, nephelometer, cloud particle spectro-
meter and solar flux radiometer experiments.
Following
Power Svs em
The power system uses a silver-zinc battery, providing4 0 ampere hours of energy at 2 8 volts. The system consists
of a battery, a power interface unit and a current sensor.
The power interface unit controls power and contains fuses
and power switching relays for vehicle systems. Power forprobe checkout and heating is provided by the Bus prior toprobe to probe separation. During this time, the batteries
are open-circuited by switches in the power interface unit.
The Small Probes
Atmosphere entry points for the Small Probes are spread
over the face of Venus--two on the night side at high northernand mid-southern latitudes, and the third at mid-southern lati-
tudes on the day side.
Like the Large Probe, each of the Small Probes consistsof a forward heat shield, a pressure vessel and an afterbody.
The three small probes are identical. Each is 0 .8 m ( 3 0 in.)in diameter and weighs 9 0 kg ( 2 0 0 lb.). Each carries three
scientific instruments, weighing 3 .5 kg (7.7 lb.). The three
Small Probe instruments return less detailed information than
the seven on the Large Probe. But except for the atmosphericcomposition measurements, made only by two Large Probe instru-
ments, Small Probe atmosphere measurements are in many respectscomparable to Large Probe data.data at 6 4 bps during flight down to 30 km (18 mi.) altitudeand 1 6 bps from there to the surface.
Neither forward aeroshell nor afterbody of the Small
Probes ever separates from the pressure vessel, nor is a para-chute used for deceleration as with the Large Probe. TheSmall Probes are slowed entirely by aerodynamic braking, and
instruments gain access to the atmosphere through doors in theintegral afterbody. Both aeroshell and pressure vessel aremade of titanium for light weight and strength at high tempera-
tures. The afterbody is made of aluminum.
Small Probe Structures
The pressure vessel nests into the aeroshell and is
permanently attached to it. The afterbody also is permanentlyattached to the pressure vessel, and its shape closely follows
the contours of the vessel's aft hemisphere, protecting it from
atmosphere heat. As in the case of the Large Probe, the pres-
sure vessels for the three small probes had to be very careful-
ly designed and machined because of weight limitations, the seven
hull penetrations required and the strength requirements at high
Venusian pressures and temperatures.
The pressure vessels are fabricated in two hemispheres
and joined with flanges, bolts and seals. The flight vesselswere tested at Venus surface temperatures and pressures, and
the test vessels tested even under more severe conditions.
Three doors in the afterbody open after entry heating
at about 7 0 km altitude (44 mi.), providing access by the threeinstruments to the atmosphere.from each of two protective housings--one for the atmospheric
structure and the other for the net flux radiometer instrument.
These housings project like ears from each side of the pres-
sure vessel sphere. The temperature sensor and atmosphericpressure inlet for the atmospheric structure instrument extend
10 cm (4 in.) from the door of one housing, and the nex flux
radiometer sensor extends similarly on the opposite side.
Two of these doors.open out
When the doors to these two housings open after atmospheric
entry at 7 0 km (44 mi.) altitude, they are retained, rather than
jettisoned, and serve to slow spacecraft spin rate. A vane, lessthan one square inch, is attached to the pressure sensor inletto assure that the vehicle will spin throughout the descent,so that instruments can see in a full circle as the probe
rotates. The cloud sensor (nephelometer) cover opens andfolds down. As with the Large Probe, a hemispherical-pattern
antenna is mounted at the aft pole of the pressure vesselsphere.
Each Small Probe pressure vessel has a total of seven
sealed penetrations: one for the antenna, one for the two
sapphire nephelometer windows, one for the atmospheric pres-
sure inlet and a hatch for ground test cooling and systemscheckout. The other three vessel penetrations are feed-
throughs for electrical cables. Each external radiometersensor on each small probe has two diamond windows.
Flight Sequence
For the three Small Probes, atmospheric entry speeds
are about 4 2 , 0 0 0 kph (26,000 mph), and peak decelerations vary
in entry flight path angles.
Twenty minutes before entry, all systems and instruments
are activated and communications with Earth are established.Just before entry, spin rates are cut about three times from48 to 14 rpm, The 48-rPm spin rate imparted by spin-off
launch from the Bus disperses the probes over the planet to
desired entry points. But it also means that the probesenter the upper atmosphere somewhat tilted to their entryflight paths. With the slower 15-rpm rotation, aerodynamic
forces quickly line up the axes of the probes with their entry
heating damage could occur on the edges of the probes conical
heat shields.
A yo-yo system spins down the probes. Two weights are
cut loose by a pyrotechnic cable cutter, and probe spin
swings the weights out on 2.4 m (8-ft.) cables. With thisweight moved radially outward, rotation rate must slow to
maintain the same rotational momentum. Weights and cablesare then jettisoned.
In order to save weight and also because a longer stay-time at upper altitudes is not needed, the small probes do
not use parachutes. On the large probe, more time is needed
for measurements of atmosphere and cloud composition. Thesmall probes do not carry atmospheric composition instruments.
As with the Large Probe, heat shield temperature and
probe acceleration data are stored for the atmospberic struc-ture experiment during the entry communications blackout. A
T h e c l o u d s e n s o r i n s t r u m e n t ( n e p he l o m e te r ) i s e n t i r e l yi n s i d e t h e p r e s s u r e v e s s e l , a nd l o o k s o u t t h r o u g h t w o s a p p h i r ewindows. For t h e n e t f l u x r ad io m e te r (h ea t d e p o s i t i o n i n s t r u -m e n t ) s e n s o r s a r e co m p l e t e l y ex t e r n a l , m ounted on a s m a l l boomex t e n d i n g f ro m t h e ex p e r i m en t h o u s in g . The r ad i o m e t e r s e n s o rw i t h i t s t w o diam ond window s t u r n s co n s t a n t l y i n a h a l f c i r c l e ,f i r s t l o o k i n g up a nd t h e n down. I n s t r u m e n t e l e c t r o n i c s a r ei n t e r n a l .
Communications
Communicat ions systems f o r t h e S m a l l probes c o n s i s t o fs o l i d s t a t e t r a n s m i t t e r s and h e m i s p h e r i c a l c o ve r ag e a n t e n n a s ,i d e n t i c a l w i t h those f o r t h e L a r g e Probe. Each t r a n s m i t t e rh a s one 10- W a t t , s o l i d s t a t e a m p l i f i e r . T h i s compares wi th40 w a t t s f o r t h e L a rg e P ro be . T h i s sy s te m c a n t r a n s m i t d a t a
t o t h e D S N ' s 6 4 - m (210- foo t ) a n t e n n a s a t a r a t e of 64 bps above30 km ( 1 9 m i . ) a l t i t u d e a nd 1 6 b p s be lo w t h a t t o impact . TheS m a ll Pr o b e s do n o t c a r r y a r e c e i v e r f o r two-way Dopple r t r a c k -
i n g as d o es t h e L ar g e P r o be , and D o p pl e r t r a c k i n g i s done us ingan o s c i l l a t o r ( s t a b l e t o a p p r o x i m a t e l y o n e p a r t i n a b i l l i o n )
on t h e p r o b e s a s a r e f e r e n c e f r eq u en c y f o r gro un d t r a c k i n gcomDutat ions .
D a t a r e t u r n e d i n c lu d e s c i e n t i f i c a nd e n gi n ee r in g i n f o r -mat ion . T h i s i n c l u d e s i n t e r n a l t e m p e ra t u r e and p r e s s u r e m ea su re -men t s , e l e c t r i c a l c u r r e n t fl o w a nd v o l t a g e s , a nd o n- of f s t a t u sof i n s t r u m en t s an d p r o be s y s t em s .
Command System
t h a t on t h e L a rg e Pr o be . I t p r o v i d e s 6 4 commands, a l l o r i g i -n a t e d o n b o a rd t h e p r o b es b y t i m e r s , programmers, G-switchesand o t h e r l o g i c s a nd d e v i c e s .
The command s ys te m on t h e S m a l l P r o b es i s i d e n t i c a l t o
Data Handl ing System
Components o f th e d a ta han d l in g sys t em on th e S m a l l
P r o b es a re i d e n t i c a l t o t h o s e f o r t h e L a r g e Pr o b e . The d a t a -h a n d l i n g u n i t c an a c c e p t 36 a n a l o g , 1 2 d i g i t a l and 24 one-b i t c h a n n e l s f ro m i n s t r u m e n t s a nd s y s t em s . L o gi c of d a t af o r m a t s a l s o i s i d e n t i c a l .
The s y s t em f o r e ach S m a l l Probe p r ov id e s f o r t h r e eh i g h - r a t e d a t a f o r m a t s : u p p e r d e s c e n t , l o w e r d e s c e n t a n de n t r y b l a c k o u t . As w i t h t h e L a rg e P ro b e, a s t o r a g e c a p a c i t yo f 3072 b i t s i s prov ided by t h e da ta memory. Fo l lowing th een t r y co m m u n i ca t i o n s b l ack o u t , s t o r e d d a t a w i l l b e p l a y e dback and t e l e m e t e r e d i n t h e u p p e r d e s c e n t f or m a t a t 6 4 bps .R e a l t i m e t r a n s m i s s i o n w i l l o c cu r i n i t i a l l y a t 6 4 bps i n t h e
u p p e r d e s c e n t format , chang ing t o 1 6 b p s a t 30 km ( 1 9 m i . )
a l t i t u d e ( lower d e s c e n t f o r m a t ) . Data r a t e a l l o c a t i o n amongt h e t h r e e S m a l l Probe i n s t r u m e n t s r a n g e s f r o m 6 t o 20 bps i nt h e upper for mat and 1 . 5 t o 7 .2 5 b p s i n t h e l o w er f o r m a t .
Power Sys e m s
S m a l l Probe power systems a re s i l v e r - z i n c b a t t e r i e sw h i c h p r o v i d e 11 ampere-hours of en e r g y a t a normal 2 8
v o l t s . The s y st e m i n c l u d e s a b a t t e r y , power i n t e r f a c e u n i ta n d c u r r e n t s e n s o r . O t h e r components a r e i d e n t i c a l t o t h o s e
C l o u d P h o t o p o l a r i m e t e r -- T h i s i n s t r u m e n t m e a su r es t h ev e r t i c a l d i s t r i b u t i o n o f c l o ud and ha ze p a r t i c l e s and ob-s e r v e s u l t r a v i o l e t a tm o s ph e ri c m ar ki ng s a nd c l o u d c i r c u l a -t i o n s . U l t r a v i o l e t images p r o v id e t h e v i s u a l r e f e r e n c e f o rd a t a from o t h e r O r b i t e r e xp er im e nt s and f o r t h i s i n s t r u m e n t ' sp o l a r i z a t i o n r e a d in g s .
A 3.7-cm ( 1 . 5 - i n . ) t e l e s c o p e w i t h a r o t a t i n g f i l t e r
w he el o b s e r v e s t h e p l a n e t a t f i x e d a n g l e s , u s i n g t h e O r b i t e rr o t a t i o n f o r s ca ns ac ros s t h e p l a n e t a nd m o ti on a l o n g t h es p a c e c r a f t t r a j e c t o r y a ro u nd V enus f o r c o mp le te p l a n e t a r ym ap pi ng . T he a n g l e o f t h e t e l e s c o p e may b e v a r i e d b y g r ou n dcommand f o r s e l e c t o b s e r v a t i o n s from a ny p o i n t i n o r b i t .
The i n s t ru m e n t u s e s a n u l t r a v i o l e t ( U V ) f i l t e r ( f o rmaximum c o n t r a s t ) t o t r a c k t h e p u z z l i n g f a s t -m o v in g UV
a b s o r b i n g m a r k in g s . F i v e p l a n e t a r y im ag es c a n b e made i ne ac h s p a c e c r a f t o r b i t . The f i e l d o f v iew i s a b o u t o n e - h a l fm i l l i r a d i a n , c o rr e sp o n di n g t o a r e s o lu t i o n of abo u t 30 km(19 m i . ) d i r e c t l y be low t h e O r b i t e r .
The i n s t r u m e n t m e as ur es s c a t t e r e d s u n l i g h t p o l a r i z a t i o nb a se d o n c l o u d an d h a ze p a r t i c l e s i z e , s h a p e a n d d e n s i t y .V e r t i c a l d i s t r i b u t i o n of c lo u d and h az e p a r t i c l e s i n r e l a -
t i o n t o a t m o s ph e r i c p r e s s u r e i s e x t r a c t e d from t h i s d a t a .
W hile t h e O r b i t e r i s a t p e r i a p s i s t h e i n s tr u m e n t o b s e r ve s
i n v i s i b l e l i g h t t h e hi gh -h az e l a y e r s of t h e a tm os ph er e.These " l imb scans ' ' have a r e s o l u t i o n a s s m a l l a s . 5 km ( . 3 m i . ) .The i n s t r um en t we ighs 5 kg (11 l b . ) a nd u s e s 5 .4 w a t t s .
S ur fa ce Radar Mapper - - The r a d a r mapping exper imentmakes f o r t h e f i r s t t i m e s t u d i e s of l a r g e p o r t i o n s of t h ep l a n e t ' s h e mi sp h er e n o t v i s i b l e from E a r t h . T h i s e x p er im e n tw i l l p r ov i de t h e o n l y d i r e c t o bs e r v a t i on s of t h e s u r f a c e t ob e o b t a i n e d fro m t h e O r b i t e r . From o b s e r v i n g t h e e c h o o fs e v e r a l r a d i o f r e q u e n c i e s , e x p er im e n te r s d e r i v e s u r f a c eh e i g h t s a lo n g t h e o r b i t a l t r a j e c t o r y t o a n a c c u r a c y o f 1 0 0 m( 3 0 0 f t . ) o r b e t t e r , g i v i n g a good e s t i m a t e o f g l o b a l to p o-
g r a p h y a n d s h a p e . S u r f a c e e l e c t r i c a l c o n d u c t i v i t y c an a l s ob e d e r i v e d from t h e r a d a r d a t a .
A l o w power ( 2 0 w a t t s peak p u ls e power) S-band (1 .757G H zlr ad ar s ys te m o b s e r v e s t h e s u r f a c e f o r o n e o u t o f e v e ry1 2 s ec on ds o f s p a c e c r a f t r o t a t i o n .
Measurements a r e made whenever th e Or b i t e r i s below3 , 0 0 0 km (1 ,860 m i . ) , s u b j e c t t o c o n s t r a i n t s s e t b y t h er e v o l v i n g r a d a r a n t e n n a an d by c o m p e t i t i o n w i t h o t h e r e x-p e ri m en ts f o r t h e l i m i t e d t e l e m e t r y c a p a c i t y . The i n s t r u -m ent a u t o m a t i c a l l y c om p en sa te s f o r D op pl er s h i f t c a us e dby t h e r a d i a l mo ti on of t h e O r b i t e r .
T e a m s c i e n t i s t s s u b t r a c t t h e o b se rv ed d i s t a n c e betw eent h e O r b i t e r and t h e s u r f a c e from t h e s p a c e c r a f t ' s o r b i t a lr a d i u s ( o b t a i n e d from DSN t r a c k i n g ) t o f i n d a b s o l u t e top o-g r a p h i c a l m ea su re me nts . S u r f a c e r e s o l u t i o n i s b e s t a t a p e r i a p s i sa l t i t u d e o f 200 km ( 1 7 4 m i . ) : 20 k m ( 1 2 m i . ) l ong and 1 6
k m ( 9 . 6 m i . ) a c r o s s t h e s u b o r b i t a l t r a c k . Data g a t h e r e dby t h e i n s t r u m e n t a nd te lemetered t o E a r t h w i l l be computer-a s se m b le d i n t o r a d a r maps o f t h e p l a n e t .
R e s o l u t i o n i s comparable t o t h e Ea r th - ba s ed r a d a rs t u d i e s ; enough t o d i s c e r n m a jo r s u r f a c e f e a t u r e s .
The in s t r um en t weighs 9 . 7 kg ( 2 1 . 3 l b . ) a nd u s e s 1 8
w a t t s .
I n f r a r e d R a d i o m e t e r -- T h i s i n s t r u m e n t m ea s ur e s t h e" h e a t " ( i n f r a r e d r a d i a t i o n ) e m i t t e d by t h e a tm os ph er e a tv a r i o u s a l t i t u d e s f rom 60 k m (36 m i . ) a t t h e t o p of t h ed e ns e c l o ud l a y e r s o u t t o 1 5 0 km ( 9 0 m i . ) . I n a d d i t i o n ,t h e i n s tr u m e nt s e a r c h e s f o r w a t e r v ap o r abo ve t h e c l o u dl a y e r s , m ea su re s t h e s i z e o f h e a t t r a p p i n g c lo u d l a y e r sand m ea su re s t h e p l a n e t a r y s o l a r r e f l e c t a n c e ( a l b e d o ) .The r a d i o m e t e r ' s d a t a y i e l d s a v e r t i c a l t e m pe ra tu re p r o f i l eo f t h e upper a tmosphere a s w e l l a s a h o r i z o n t a l t e m p e r a t u r e
p r o f i l e a l on g t h e s u b o r b i t a l t r a c k . Such i n fo r m a ti o n i si m p o rt a n t i n u nc ov er in g t h e e x t e n t a n d d r i v i n g f o r c e s o ft h e seemi ng f o u r- d ay c i r c u l a t i o n o f t h e up p er a tm o s p h er e .
The i n s t r u m e n t f e a t u r e s e i g h t d e t e c t o r s , e ac h s e n s i t i v et o a d i f f e r e n t f r a c t i o n of t h e i n f r a r e d s pe ct ru m. F i v ed e t e c t o r s m ea su re t h e i n f r a r e d e m i s s io n s a t f i v e s e l e c t edw av e l en g t h s o f t h e p n ( m i c r o m e t e r s ) , a b s o r p t i o n b an d o fcarbon d i ox id e . Each wave leng th samples a s p e c i f i c d e p thi n t h e a t m o sp h er e , d ep en di ng o n h e a t a b s o r b i n g c h a r a c t e r -i s t i c s o f t h e C 0 2 m o le c ul e an d t h e v a r i a t i o n o f t e m p er a t ur ew i t h a l t i t u d e . One d e t e c t o r e x c l u s i v e l y d e t e c t s an d mapst h e d i s t r i b u t i o n of water v ap or ( i f i t e x i s t s ) i n t he
u p p e r a t m o sp h e r e. A n o t he r d e t e c t o r m eas u re s t h e s i z e ands h a p e o f c l o u d l a y e r s , a nd t h e l a s t d e te c to r m eas u r e s t h e
A 48-mm-ape rtu re t e l e sc op e m i r ro r f e e ds a l l e i g h tc h a n n e l s . The t e l e s c o p e i s s e t a t 45 d e g r e e s t o t h eO r b i t e r s p i n a x i s so t h a t s ca ns a re made b y s p a c e c r a f tro t a t i o n . When l ook ing a t o n e p l a n e t ' s l im b t h e n ar ro wf i e l d of v iew g i v e s v e r t i c a l r e s o l u t i o n o f 5 km ( 3 m i . )a t p e r i a p s i s . When t h e O r b i t e r i s i n b e s t p o s i t i o n f o r
l im b s c a n ni n g o f t h e p l a n e t ' s a t m o sp h e r ic " ed ge , I' t h ei n s tr u m e nt o b t a i n s a d d i t i o n a l d a t a o n cl o u d l a y e r s a n dt h e v e r t i c a l d i s t r i b u t i o n of w a t e r v a p o r .
The i n s t ru me n t we ighs 5.9 kg ( 1 3 l b . ) a nd u s e s 5 . 2w a t t s .
A ir gl ow U l t r a v i o l e t S p e c t r o m e te r -- The u l t r a v i o l e ts p e c t r o m e t e r o b s e r v e s t h e num erous a t m o s p h e r i c m a r k in g sw hich c a n b e s e e n o n l y th r ou g h u l t r a v i o l e t ( U V ) f i l t e r s .The i n s t r u m e n t t r a c k s t h e UV abso rb ing masses wh ich r o t a t ei n f o u r d a y s, m e as ur es t h e e s c a p e r a t e of atomic hydrogenfrom t h e o u t e r a tm o sp he re an d m e as ur es t h e u l t r a v i o l e t
s c a t t e r i n g p r o p e r t i e s of t h e c l o u d t o p s a n d ha z e s a t a b o u t80 km (50 m i . ) a l t i t u d e .
A b s o r p t i o n o f UV r a d i a t i o n i n t h e u pp er a tm os ph er ep ro d uc es o p t i c a l UV emiss ions known a s t h e " a i r g l o w " .V a r i ou s a i r g l o w e m i s s i o n s a r e c au se d by d i f f e r e n t p h y s i c a lp r o c e s s e s (e . g . , s p l i t - u p o f m o le c ul es i n t o e l e c t r o n i c a l l ye x c i t e d a to m s ) . By v iewing day and n i g h t a i rg low a t wave-l e n g t h s b e t w e e n 1 , 1 0 0 Angstroms and 3,400 Angstroms, t h es p e c t r o m e t e r c a n t h u s i d e n t i f y t h e mechanism w hi ch e x c i t e st h e g a s e s o f t h e u p p e r a t m o sp h e r e. The t e m p e r a t u r e s o ft h e u p p e r a t m o s p h e re a t v a r io u s a l t i t u d e s c an a l s o b ei n f e r r e d f ro m d a t a f ro m l im b s c a n s a t t h e a t m o s p h e r e ' s
edge , a t s e le c te d w a v e l e n g t h s .
The i n s t r um en t measu res t h e Lyman Alpha co rona t o f i n dh yd ro ge n e s c a p i n g f rom t h e f a r t h e s t r e a c h e s o f V e nu s'a t m o sp h e r e. T h e s e d a t a a r e i m p o r t a n t b e c a u s e e s c a p i n ga tomic hydrogen i s t h e l a s t s t e p when a p l a n e t i s l o s i n gwater .
The s p e c t r o m e t e r f e a t u r e s a 125-mm t e l e s c o p e and mono-chroma to r t o r e s t r i c t (upon ground command) t h e vi ewi ngspec t rum t o any UV w a v el en g th . P h o t o m u l t i p l i e r t u b e s co n-v e r t t h e i m pi ng in g W r a d i a t i o n t o e l e c t r i c a l i m p u l s e s ,which a re t h e n t e l e m e t e r e d t o E a r t h f o r c o n s t r u c t i o n i n t o
u l t r a v i o l e t p l a n e t a r y maps.
The i n s t r um en t we ighs 3 . 1 kg ( 6 . 9 l b . ) a nd u s e s 1 . 7
N e u t r a l Mass S p e c t r o m e t e r -- T h i s i n s t r u m e n t m ea s u re st h e d e n s i t i e s of n e u t r a l i o n i z e d a t o m s a nd m o l e c u l e s i n
Venus ' uppe r a tmos phere be tween 150 km ( 9 0 m i . ) a t p e r i -a p s i s and 200 km ( 1 2 0 m i . ) . F i n d i n g t h e v e r t i c a l and h o r i -
z o n t a l v a r i a t i o n s i n t h e n e u t r a l g a s m ol ec ul es w i l l h e l pd e f i n e t h e c h em ic al s t a t e of t h e u p p e r a t m o s p h e re . V a r i a -
t i o n s o f h yd ro ge n an d he li um c o n c e n t r a t i o n s w i l l t e l l t h ee x t e n t o f g a s e s c a p e f ro m t h e a t m os p h er e . R e s e a r c h e r s w i l lf i n d t h e h e i g h t o f t h e ho mo pau se ( a bo v e w hi ch a t m o s p h e r em ix in g s t o p s ) by co mp ari ng t h e d e n s i t i e s o f i n e r t g a s e s a tt h e O r b i t e r a l t i t u d e s w i t h m e as ur em en ts made by t h e L a r g eP r o b e a nd Bus n e u t r a l mass s p e c t r o m e t e r s b e l o w 1 5 0 km ( 9 3 m i . ) .
N ob le g a s e s , o t h e r n o n - r e a c t i v e g a s e s an d c h e m i c a l l y
a c t i v e g a s e s up t o 4 6 atomic mass u n i t s a r e i d e n t i f i e d a ndm easu red . Gas molecules a re f i r s t i o ni ze d and t he n d e f l e c t e dby a m a g n e t ic f i e l d a c c o r d i n g t o t h e i r m a s s . The ave ragev e r t i c a l s pa ci ng of s a m p l e p o i n t s i s a p p r o x i m a t e l y 400 m
( 2 4 0 f t . ) a t 500 km ( 3 0 0 m i . ) a l t i t u d e w hi l e t h e h o r i z o n t a l
s p a c i n g f o r s am pl in g a l o n g t h e O r b i t e r p a t h i s a b o u t 2 km( 1 . 2 m i . ) .
The i n s t r umen t weighs 4 . 5 kg ( 9 . 8 l b . ) a nd u s e s 1 5 w a t t s .
I o n Mass S p e c t r o m e t e r -- The i o n m a s s s p e c t r o m e t e rm ea su re s t h e d i s t r i b u t i o n a nd c o n c e n t r a t i o n of p o s i t i v e l yc h a r g e d i o n s i n t h e V en u si an u p p e r a t m os p h er e fr om 1 5 0 km
( 9 0 m i . ) t o t h e i o n o sp h e r e . The in s t r u m e n t d i r e c t l y mea-s u r e s io n s i n a m a s s r a n g e f ro m h y dr og e n i o n ( p r o t o n ) t oi o n s of i r o n , c o r r e sp o n d i n g t o f ro m 1 t o 56 a to mi c massu n i t s . Such d a t a a r e i m p o r ta n t i n u n d er s ta n d in g t h e b a s i cn a t u r e of t h e i o n o s p h e r e an d i t s r e l a t i o n w i th t h e s o l a r wind.
The in s t r um en t m akes f i r s t a n e x p l o r a t o r y sweep o f 1 . 5
s e c o n d s , d u r i n g w hic h a s e a r c h i s made f o r up t o 1 6 d i f -f e r e n t i o ns . I t then makes a s e r i e s o f s w e e p s , r e p e a t i n gt h e s a mp li ng of t h e e i g h t m os t p ro m in e nt i o n s i d e n t i f i e dd u r i n g t h e e x p l o r a t o r y s we ep . ( Th e Bus i n s t r u m e n t i s i d e n -t i c a l t o t h e O r b i t e r v e r s i o n e x c e p t t h a t t h e s e o p e r a t i n gs e q u e n c e s c a n n o t b e modified by ground command a s t h e y c a non t h e O r b i t e r . )
I n f l i g h t , a s e ns o r i s e x po s ed t o a s t r e a m of atmos-p h e r i c i o n s , w h ic h f l o w i n t o a n aluminum c y l i n d e r e n c l o s i n ga s e r i e s of p a r a l l e l w i r e g r i d s . Each i o n s p e c i e s i s a c c e l -
e r a t e d by a s p e c i f i c v o l t a g e a p p l i e d t o t h e g r i d s so t h a tt h e i o n s i m pi ng e o n a c o l l e c t o r a t t h e r ea r o f t h e s e n s o rc y l i n d e r . The i o n s t r e a m ' s a c c e l e r a t i n g v o l ta g e w i l l y i e l di t s i d e n t i t y and i t s a m p l i t u d e w i l l r e v e a l i t s c o n c e n t r a t i o n .
T he i n s t r u m e n t w e ig h s 3 kg ( 6 . 6 l b . ) a nd u s e s 1 . 5 w a t t s .
S o l a r Wind P l a s m a A n a l y z e r -- T h i s i n s t r u m e n t m e a su r esp r o p e r t i e s of t h e s o l a r wind and i t s i n t e r a c t i o n s w i t hV e nu s ' i o n o s p h e r e a n d u p p e r a t m o s p h e re . T he i n s t r u m e n tm e as ur es v e l o c i t y , f l ow d i r e c t i o n an d t e m p e r at u r e of t h es o l a r w in d. Such f i n d i n g s s h o u l d h e l p e x p l a i n h o w t h e
i o n o s p h e r e r e a c t s w i t h t h e s o l a r w in d an d p o s s i b l y t h er o l e t h e s o l a r w ind p l a y s i n V en us ' weather p a t t e r n s .
The r e g i o n a r o u nd V en us , t h e c a v i t y "shadowed" b y t h es o l a r w in d, i s d e te r m in e d t o t h e e x t e n t a ll o w ed b y t h es p a c e c r a f t o r b i t . The i n s t r u m e n t s e a r c h e s f o r streams ofs o l a r p a r t i c l e s i n t h i s r e g i o n .
The p lasm a ana lyze r i s a n electrostatic/energy-per-u n i t c h ar g e s p e c t r o m e t e r . The s o l a r wind f l u x ( r a t e off l o w of t h e s o l a r wind) i s m ea su re d by t h e d e f l e c t i o n o fi n - r u s h i n g p a r t i c l e s by an e l e c t r o s t a t i c f i e l d be tw ee ntw o m e t a l p l a t e s . I f t h e p a r t i c l e s a re w i t h i n t h e e n er gy
r a n ge d et er m in ed by t h e p l a t e s ' v o l t a g e d i f f e r e n c e s , t h eye x i t be tween t h e p l a t e s , h i t t i n g o ne o f f i v e d e t e c t o r s .Which t a r g e t th e p a r t i c l e s h i t d e t e r m i n e s t h e s o l a r windd i r e c t i o n . By v a r y i n g t h e v o l t a g e b et we en t h e p l a t e s , t h ei n s tr u m e n t y i e l d s a c o m p l e t e p a r t i c l e s p e c t r u m of t h e s o l a rwind.
T h e i n s t r u m e n t w e ig h s 3 .9 kg ( 8 . 6 l b . ) a nd u s e s 5 . 2
w a t t s .
Magnetometer -- T h e m a gn e to m et er s t u d i e s V e nu s ' m a g -n e t i c f i e l d and t h e i n t e r a c t i o n of t h e s o l a r w i n d w i t h t h ep l a n e t . I t " s ea r ch e s ". f o r s u r f a c e - c o r r e l a t e d m a gn et ic fea-
t u r e s , s u ch a s r e g i o n s of c r u s t ma gn et iz ed i n t h e p a s t p e r -haps when Venus had much s t r o n g e r m a g n e t ic p r o p e r t i e s . Themeasurements of t h e m a gn e ti c f i e l d of E a r t h ' s s i s t e r p l a n e tmay s h e d l i g h t on w h a t i n t e r n a l f l u i d m o ti on s pr od uc e p la n e -t a r y m a g n e t i c f i e l d s . ( I t i s s t i l l not known what mot ionsa r e r e s p o n s i b l e f o r E a r t h ' s m a gn e ti c f i e l d s . )
I t a p p e a r s V en us h a s a v e r y weak m ag n e ti c f i e l d ; y e t ,it may p l a y a n i m p o r t a n t r o l e i n t h e i o n o s p h e r e - s o l a r w in di n t e r a c t i o n . T h e m a gn e to m et er s h o u l d f i n d whether it i st h e weak i n t r i n s i c m ag ne ti c f i e l d , a n i n d u c e d m a g n e t i cf i e l d o r t h e i o n os p he re i t s e l f w hich d e f l e c t s t h e s o l a rwind.
The i n s t r u m e n t c o n s i s t s of t h r e e s e n s o r s on 4.7-m( 1 5 . 5 - f t . ) booms, long enough t o i s o l a t e t h e m from mucho f t h e s p a c e c r a f t ' s own m a g ne t ic f i e l d . The inboard s e n s o r ,t i l t e d 45 d e g re e s t o t h e s p i n a x i s e x c l u s i v e l y m e as u re st h e O r b i t e r ' s m ag ne ti c f i e l d , w hich w i l l be s u b t r a c t e d f r o mt h e o u tb o a rd s e n s o r s ' r e a d i n g s .
Each s e n s o r c o n s i s t s of a r i n g a r o u n d which i s wrapped
a r i b b o n of perm eab le metal . Any e x t e r n a l ma g n et ic f i e l dc a u s e s t h e core t o p r o d u c e a n e l e c t r i c a l s i g n a l . A f e e d -
b ack s i g n a l t he n c a n c e ls t h e e x t e r n a l f i e l d so t h a t t h em a gn e to m et er a l w ay s o p e r a t e s i n a z e r o f i e l d c o n d i t i o n .The s t r e n g t h of t h e f e ed b ac k s i g n a l i s a measure of t h ee x t e r n a l m ag ne ti c f i e l d .
T h e i n s t r u m e n t weighs 2 kg ( 4 . 4 l b . ) a nd u s e s 2 . 2w a t t s power.
E l e c t r i c F i e l d Detec tor=- - T h i s i n s t r u m e n t w i l l h e l pa n sw e r q u e s t i o n s c o n c e r n i n g the c h a r a c t e r i s t i c s o f t h e i n t e r -a c t i o n s betw een Venus and t h e s o l a r wind, t h e m i l l i o n - m i l e -
an-hour i o n i z e d g a s t h a t c o n t i n u a l l y streams outward fromt h e S un t o t h e s o l a r s y st em .
T h e d e t e c t o r w i l l d e te r m in e t h e k i n d s o f i n t e r a c t i o n sbetween th e plasma ( t h e mass of i o n s a nd e l e c t r o n s ) of
V en us ' i o n o s p h e r e a n d t h e s o l a r w in d, t h e e x t e n t t o whichth e s o l a r w ind i s d e f l e c t e d around Venus , t h e e x t e n t t owhich the s o l a r wind h e a t s t h e i o n os p he r e, t h e e x t e n t ofi o n i z a t i o n c a u se d by e x o s p h e r e- s o l a r w ind i n t e r a c t i o n an ds o l a r w i n d t u r b u l e n c e . T h e i n s t r u m e n t a l s o searches f o r
" w h i s t l e r s " -- e l e c t r o m a g n e t i c d i s t u r b a n c e s which t r a v e la l o n g a p l a n e t ' s m a gn e ti c f i e l d l i n e s .
T h e i n s t r u m e n t m e as u re s e l e c t r i c components of plasmawaves and r a d i o e m i s s io n s i n th e f r e q u e n c y r e g i o n f r o m1 0 0 t o 1 0 0 , 0 0 0 H e r t z w hi ch i n d u c e a c u r r e n t i n t h e i n s t r u -m ent ' s V - type e l e c t r i c d i p o l e a n te n n a. T h e c u r r e n t i sa m p l i f i e d an d t h e i n f o r m a t i o n p r o ce s s e d a nd r e l a y e d b a ck
t o E a r t h . Four 30 p e r c e n t b an d wi dt h c h a n n e l s a re employed;each i s u s e f u l a t d i f f e r e n t p o i n t s a lo ng t h e O r b i t e r t r a -j e c t o r y , as i t p a s s e s t h r ou g h v a r y i ng d e n s i t i e s of t h es o l a r w in d . T he 0.6-m ( 2 6 - i n . ) - l o n g a n t e n n a i s d e s i g n e dt o l e a n on t h e O r b i t e r s h r o u d a n d d e p l o y a u t o m a t i c a l l ywhen th e sh roud i s e j e c t e d .
T h e i n s t r u m e n t w e ig h s 0 . 8 kg (1.74 l b ) a n d u s e s 0 . 7w a t t s of power.
E l e c t r o n T e m p e ra t u re P r o b e s -- T h e p r o b e s m e a su r e t h et h e r m a l c h a r a c t e r i s t i c s of V e n u s ' i o n o s p h e r e : e l e c t r o nt e m p e r a t u r e a nd c o n c e n t r a t i o n a nd i o n p la sm a mass and con-c e n t r a t i o n , a s well a s t h e s p a c e c r a f t ' s own e l e c t r i c a l
p o t e n t i a l . S uc h m e as ur em e nt s w i l l h e l p s c i e n t i s t s under-s t a n d t h e h e a t i n g m ec ha nism s of V en us ' i o n o s p h e r e , c u r r e n t l yb e l i e v e d t o i n c l u d e h e a t i n g a t h i gh e r a l t i t u d e s by t h e s o l a rwind and a t low er a l t i t u d e s by s o l a r u l t r a v i o l e t r a d ia t i o n .
T w o c y l i n d r i c a l probes 7 c m ( 3 i n . ) b y 0 . 2 5 c m ( 0 . 5
i n . ) a r e use d. One pro be i s mounted p a r a l l e l t o t h e space -c r a f t s p i n a x i s on a 0 . 4 - m (16-in. ) boom, and t h e o t h e rp r o b e i s m o u n t e d p e r p e n d i c u l a r t o t h e s p i n a x i s on a 1-m( 4 0 - i n . ) boom. (The booms a r e long enough t o p l a c e t h ese ns or s beyond m uch of t h e p h o t o e l e c t r o n c l o u d an d i o n
s h e a t h s u rr o un d in g t h e s p a c e c r a f t w h i c h m i g h t d i s t o r t read-i n g s . ) T h e l o n g e r boom a l l o w s measurement of e l e c t r o n c o n -t e n t a n d t e m p er a tu r e f o r c o n d i t i o n s of v e r y l o w e l e c t r o nc o n c e n t r a t i o n s .
E a c h p r o b e h a s i t s own power g e n e r a t o r w h i l e s h a r i n gi n f l i g h t d a t a a n a l y s i s c i r c u i t r y . A s a w t o o t h v o l t a g e s w ee pseach p r o b e t w i c e p e r s e c o n d a nd i s e l e c t r o n i c a l l y a da pt ed t om atch t h e e x i s t i n g e l e c t r o n d e n s i t y an d te m p e r a tu r e b e i n gmeasured.
T he i n s t r u m e n t w e i gh s 2 . 2 kg ( 4 . 7 6 lb.) a n d u s e s 4 . 8
w a t t s of power.
Charqed P a r t i c l e R e t a r d i n g P o t e n t i a l A na ly ze r -- T h i s
i n s t r u m e n t m e a su r es t h e t e m p e r a t u r e , c o n c e n t r a t i o n a n d v e l o -c i t y of t h e m o s t a bu n da nt i o n s i n t h e i o n o s p h e r e ( p r e s u m a b l yc a r b o n d i o x i d e an d ox yg en i o n s . ) I t a l s o m e a s u r e s t h e c o n -c e n t r a t i o n , t e m p e ra t u re an d e n e r g y of s u r r o u n d i n g p h o t o -e l e c t r o n s i n t h e i on o sp he re .
T h e i n s t r u m e n t i s d es ig ne d s p e c i f i c a l l y f o r d e t e c t i n gt h e l o w e ne rg y pla sm a p a r t i c l e s i n V en us ' i o n o sp h e r e , a sopposed t o t h e much more h i g h l y e n e r gi z e d s o l a r wind par-t i c l e s . However, t h e a n a l y z e r s h o u ld p r o v i d e d a t a c on c er n-i n g t h e s o l a r w in d- io no sp he re i n t e r a c t i o n a t a n a l t i t u d e o f
400 t o 500 k m ( 2 4 0 t o 300 m i . ) a t t h e p o i n t where t h e s o l a rwind streams i n t o t h e i o n o sp h er e .
By v ar yi ng e l e c t r i c a l p o t e n t i a l s , c o l l e c t o r g r i d s of6 c m ( 2 .5 i n . ) d i am e te r s e l e c t i v e l y allow v a r i o u s i o n o s p h e r i cp a r t i c l e s t o s t r i k e a d e t e c t o r . C u r r e n t i nd uc ed i n t h e de-
t e c t o r i s a m p l i f i e d by a n e l e c t r o m e t e r .
Measurements a r e t a k e n a t i n t e r v a l s a 1 on g .a 120-km ( 7 2 -
m i . ) o r b i t s eg me nt t h r o ug h t h e i o n o s p h e r i c pla sm a r e g i o n .O n b o a r d a n a l y s i s s e l e c t s t h e optimum p o i n t i n t h e s p a c e c r a f t
r o t a t i o n a t which t o sample t h e i o n o s p h e r i c plasma, s o t h a teach s c a n i s c o m pl e te d i n a small f r a c t i o n of a s p i n p e r i od .
The i n s t r u m e n t a c h i e v e s a 20-km ( 1 2 - m i . ) r e s o l u t i o n f o rt o t a l i o n c o n c e n t r a t i o n .
T he i n s t r u m e n t w e ig h s 2 . 8 kg ( 6 . 3 l b . ) a nd u s e s 2 . 4w a t t s of power.
G a m m a R a y B u r s t D e t e c t o r -- T h e gamma r a y b u r s t d e t e c t o ro b s e rv e s t h e i n t e n s e s h o r t d u r a t i o n ( on e -t e nt h s ec on d t o a
f e w t e n t h s of s e c o nd s ) " b u r s t s " of h i g h en e r q y p r o t o n s f ro mo u t e r s p a c e . T h i s phenomenon w a s n ' t d i s c o v e r e d u n t i l 1 97 3,a nd t h e n a t u r e a nd o r i g i n o f t h e s o u rc e s a r e s t i l l unknown.T h e gamma ray b u r s t s occ ur randomly i n t i m e ( r o u g h l y 1 0 p e ry e a r ) a nd a pp e ar t o o r i g i n a t e from random p o i n t s i n t h e u n i -v e r s e . T he gamma r ay b u r s t d e t e c t o r i s t h e o n l y ex p e r i m en ton Pi on ee r Venus which i s n o t i n vo lv ed i n t h e d i r e c t s tu dyof Venus and i t s e n v i r o n s .
The Venus O r b i t e r , s e p a r a t e d f r o m E a r t h b y r o u g h l y o n ea s t ro n o m ic a l u n i t ( 1 4 9 m i l l i o n km o r 93 m i l l i o n m i . ) p r o v i d e sa means t o o b t a i n a " f i x " o n t h e s t r a n g e b u r s t s , by c o r r e l a -t i n g i t s o b s e r v a t i o n s w i t h t h o s e made by o r b i t i n g E a r t hs a t e l l i t e s . Measurements of t h e gamma r a y so ur ce s w i l l b emade w i t h a n a c c u r a cy of l e s s t h a n o n e a r c m i n u t e , p r e c i s eenough f o r a n a t t e m p t a t o p t i c a l i d e n t i f i c a t i o n of t h e s o ur ce s .
Two sodium i o d i d e p h o t o m u l t i p l i e r d e t e c t o r u n i t s s en -s i t i v e t o p ho to ns i n t h e 0 . 2 t o 2 . 0 m i l l i o n e l e c t r o n v o l t s( M e V ) e n e r g y r a n g e p r o v i d e a co n t i n u o u s t i m e h i s t o r y f o rt h o s e b u r s t s i n t e n s e enough t o be d e t e c t e d a n d g i v e a c o a r s ep r o f i l e of t h e gamma b u r s t energ y range . A memory u n i t of
2 0 , 0 0 0 " b i t s " f o r s t o r i n g d a t a f o r l a t e r r e a d o u t i s r e q u i r e dt o accom modate t h e v e r y h i g h d a t a r a t e s t h a t o cc u r d u ri n g a
b r i e f b u r s t .
The in s t r um en t weighs 2 . 8 kg (6.35 l b . ) a n d u s e s 1 . 3
w a t t s of power.
O r b i t e r Rad io Sc ience
I n t e r n a l D e n s i ty D i s t r i b u t i o n E xp er im en t -- T h i s e x p e r i -m en t d e t e r m i n e s V e n us 's i n t e r n a l m a s s d i s t r i b u t i o n , t h e pro -cesses which have p ro du ce d t h a t d i s t r i b u t i o n , t h e p l a n e t ' sg l o b a l s ha p e a nd t h e r e l a t i o n s h i p b etw ee n V enu s' s u r f a c ef e a t u r e s and t h e i r c o r re sp on di ng i n t e r n a l d e n s i t i e s . R e -s e a r c h e r s h op e t o c o n s t r u c t a model of t h e p h y s i c a l p r o c e s s e sw hi ch go v er ne d V en us ' p l a n e t a r y e v o l u t i o n w i t h t h e h e l p o ft h i s e x pe r im e n t ' s d a t a .
S c i e n t i s t s u s e t h e two-way D op pl er t r a c k i n g o f t h eO r b i t e r , w h i c h i s also u se d f o r n a v i g a t i o n , t o f i n d v e ry
s m a l l ch an g es i n i t s o r b i t . They u s e t h e s e o r b i t c ha ng est o c h a r t V enus ' g r a v i t y f i e l d . T h i s g r a v i t y i n f o rm a ti o nc a n t h e n b e used t o c a l c u l a t e v a r i a t i o n s i n p l a n e t d e n s i t y .
An S-band s i g n a l of 2 . 2 G H z i s t r a n s m i t t e d f r o m a D S N
a n te n na , r e c e i v e d by t h e O r b i t e r s p a c e c r a f t and r e t r a n s -m i t t e d back t o t h e DSN an t en n a . D o p p l e r s h i f t s i n f re qu en cyof these s i g n a l s mean c h an ge s i n s p a c e c r a f t v e l o c i t y . Mostof th e v e l o c i t y c h a n g e s a r e due t o t h e r e l a t i v e o r b i t a l
mo t ions of E a r t h , Venus and t h e Pi on ee r Venus O r b i t e r . How-e v e r , l o c a l a no ma li e s i n t h e i n t e r n a l mass d i s t r i b u t i o n ofVenus i n d u c e a d d i t i o n a l v e l o c i t y c h an g es . A n a l y s i s of t h ev e l o c i t y c ha ng es t h e r e f o r e p ro v i d e s i n f o r m a t i o n on t h e i n -t e r n a l m a s s d i s t r i b u t i o n of Venus.
Comparison of t h i s d a t a w i t h t h e r a d a r mapping d a t amay s u p p o r t t h e e x i s t e n c e of b a s i c o n- go in g p h y s i c a l p r o -cesses , s u ch a s E a r t h - l i k e p l a t e t e c t o n i c s ( t h e movementof m a ss iv e c r u s t a l p l a t e s s l o wl y p a s t o ne a n o t h e r ) . Thed a t a a l s o w i l l i n f e r t h e l i k e l y co m p o s it i o n a nd t e m p e r a t u r eo f V enus' i n t e r i o r .
C e l e s t i a l Mechanics Exper iment -- T h e c e l e s t i a l mechanicse x pe r im e n t s t u d i e s V en us ' g r a v i t y f i e l d , l e a d i n g t o c a l c u -l a t i o n s of i t s g l o b a l s h a p e a nd i n f e r e n c e s a b o u t t h e d y na mi csof t h e p l a n e t ' s u p p e r a t mo s ph e re a nd i o n o s p h e r e . T he e x p e r i -ment a l s o m ea su re s t h e d i r e c t i o n of V en us ' s p i n a x i s , r o t a -t i o n o f t h e p l a n e t ' s p o l e s , d e n s i t y of t h e u p p er a t m o s ph e r e ,r e l a t i v i s t i c e f f e c t s of s o l a r g r a v i t y on t h e O r b i t e r t r a c k i n gs i g n a l and improves ou r knowledge of t h e e x a c t p l a n e t a r y t r a -j e c t o r i e s of Venus and E a r t h .
S c i e n t i s t s u s e Do pp le r t r a c k i n g t o c h a r t t h e p l a n e t ' sg r a v i t y f i e l d . A DSN an t en n a o n E a r t h t r a n s m i t s a r a d i os i g n a l of 2 . 2 G H z t o t h e O r b i t e r , w hich r e t r a n s m i t s t h a t
s i g n a l , m u l t i p l i e d by 2 4 0 / 2 2 1 ( t o d i s c r i m i n a t e o ut g o in g fromi nc om in g s i g n a l s ) . U ne xp ec te d f r e q ue n c y s h i f t s i n t h e s e
s i g n a l s mean c h an ge s i n s p a c e c r a f t p o s i t i o n . T h e s e changesa r e cau s ed b y t h e m a s s and g r a v i t a t i o n a l f i e l d of Venus,g r a v i t y f i e l d of t h e Sun and Venus ' own atm os ph ere , w h i c h
e x e r t s a d r a g o n t h e O r b i t e r . More d e t a i l e d s t u d i e s of t h ea tmosphere a r e p o s s ib l e j u s t b e f o r e and a f t e r t h e o c c u l t a -t i o n s of t h e O r b i te r by Venus , when t h e r a d i o s i g n a l mustp a ss q u i t e c l o s e t o t h e p l a n e t s u r f a c e on i t s way t o E a r t h .
D i s t o r t i o n s ( s c i n t i l l a t i o n s ) of t h e O r b i t e r s i g n a l d u ri n gthese p e r i o d s r e v e a l v a r i a t i o n s i n up pe r a tm os ph er e d e n s i t y .
S i m u l t an eo u s r a d i o t r a c k i n g of the O r b i t e r w i t h e x t r a -g a l a c t i c r a d i o s o ur c es w i l l a l l o w v e r y p r e c i s e d e t e r m i n a t i o nof t h e o r b i t s of E a r t h a n d V enus w i t h r e s p e c t t o t h e s e e x t r a -g a l a c t i c o b j e c t s .
D u al F r eq u e nc y R a d io O c c u l t a t i o n E x p er im e n t -- T h i se x p e r i m e n t s t u d i e s t h e a tmos phere of Venus by obse rv inghow O r b i t e r X- a nd S -b and r a d i o s i g n a l s p e n e t r a t e V en us 'atmosphere on t h e way t o r e c e i v e r s o n E a r t h . T h e 4 0 o c c u l -t a t i o n s w i t h Venus which t h e O r b i t e r t r a j e c t o r y e n c o u n t e r s
o v e r i t s m i s s i o n l i f e t i m e w i l l produce 80 p r o f i l e s of t h es i g n a l d i s t o r t i n g p r o p e r t i e s of t h e p l a n e t ' s lower and upper
a t m o s p h e r e a n d i o n o s p h e r e .
By an a l yz in g t h e s c i n t i l l a t i o n s i n r a d i o s i g n a l s c au se dby v a r i o u s a t mo s ph e ri c l a y e r s , i n v e s t i g a t o r s c a n i n f e r t h er e f r a c t i o n , t e mp e ra tu r e , p r e s s u r e a nd d e n s i t i e s of t h e atmos-p h e r e f r o m 3 4 k m ( 2 0 m i . ) a l t i t u d e u p t hr ou gh t h e i o n o s p h e r e .A s t h e r a d i o s i g n a l s p i e r c e t h e i o no s ph e re , i n v e s t i g a t o r sc an me as ur e s i g n a l d i s t o r t i o n d ue t o v a r y i n g e l e c t r o n d en -s i t i e s i n t h i s b ar el y- kn o wn r e g i o n . S i n c e m o s t of t h e se
measurements a re made o n V e n u s t : n i g h t s i d e , d a t a i s p r o v i d e don t h e r e p o r t e d l y v a r i a b l e V e nu sia n n i g h t t i m e i o n o s p h e r e .
T h e O r b i t e r hi g h- g ai n a n t en n a i s s p e c i a l l y aimed d u r i n go c c u l t a t i o n s s o t h a t t h e r e f r a c t e d r a d i o s i g n a l i s o p t i m a l l yaimed a t E a r t h . D S N s t a t i o n s on E a r t h a r e e qu ip p ed w i t hs p e c i a l r e c e i v e r s t o t r a c k t h e i n c o m i n g s i g n a l s a s t h e i rp h a se a nd f r e q u e n c i e s a r e m o d i f ie d d u r i n g t r a n s m i s s i o nthrough Venus I s atmosphere.
Atmospher ic and S o l a r Wind Turb ulen ce Exper imen t -- T h ee x p e r im e n t o b s e r v e s t h e s m a l l s c a l e t u r b u l e n c e ( l e s s t h a n 1 0
km o r 6 m i . ) i n t h e Venus ian a tmosphe re above 3 5 k m ( 2 2 m i . )a l t i t u d e . I t w i l l r e v e a l t h e v a r i a t i o n of a t m o s p he r i c t u r -
b u le n ce w i t h l a t i t u d e , l o n g i t u d e an d a l t i t u d e c h an ge s d u r in g
t h e 40 o c c u l t a t i o n s w hen O r b i t e r s p a c e c r a f t s i g n a l s m us tp a s s t h r o u g h V e nu s ' a t m o s p he r e o n t h e i r way t o E a r t h t r a c k -i n g s t a t i o n s . B ec au se t h e s i g n a l s t r a v e l t hr ou g h t h e i ono-s p h e r e a s w e l l , f l u c t u a t i o n s i n e l e c t r o n d e n s i t y ca n a l s o b e
i n f e r r e d from t h e d a t a .
F o l l o w i n g c o n c l u s i o n of t h e n or ma l m i s s i o n l i f e t i m e(a round Augus t 19791, t h e O r b i t e r w i l l p r o v i d e d e n s i t y an dv e l o c i t y m e a s u r e m e n t s of t h e s o l a r wind near t h e Sun. Venusw i l l t h e n a p pr o ac h s u p e r i o r c o n j u n c t i o n ( E a r t h a nd Venus
w i l l b e on o p p o s i t e s i d e s of t h e S u n ) . T h i s i s a n i d e a lt i m e t o i n v e s t i g a t e t h e s o l a r wind , t h e s t ream of i o n i z e dp a r t i c l e s c o n s t a n t ly s w i r l i n g o f f t h e Sun . Because t h e s o l a r
wind i s s o c h a n g e a b l e , r e p e a t e d O r b i t e r o b s e r v a t i o n s of t h es o l a r w in d n e a r t o a nd f a r f rom t h e Sun w i l l p r o v i d e n e e d e di n f o r m a t i o n a b o u t s o l a r wind d e n s i t y , t u r b u l e n c e a nd v e l o c i t yu n i f o r m i t y . T w o D S N s t a t i o n s w i l l a n al y ze t h e f l u c t u a t i o n s( s c i n t i l l a t i o n s ) i n the O r b i t e r S- and X-band s i g n a l s a s t h e yp a s s t h e s o l a r w ind o n t h e i r way t o E a r t h .
Atmospher ic Drag Exper iment -- T h i s i n v e s t i g a t i o n t a k e sd r a g m easurement f o r t h e f i r s t t i m e of a n o t h e r p l a n e t ' s a tm os -p h e r e , a s t h e a tm o sp he re " f r i c t i o n " o f Venus s l o w s t h e O r b i-t e r . E x p e r i m en t e r s w i l l us e d r ag measu remen ts th roughou tt h e O r b i t e r m i ss i on t o s e a r c h f o r a ny v a r i a t i o n s i n a tmos-p h e r i c d e n s i ty t h a t c o r r e l a t e w i t h s o l a r w ind a c t i v i t y c h a ng e si n s o l a r u l t r a v i o l e t r a d i a t i o n and d i f f e r e n c e s i n d e n s i t y on
t h e p l a n e t ' s n i g h t s i d e . I n a d d i t i o n , p r o j e c t s c i e n t i s t s a r el o o ki n g f o r e vi d e n ce t h a t t h e s ee ming f ou r- da y r o t a t i o n o f t h el o wer a t m o sp h e r e ex t en d s i n t o t h e u pp e r a t m o s p he r e .
D S N s t a t i o n s a n a l yz e t h e D o pp le r e f f e c t on t h e s pa ce -c r a f t ' s X- an d S-band r a d i o s i g n a l , c au s ed b y a t m o s p h e r i cd r ag -i n d u ced ch an g e i n t h e O r b i t e r ' s d i r e c t i o n a nd s p ee d .
The e n t i r e s p a c e c r a f t , e s s e n t i a l l y t h e s ha pe of a c y l i n -d e r , a c t s a s t h e t e s t i n s t r u m e n t . A t mo sp he ri c d e n s i t y i sd et er mi ne d b e s t i n t h e v i c i n i t y of p e r i a p s i s ( be tw ee n 1 5 0
and 250 km o r 9 3 and 1 5 5 m i . ) , w he re t h e d ra g e f f e c t i s muchg r e a t e r t h an e l s ew h er e a l on g t h e O r b i t e r t r a j e c t o r y . A s t h e
p e r i a p s i s a l t i t u d e c ha ng es , v a r i a t i o n s of a t m o s p h e r i c d e n s i t yw i t h a l t i t u d e c an be p l o t t e d .
Knowledge o f a tmo sphe r i c d e n s i t y a i d s i n t e r p r e t a t i o n o fm a s s s p e c tr o m e te r f i n d i n g s , i n f e r s t h e c om p os it io n a nd t e m -p e r a t u r e of t h e u p pe r at mo sp he re a nd a i d s i n c o n s t r u c t i n g amodel of Venus ' upper a tmosp here .
Large P robe Exper imen t s
N e u t r a l Mass S p e c t r o m e t e r -- T h e n e u t r a l mass s p e c t r o -m e t e r m eas u r es t h e a t m o s p h e r i c co m p o s i t i o n o f t h e l o w e r 60 km( 3 6
m i . ) of Venus' a tm osp here ( l a r ge1 y t h e a tmosphere belowt h e ma s si v e c l o u d l a y e r s ) a s t h e L a r g e P r ob e d e s cen d s b yparachute . Knowledge of t h e r e l a t i v e a bu nd an ce s of g a s e sw i l l h e l p a ns we r q u e s t i o n s a b o ut t h e e v o l u t i o n , s t r u c t u r ea nd h e a t b a l a n c e of Venus.
The i n s t r u m e n t d e t e r m i n e s t h e v e r t i c a l d i s t r i b u t i o n andc o n c e n t r a t i o n o f n o n - r e a c t i v e g a s e s , c h e m i c a l l y g c t i v e g a s e s
and r a t i o s of i n e r t ga s i s o to p e s . Water v ap or ( i f i t e x i s t s )i s a l s o measured .
T h e i n s t r u m en t i s m ounted i n s i d e t h e L a r g e Pr o be p r e s -s u r e v e s s e l . I t r e c e i v e s a co n t i n u o u s f l o w o f a t m o s p h e r i c
g a s t h r o u g h t w o u n i q u e ceramic i n l e t t ub es t h a t p r o t r u d et hr ou gh t h e p r e s s u r e v e s s e l w a l l . The i n l e t tu b e s a re c a l l e dC e r a m i c Micro Leaks (CMLS) and a r e made t o g r e a t l y l i m i t t h eamount of g a s e n t e r i n g t h e i n s tr u m e nt , w i t h o u t c he m i ca l lya l t e r i n g it.
The C M L s a r e p a s s i v e d e v i c e s a nd t h e a mount of g a sf l o w in g t h r o u gh t hem i n c r e a s e s w i t h i n c r e a s i n g a t m o s p h e ri cp r e s s u r e . T o p r e v e n t " f l o o d i n g " of t h e i n s t r u m e n t , o n e CML
i s sea l ed when t h e a t m o s p h e r i c p r e s s u r e i s a b o u t 1 . 5 b a r s .
A f t e r e n t e r i n g t h e i n s tr u m e nt , t h e a t m o s p h e r i c g a s i s f i r s ti o n i z e d a n d t h e s e p a r a te d i o n s s o r t e d o u t f o r m a s s andq u a n t i t y o f each c o n s t i t u e n t by t h e i r d i f f e r e n t d e f l e c t i o n si n p a s s i n g t h ro u gh m a g n et ic f i e l d s .
The spec t rometer c an i d e n t i f y g a s e s w i t h masses up t o208 atomic m a s s u n i t s , b e l ie v e d t o be a l a r g e e n o u g h m a s sr an g e f o r a l l m o le cu le s l i k e l y t o b e e n c ou n te r ed i n t h el ow e r a tm o sp h er e. S e n s i t i v i t y i s o ne p a r t p e r m i l l i o n .
S i x t y a t m o s p h e r i c s a m p l i n g s a re p l a n n e d , w i t h a mass spec t rumt a k i n g 6 4 s e c o n d s . An o n b o ar d m i c r o p r o c e s s o r c o n t r o l s t h ei n s t r u m e n t a n d a c c u m u l a t e s d a t a f o r t e le m e t r y t o E a r t h .
The i n s t r um en t we ighs 1 0 . 9 kg ( 2 4 l b . ) a n d u s e s 14 w a t t s .
G a s Chromatograph -- T h e ga s ch roma tograph measures t h eg a s e o u s c o m p o s i t i o n of Venus ' l o w e r atmosphere. B y f i n d i n qt h e m a j o r s o u r c e s o f i n f r a r e d o p a c i t y ( t h o s e gases t h a t t r a ph e a t ) , s c i e n t i s t s s ho uld b e t t e r un de rs ta nd why Venus h as 480-d e g r e e C (900-degree F . ) s u r f a c e t e m p e r a t u r e s . From t h em e as ur em e nt o f g a s e s pr od u ce d b y r a d i o a c t i v e d e c ay , s c i e n -t i s t s c a n i n f e r t h e d e gr ee of d i f f e r e n t i a t i o n w i t h i n V enus'i n t e r i o r . E x p e r i m e n t e r s w i l l a l s o b e a b l e t o d e d u c e t h es i m i l a r i t y of t h e c om p os it io n o f t h e s o l i d p a r t s of Venusa nd E a r t h by t h e i d e n t i f i c a t i o n of v a r io u s s u l f u r i c g ases .
T h e i n s t r u m e n t s a m p l e s t h e lower a t m o s p h e r e t h r e e t i m e sd u r i n g t h e L a rg e P r o b e ' s d e s c e n t . T h e a tmosphe re f l o w s i n t oa t u b e p e n e t r a t i n g t h e e x t e r i o r of t h e L a r g e P r o b e a nd i n t oa hel ium g a s s t ream, w h i c h sweeps t h e s a m p le i n t o t w o chroma-tog ra ph co lumns. Atmosphe r ic ga se s a re i d e n t i f i e d by t h et i m e it t akes t hem t o f l ow th rough t h e co lumns. As a c a l i -b r a t i o n c h e c k , t w o sam ples o f f r e o n ( a g a s n o t l i k e l y t o be
e n c o u n t e r ed i n t h e a t m o sp h er e) a r e added t o t h e t h i r d sample ,and t h e i r r e s o l u t i o n n o te d .
T h e i n s t r u m e n t w e i g h s 6 . 3 kg ( 1 3 . 8 l b ) a n d u s e s 4 2w a t t s , t h e m o st of any P io nee r Venus i n s t ru me n t .
S o l a r F l u x R a d io m e te r - - Th is i n s t r u m e n t m e a s ur e s w h er es o l a r ene rgy i s d e p o s i t e d i n t h e l ow e r V e nu si an a tm o s ph e re ,g i v i n g a v e r t i c a l p r o f i l e of s u n l i g h t i n p u t . I t r e v e a l show much sunlight i s abs orbe d by t h e c lo ud s and how muchs u n l i g h t r e a c h es t h e s u r f a c e , i m p o rt a n t i n f or m a t i o n f o r re -so lv in g whe the r Venus ha s a greenho use wea the r mach ine ande x p l a i n i n g w h y i t s s u r f a c e i s s o h o t .
The i n s t r u m e n t c o n t i n u a l l y m e as ur es t h e d i f f e r e n c e i ns u n l i g h t i n t e n s i t y d i r e c t l y a bo ve a nd below t h e Large Probeh o r i z o n a s t h e pro be d r i f t s t o t h e p l a n e t s u r f a c e . F i v eq u a r t z l e n s e s of 3 mm ( 1 / 8 i n . ) d ia m et er i n s i d e f i v e f l a tsapphi re windows c o l l e c t t h e l i g h t a n d t r a n s m i t it b y q u a r t zr o d s t o a n e l e c t r o n i c l i g h t d e t e c t o r . S u n l i g h t i n t e n s i t y i sd e t ec t e d i n t h e s p e c t r a l r a ng e of 0 . 4 t o 1 .8 p m ( m i c r o m e t e r s ) ,t h e w a ve le n gt h r a n g e f o r m os t s o l a r e n e r g y . V e r t i c a l r e s o -l u t i o n i s 700 t o 1,000 m ( 2 , 3 0 0 t o 3 , 3 0 0 f t . ) . L e n s e s a r ep o s i t i o n e d b o t h u p a n d down t o f i n d t h e a m o u n t of s o l a re n e r g y absorbed i n l a y e r s o f t h e a t m os ph er e. T o a v o i d h a v i ngt h e p r ob e o r i t s p a ra c hu te i n t h e f i e l d o f v iew , t h e r a d io -
m e t e r s a mp l es s u n l i g h t i n na rr ow 5 -d eg re e f i e l d s o f v ie w .
The i n s t ru me n t we ighs 1 . 6 kg ( 3 . 5 l b . ) a n d u s e s 4 w a t t s .
I n f r a r e d R a d i o m e t e r -- The i n f r a r e d r a d i o m e t e r me a su r est h e v e r t i c a l d i s t r i b u t i o n o f i n f r a r e d r a d i a t i o n i n t h e atmos-phe re f rom Large P robe pa ra ch u t e dep loymen t a t 6 7 km ( 4 0 m i . )down t o t h e s u r f a c e . I t a l s o d e t e c t s c lo ud l a y e r s a nd w a t e rva po r , bo th of which may w e l l be t rapping enormous amountso f h e a t an d p r e v en t i ng t h e i r r e r a d i a t i o n b ac k i n t o s p a c e.F i n d i n g major h e a t s o u r c e s ( a nd t r a p s ) i s e s s e n t i a l t o prov ingVenus has a greenhouse hea t i ng mechan i sm.
S i x p y r o e l e c t r i c i n f r a r e d d e t e c t o r s were c h o s e n b e c a u s et h e y do n o t need s p e c i a l c o o l i n g eq ui pm en t f o r t h e i r u s e i nt h e e xt r e m e a t m o s p h e r i c h e a t . Each d e t e c t o r v ie w s t h e a tm os -p h e r e v i a r o t a t i n g l i g h t p i p e s t hr ou gh a d i f f e r e n t i n f r a r edf i l t e r b etw een 3 and 50 mic rons . The v i ews o f t h e d e t e c t o r si s d i r e c t e d a t 4 5 d e g r e e s a b o v e a nd b el ow t h e p r o b e h o r i z o nt h r o u g h a diamond window heated t o p r e v e n t p a r t i c l e c on tam i-n a t i o n d u r i n g p as s a g e t h ro u gh c l o u d s . T h e d i f f e r e n ce i n i n f r a -r e d r a d i a t i o n , c l o u d o p a c i t y and water vapor between t h e t w ov i e w i n g a n g l e s i s t e l e m e t e r e d t o E a r t h e v e r y 6 s e c o n d s , g i v i n ga v e r t i c a l i n f r a r e d s p a t i a l r e s o l u t i o n of 250 m (8 25 f t . ) o rb e t t e r .
Two of t h e s i x d e t e c t o r s m o n it o r t h e t em p e ra t ur e an do p t i c a l u n i f o r m i t y o f t h e d ia mo nd v i e w i ng window , t w o d e t e c -t o r s d e t e c t a n d m e a s u r e w a t e r v a p o r , o n e d e t e c t o r measuresc l o ud o p a c i t y a nd t h e r e m ai n in g d e t e c t o r me as ur es t h e i n f r a r e d
i n t e n s i t i e s of t h e a tm o s ph e ri c l a y e r s t h e L ar ge Pr obe p a s s e st h r o u g h .
The i n s t r um en t weighs 2 . 6 k g ( 5 . 8 l b . ) a nd u s e s 5 . 5 w a t t s .
Cloud P a r t i c l e S i z e S p e c tr o m e te r -- T h i s i n s t r u m e n t
measures t h e p a r t i c l e s i z e an d s ha p e a nd d e n s i t y of V enus'c lo ud s i n t h e lower a tmosphe re from 67 k m ( 4 0 m i . ) down t ot h e s u r f a c e .
Through measurements of p a r t i c l e s i z e and m a s s , t h e
i n v e s t i g a t i o n p ro v i de s a v e r t i c a l p r o f i l e of p a r t i c u l a t ec o n c e n t r a t i o n f o r 3 4 d i f f e r e n t s i z e c l a s s i f i c a t i o n s , r a n g i n g
f r o m 1 t o 500 m i cr o ns i n d i am e t e r ( a micron i s o ne m i l l i o n t hof a m e t e r o r r o u g h l y t w o t e n - t h o u s a n d t h s of a n i n c h ) . Suchmeasurements w i l l g i v e c l u e s t o b a s i c c l o ud f o r m a t i o n p r o-
cesses a nd c l o u d - s u n l i g h t i n t e r a c t i o n s on V en us.meter a l s o d i f f e r e n t i a t e s i c e c r y s t a l s -- i f any a re p r e s e n t-- from o t h e r c r y s t a l l i n e p a r t i c u l a t e s by d e te rm in in g t h ei c e ' s c h a r a c t e r i s t i c " a s p e c t r a t i o " -- t h e r a t i o of p a r t i c l e
t h i c k n e s s t o s i z e .
The s p e c t r o -
T h e i n s t r u m e n t d i r e c t s a l a s e r beam o n t o an e x t e r n a lm i r r o r s u p p o r t e d 1 5 c m ( 6 i n . ) from t h e p r e s s u r e v e s s e l ' s
o u t e r s u r f a c e . T h e m i r r o r d i r e c t s t h e beam b ac k i n t o ab a c k s c a t t e r d e t e c t o r . A s a p a r t i c l e e n t e r s t h e i n s t r u m e n t ' sf i e l d of view, i t s shadow i s imaged onto a p h o t o d i o d e a r r a yd e t e c t o r , w h e r e i t s shadow s i z e i s measured and recorded .
The i ns t ru me n t weighs 4.4 kg ( 9 . 6 l b . ) a nd u s e s 2 0 w a t t s .
Larqe and S m a l l P r o b e I n s t r u m e n t s
A t m os p he ri c S t r u c t u r e E x p e ri m e n ts -- T h e s e i n v e s t i g a t i o n s
d e t e r m i n e V en us ' a t m o s p h e r i c s t r u c t u r e f ro m 200 km ( 1 2 0 m i . )
t o impac t a t f o u r e n t r y s i t e s w e l l s e p a r a t e d from o n e a n o t h e r .
T em p er at ur e, p r e s s u r e a nd a c c e l e r a t i o n s e n s o r s on a l l f o u r
p r o b e s y i e l d d a t a on t h e l o c a t i o n a nd i n t e n s i t i e s of atmos-p h e r i c t u r b u l e n c e , t h e v a r i a t i o n of t e m p e r a t u r e s w i t h pres-
s u r e and a l t i t u d e , th e a v e r a g e a t m o s p h e r i c m o l e c u l a r w e i g h tand the r a d i a l d i s ta n c e t o t h e c e n t e r o f V enus. I f t h eP r ob e s s u r v i v e i mp a ct ( a remote p o s s i b i l i t y ) , t he y w i l l re -
v e a l a n y se i smic a c t i v i t y i n t h e c r u s t of t h e p l a n e t .
Each h as o n e f r e e w i r e e le m en t p r o t r u d i n g i n t o t h e a tm o sp he ref o r maximum s e n s i t i v i t y and one e l emen t bonded t o t h e s u p p o r tframe f o r maximum s u r v i v a b i l i t y .r a n g e p e r m i t s it t o r e c o r d t e m p e r a t u r e s f r o m b e l o w f r e e z i n gt o 4 70 d e g r e e s C ( 9 0 0 d e g r e e s F.).
T h e t e m p er a tu r e s e n s o r s a re d u a l resistance.thermometers.
I t s e x t r e m e t e m p e r a t u r e
P r e s s u r e s e n s o r s a r e m u l t i p l e r an ge , m i n i a t u re s i l i c o n
d iaphragm se ns o r s . The w i d e r ange needed f r o m 30 m i l l i b a r st o 1 0 0 b a r s p r e s s u r e i s a c h i e v e d b y 1 2 s e n s o r s of o v e r l a p p i n gs e n s i t i v i t y . T h i s a l s o p r o v i d e s r e d u nd a nc y i n case of a
A c c e l e r a t i o n s e n s o r s ( f o u r o n t h e l a r g e r pr o b e, o nech of t h e s m a l l p r o b e s ) h a v e p e n d u l o u s mass, main-
t a i n e d i n n u l l ( z e r o ) p o s i t i o n by t h e i n t e r a c t i o n of ac u r r e n t i n a c o i l i n s i d e t h e m a s s w i t h a m a gn e ti c f i e l d .The n u l l i n g c u r r e n t i s t h e measure of a c c e le r a t ion .
An e l e c t r o n i c s p a ck a ge d i s t r i b u t e s power t o a l l sen-s o r s , s am ple s t h e i r o u t p u t , c ha ng es t h e i r r a n g e s a n d s t o r e s
d a t a .
T he i n s t r u m e n t s o n t h e L a r g e P r o b e w ei gh 2 . 3 k g ( 5 . 1l b . ) a nd u s e 4 . 9 w a t t s . On each o f t h e Sm al l P rob es t h ei n s t r u m e n t s w ei gh 1 . 2 kg ( 2 . 7 l b . ) a nd u s e 3 .5 w a t t s .
Nephelometer -- The n e p he l om e t er s e a r c h e s f o r c l o u d
p a r t i c l e s ( s o l i d o r l i q u i d ) i n t h e l o w e r atmosphere f rom6 7 k m ( 4 0 m i . ) t o t h e s u r f a c e . By p r o v i d i n g a l l f o u r p r o b e sw i t h n e p he l om e t er s , i n v e s t i g a t o r s can d e t e r m i n e w h e t h e r
c l o u d l a y e r s v a r y fro m l o c a t i o n t o l o c a t i o n o r a re u n i f o r m l yd i s t r i b u t e d a c r o s s t h e p l a n e t .
A l i g h t e m i t t i n g d i o d e (LED) of 9 , 0 0 0 A n g s t r o m s t o g e t h e rw i t h a p l a s t i c F r e s n e l l e n s f o r f o cu s i ng t h e l i g h t i l l u m i n a t et h e a t m o s p h e r e t h r o u g h a window mounted i n t h e p ro b e p r e s s u r e
vesse l . The t r a n s m i t t e d l i g h t beam i s p r o j e c t e d a d i s t a n c eb ey on d t h e t u r b u l e n t a t m os p h er e s u r r o u n d i n g t h e p r o b e s a st he y descend . Through a second window, a r ece ive r m easurest h e i n t e n s i t y of l i g h t b a c k s c a t t e re d ( a bo u t 1 7 5 d e g r e e s ) b ya t m o s p h e r i c p a r t i c l e s . B ot h windows a r e p r o t e c t e d f ro m t h es e a r i n g t e mp e r at u re s of t h e Venusian a tmosphere and f roms t r a y l i g h t .
I n v e s t i g a t o r s w i l l u s e t h e backw ard l i g h t s c a t t e r i n g
p r o p e r t y of c l o u d s an d h a z e s t o c o n s t r u c t a v e r t i c a l p r o -f i l e o f p a r t i c l e d i s t r i b u t i o n i n t h e l ow er a tm osphere . I n
a d d i t i o n , t h e t w o s m a l l p r o b e s d e s c e n d i n g i n t h e s u n l i ts i d e w i l l b e m ea su rin g t h e v e r t i c a l d i s t r i b u t i o n of s o l a r
s c a t t e r e d l i g h t a t 3,500 Angstroms and 5,300 Angstroms.
T he i n s t r u m e n t w e ig h s 1.1 kg ( 2 . 4 l b . ) a nd u s e s 2 . 4w a t t s .
N e t F l u x Radiometer -- T h i s i n s t ru m e n t maps t h e p la n e-t a r y p o s i t i o n s of s o u r c e s a nd absorbe rs of r a d i a t i v e e n e r g y
and t h e i r v e r t i c a l d i s t r i b u t i o n . T h e d i s t r i b u t i o n of r a d i a -t i v e e n er gy ( h e a t a nd s u n l i g h t ) p ow er s t h e a t m o s p h e r i c c i r -
c u l a t i o n o n V e n u s a s w e l l a s E a r t h . T h e i n s t r u m e n t d a t a w i l lb e r e l a t e d t o t h e o b s er v e d a t m o s p h e r i c m o t i o n s , t e m p e r a t u r es t r u c t u r e and c lo u d c h a r a c t e r i s t i c s f r o m o t h e r P i o n e e r V e n u se x p e r i m e n t s t o g a i n a more a c c u r a t e p i c t u r e o f V en us ' w e a -t h e r m a ch in e .
The i n s t r u m e n t s o n each of t h e t h r e e S m a l l P r o b e s a r e
i d e n t i c a l a nd c a n o p e r a t e e qu a l ly i n e i t h e r day o r n i g h th e m i s ph e r e s . F o l l ow i n g d e s c e n t i n t o t h e lower a tm ospherebelow 72 km ( 4 5 m i . ) t h e i n s t r u m e n t ' s s e n s or i s dep loyedf r o m a p r o t e c t i v e e n c l os u r e t o a p o s i t i o n l o c a t i n g it beyond
t h e t u r b u l e n c e n e a r t h e b a s e of t h e h e a t s h i e l d . Data col-l e c t i o n c o n t in u e s u n t i l i mp ac t.
The i n s t r u m e n t ' s f l u x p l a t e i s o r i e n t e d p a r a l l e l t o t h ep l a n e t ' s s u r fa c e . A d i f f e r e n c e be tween upward and downwardr a d i a n t e ne rg y f a l l i n g o n t h e two s i d e s of t h e p l a t e p ro du ce sa t e m p e r a t u r e g r a d i e n t t h r o u g h it , w hich indu ces an e l e c t r i c a l
c u r r e n t . The p l a t e i s f l i p p e d 1 8 0 d e g r e e s e v e r y s e co n d t oa s s u r e e ve n d a t a c o l l e c t i o n .
The i n s t r u m e n t w e ig hs 1.1 kg ( 2 . 4 l b . ) a n d u s e s 3 . 8 w a t t s .
MultiDrobe Bus ExDeriments
N e u t r a l Mass S p e c t r o m e t e r -- T h e n e u t r a l m a s s s p e c t r o -m e t e r m e a s u r es t h e v a r i o u s co mp on en ts (atoms a n d m o l e c u l e s )of t h e a t m o s p he r e s a nd t h e i r v e r t i c a l d i s t r i b u t i o n f r o ma b o u t 1 , 0 0 0 km ( 6 0 0 m i . ) t o 130 km (8 0 m i . ) , em phas iz ingt h e a l t i t u d e r an ge 1 5 0 t o 1 2 0 km (90 t o 75 m i . ) w h i c h n e i t h e rt h e O r b i t e r n o r t h e f o u r p r o b e s reach . ( T h e Bus i s e x p e c t e dt o b u r n u p a t a n a l t i t u d e of a b o u t 1 2 0 km ( 7 5 m i . ) .
From t h e i n s t r u m e n t d a t a , i n v e s t i g a t o r s c a n d e r i v e t h eh e i g h t o f th e t u r b op a u s e ( t h e r e g i o n a bo ve w hi ch a t m o s p h e r i cg a s e s do n o t m i x ) , f i n d t h e r a t i o s of a t m o s p h e r i c i s o t o p e sa nd d e r i v e eddy d i f f u s i o n c o e f f i c i e n t s (mathematical e x p r e s -
s i o n s d e s c r i b i n ghow
r a p i d l y t h e a tm o sp h er e i s m i x e d ) .T h e
c o m p o s i t i o n of t h e i o n o s p h e r e ' s maximum d e n s i t y c a n a l s o b e
d e t e r m i n e d , a s w e l l a s t h e t e m p e r a t u r e o f t h e e x o s p he r e, t h eo u t e rm o s t f r i n g e of Venus ' a tm osphere .
T he i n s t r u m e n t i o n i z e s a t m o s p h e r i c co mp on en ts u p t o 4 6
a t o m i c mass (hydrogen t o i r o n ) by e l ec t r o n bombardment. I tt h e n s e p a r a t e s them a c c o r d i n g t o t he i r masses by how f a rt h e y a re d e f l e c t e d by a m a g n e t i c f i e l d . The i n s t r u m e n t f e a -t u r e s a f a s t d a t a s am p li ng an d t e l e m e t e r i n g c a p a c i - ty t o copew i t h t h e 3 km-per-second ( 1 1 0 mph) Bus d e sc en t sp ee d. One
d ay b e f o r e Venus e n c o u n t e r , a known amount of g a s i s r e l e a s e di n t o t h e i n s tr u m e n t f o r i d e n t i f i c a t i o n a nd measurem ent, t o b eused a s a r e f e r e n c e f o r t h e s pe c t ro m e t er ' s s e n s i t i v i t y .
T he i n s t r u m e n t w e ig h s 6 . 8 kg ( 1 5 l b . ) a nd u s e s 5 w a t t s .
I o n Mass S p e c t r o m e t e r -- The ion mass s p e c t r o m e t e rm ea su re s t h e d i s t r i b u t i o n an d c o n c e n t r a t io n of p o s i t i v e l yc h a r g e d i o n s i n t h e u p p e r Venus a t m o s ph e r e fr om 1 2 0 km ( 75m i . ) up t h r o u g h t h e i o n o s p h e r e .
(See O r b i t e r I o n Mass S p e c tr o m e te r f o r i n s t r u m e n td e s c r i p t i o n . )
M u l t i p r o b e R a d io S c i e n c e E x p e ri m e n ts
D i f f e r e n t i a l Long B a s e l i n e I n t e r f e r o m e t r y -- T h i si n s t r u m e n t m e as ur es t h e v e l o c i t y a nd d i r e c t i o n o f V en us 'winds a s t h e f o u r p r o b e s d e sc e nd t h r o u g h t h e a t m o sp h e re .By c om p ar in g t h e d e s c e n t p a t h s o f t h e p r o b e s w i t h s i m u l -t a n e o u s m e as ur em e nt s o f a t m o s p h e r i c t e m p e r a t u r e a n d p r e s -s u r e fro m p r ob e s e n s o r s , i n v e s t i g a t o r s c a n a ss e mb l e a b e t t e rmodel of V enus ' a t mo s ph e ri c c i r c u l a t i o n , p a r t i c u l a r l y i nr e g a r d t o wind speeds .
W h il e t h e f o u r p r o b e s d e s c en d t o t h e s u r f a c e , t h e Busf o l l o w s a b a l l i s t i c t r a j e c t o r y i n t h e up pe r atm os ph ere .T h i s t r a j e c t o r y s e r ve s a s a r e f e r e n c e . P r o b e v e l o c i t i e sc a n b e r e c o n s t r u c t e d and mea su re d v e r y a c c u r a t e l y r e l a t i v e
t h e b u s , an d a b s o l u t e p ro b e v e l o c i t i e s c an b e r e c o n s t ru c t e df ro m t h e known b u s v e l o c i t y .t i o n s of t h e p ro b e t r a j e c t o r i e s fro m a n a t m o sp h e r el e s sma thema t i ca l mode l a r e c a u s e d by a t m o s p h e r i c w in d s .
I n v e s t i g a t o r s assume d e v i a -
Two w i d e l y s e p a r a t e d D S N s t a t i o n s s im u l t a ne o us ly t r a c k i n ga l l s p a c e c r a f t d et er mi ne t h a t p a r t o f t h e v e l o c i t y v e c t o ra l o n g t h e E arth -V en us l i n e o f s i g h t . D i f f e r e n t i a l l on g -b as edi n t e r f e ro m e t r y u se s t h r e e D S N s t a t i o n s t o f i n d t h e o t h e r two
c om po ne nts of t h e v e l o c i t y v e c t o r t o t r i a n g u l a t e o r g e t a" f i x 1 ' i n t h r e e d im e ns io ns o n t h e c o n s t a n t l y c h an g in g p a t h sof t h e f a l l i n g p r ob es .
Atmospher i c P ro pa ga t io n Exper imen t -- T h i s i n v e s t i g a -t i o n a t te m p t s t o g l e a n in f o rm a t i on a b o u t V enus ' s u r f a c ea nd a tm o sp h er e by t h e e f f e c t s o f t h e a t m os p he r e o n t h ep r o be s ' r a d i o s i g n a l s . A s t h e p r ob e s d es c e nd , P i o n e e rs c i e n t i s t s s e a r c h f o r e v id en ce of a v e r y weak s i g n a l t h a tt r a v e l s downward, r e f l e c t s o f f t h e s u r f a c e of Venus and
t h e n b o u n ces t o E ar t h . S u ch a d i s t o r t e d s i g n a l i s Dopplers h i f t e d away fr om t h e pr o b e s i g n a l of 2 , 3 0 0 M H z ( m i l l i o nHe r t z ) by l e s s th a n Hz and i s a lm os t u n d e te c t a b l e . I f t h i ss i g n a l i s d i s c o v e r e d , i t s h o u ld r e v e a l i n f o r m a t i o n a b o u tt h e V e nu si an s u r f a c e -- h en ce , a i d i n t h e i n t e r p r e t a t i o no f t h e r a d a r m ap pin g d a t a .
The descend ing p robe s a l s o r e v e a l i n f or m a t io n a b o u tt h e a t m o s ph e r e . P r o b e r a d i o s i g n a l s weaken w i t h d e c r e a s i n ga l t i t u d e due t o C 0 2 a b s o r p t i o n , a tm o sp he re r e f r a c t i o n a nda d d i t i o n a l a b s o r p t i o n from c l o u d l a y e r s o r some o t h e r a b-s o r b e r . The s t r e n g t h o f t h e p r ob e s i g n a l s s h o u ld r e v e a lth e unknown ab so rbe r : i f i t i s a c l ou d l a y e r , i n v e s t i g a t o r sc a n m ea su re t h e h e i g h t and t h i c k n e s s o f t h e l a y e r .
Atmospher i c Turbu lence Exper imen t s -- T h i s i n v e s t i g a -t i o n s t u d i e s t h e t u r b u l e n c e i n t h e Ve nu si an a tm o sp he re , t h u sa i d i n g i n t h e u n d e r s t a n d i n g o f t h e d yn am ic s o f V en us 'p h er e c i r c u l a t i o n .t h e i r t r a n s m i t t i n g s i g n a l s w i l l l i k e l y be d i s t o r t e d by s m a l lr e g i o n s o f t u r b u l e n c e c a u s e d b y t e m p e r a t u r e , p r e s s u r e andv e l o c i t y f l u c t u a t i on s . D S N r e c e i v i n g s t a t i o n s o n E a r t h w i l la n a l yz e t h e s i g n a l s f o r d i s t o r t i o n c a u se d by a t m os p he r ic t u r -bu le nce . The p robe d a t a com pl em en ts a t m o s p h e r i c t u r b u l e n c ed a t a a b o v e 35 km ( 2 1 m i . ) t a ke n by t h e O r b i t e r .
atmos-A s a l l f o u r p r ob es d es ce nd t o t h e s u r f a c e ,
R a d io s c i e n c e e x p e r im e n t s m ea su re i n t e r a c t i o n ofs p a c e c r a f t r a d i o s i g n a l s w i t h V enus a nd i t s a tmosphe re ,u s i ng t h e O r b i t e r a n d f i v e pr ob e c r a f t a s i n s t r u m e n t s .D r . Gordon P e t t e n g i l l , M a s sa c hu s et ts I n s t i t u t e of Tech-
nology, i s team l e a d e r .
D r . R og er P h i l l i p s Venus I n t e r n a l D e n s i tyJ e t P r o p u l s i o n L a b or a to r y D i s t r i b u t i o n
D r . I . I . S h a p i r oM a ss ac hu se tt s I n s t i t u t e
of Technology
C e l e s t i a l Mechanics
D r . A rv yd as K l i o r e R ad io O c c u l t a t i o nJ e t P r o p u l s i o n L a b o r a t o r y
D r . Thomas C r o f t R a d io O c c u l t a t i o n
S t a n f o r d Research I n s t i t u t e
D r . R i c h a r d Woo Atmospher i c and S o l a r CoronaJ e t P r o p u l s i o n L a b o r a t o r y
D r . G e r a l d K e a t i n gL a n g l e y R e s e a r c h C e n t e r
Atmospheric Drag
M u l t i p r o b e S p a c e c r a f t ( L ar g e P r ob e )
D r . John Hoffman Mass S p e c t r o m e t e rU n i v e r s i t y of Texas , D a l l a s
Vance OyamaAmes Research C e n t e r
A l v i n S e i f fAmes R e s e a r c h C e n t e r
D r . M a r t i n TomaskoU n i v e r s i t y of A r i z o n a
R o b e r t B o eseAmes R e s e a r c h C e n t e r
G a s Chromatograph
A t m o s p h e r e S t r u c t u r e
S o l a r Flux Radiometer
I n f r a r e d R a d i o m e t e r
D r . R o b e r t K n o l l e n b e r g Cloud P a r t i c l e S i z eP a r t i c l e Measur ing Sys tems , I n c . S p e c t r o m e t e r
M u l t i p r o b e S p a c e c r a f t (Three S m a l l P r o b e s )
A l v i n S e i f f
Ames R e s e a r c h C e n t e r
D r . B o r i s R a ge ntA m e s R e s e a r c h C e n t e r
D r . Jacques BlamontU n i v e r s i t y of P a r i s
A t m o s p h e r e S t r u c t u r e
Nephelometer
Nephelometer
D r . V e r n e r Suomi N e t F l u x R a d i o m e t e r
U n i v e r s i t y o f W i sc o ns i n
M u l t i p r o b e S p a c e c r a f t (B us )
D r . Ulf von ZahnU n i v e r s i t y of Bonn,W e s t Germany
Mass Spec t rometer
H a r r y T a y l o r I o n M a s s S p e c t r o m e t e r
Goddard S p ac e F l i g h t C e n t e r
M u l t i p r o b e R a di o S c i e n c e ( A l l Probes )
D r . C h a r l e s C . C ou nse lm an D i f f e r e n t i a l L o n g- B a se l in eM a s sa c hu s et ts I n s t i t u t e of I n t e f e r om e tr y
Technology
D r . Thomas C r o f t A t m o s p h e r i c A t t e n u a t i o n
S t a n f o r d R es ea rc h I n s t i t u t e
D r . R i c h a r d Woo Atm osp heri c Tu rb ul en ce
J e t P r o p u l s i o n L a b o r at o r y
I n t e r d i s c i p l i n a r y S c i e n t i s t s
I n t e r d i s c i p l i n a r y s c i e n t i s t s hav e b een se lec ted f o rb o t h t h e Mul t ip robe and O r b i t e r M i s s i o n s t o p r o v i d e a s s i s -t a n ce i n a n a l y se s of t h e Venus ian a tmosphe re . They are :
D r . S i e g f r i e d B a u e rG od dard S p a c e F l i g h t C e n t e r
F o r Pioneer V en us , m i s s i o n c o n t r o l l e r s w i l l be o p e r a t -i n g s i m u l t a n e o u s l y t w o d i f f e r e n t s p a c e c r a f t on t w o d i f f e r e n tm i s s i o n s . T h e O r b i t e r a n d M u l t i p r o b e w e r e l a u n c h e d w i t h i n
t h r e e m on th s of e a ch o t h e r to a r r i v e a t t h e p l a n e t l e s st h a n a week apar t .
D u ri n g t h e Venus e n c o u n t e r p e r i o d , l a u n c h o f t h ef o u r p ro b es from t h e t r a n s p o r t e r Bus t o t h e i r a t m o s p h e r i ce n t r y p o i n t s w i l l be accom pl i shed ; th e Bus w i l l b e r e t a r -g e t ed f o r i t s e n t r y : t h e O r b i t e r w i l l be p l a c e d on i t s
24-ho ur, h i g h - i n c l i n a t i o n , h i g h l y e l l i p t i c a l o r b i t . F i ved ay s a f t e r O r b i t e r e n c o u nt e r , p ro be e n t r y w i l l be moni-t o r e d , and t h e c r i t i c a l p ro be d a t a r e c e i v e d and s t o r e df o r l a t e r a n a l y s i s .
W ith c o m p le t i o n o f t h e M u l t ip r o b e m i s si o n - - a f t e r i m -
p a c t o f t h e p r o b e s on t h e s u r f a c e an d b u rn -u p o f t h e Busc o n t r o l l e r s w i l l c o n t i n u e t o o p e r a t e t h e O r b i t e r f o r t h e
e i g h t m on th s o f i t s pr im ary m iss ion . C o n t ro l l e r s may makes i g n i f i c a n t changes i n t h e o r b i t d u r i n g t h i s e xt en de dm i s s i o n p e r i o d .
c r a f t , m i s si o n o p e r a t i o n s o f t e n r e q u i r e 24-hour-a-dayc o n t r o l and c a r e f u l a n a l y s i s a nd p la n ni n g i n s h o r t t i m e
s p a n s. G r ou n d- c on t ro l le d s p a c e c r a f t p r o v id e f l e x i b i l i t yf o r c h an g in g p l a n s and o b j e c t i v e s . They a l s o o f f e r e co n-o mies i n s p a c e c r a f t d e s i g n and c o n s t r u c t i o n .
S i n c e a l l P i o n e e r s a r e r e l a t i v e l y u n au to ma te d sp a ce -
P i o n e e r Venus c o n t r o l an d s p a c e c r a f t o p e r a t i o n s w i l l
be a t t h e P i o n e e r M i s s i o n O p e r a t i o n s C e n t e r ( P M O C ) , Ames
R esea rch C en te r , M ounta in V i e w , C a l i f . , fro m t h e t i m e b o t hs p a c e c r a f t s e p a r a t e from t h e i r l au nc h v e h i c l e s t h ro ug ht h e e nd of t h e O r b i t e r m is s i on .
P i o n e e r Venus o p e r a t i o n s w i l l be made somewhat morecom plex by t h e c o n t i n u e d o p e r a t i o n a t t h e PMOC o f t h ep r e v i o u s l y l a un c he d P i o n e e r s p a c e c r a f t . P i o n e e r s 6 t o 9c o n t i n u e t o c i r c l e t h e Sun and r e t u r n i n t e r p l a n e t a r y d a t a .P i o n e e r 1 0 c o n t in u e s t o e n t e r p r e v i o u s l y u ne xp lo re ds p a c e o n i t s way o u t o f t h e s o l a r s y st e m (it s now ap-p r a c h i n g U ra nu s' o r b i t ) . P i o n e e r 11 i s descend ing backto war d t h e e c l i p t i c and ma n's f i r s t e n co u nt e r w i t h S a t u rni n September 1 9 7 9 .
The PMOC i s t h e c e n t r a l mi s si o n c o n t r o l c e n t e r . I t
i s u nd er o p e r a t i o n a l d i r e c t i o n of t h e F l i g h t D i r e c t o r .T h i s a r e a will o r i g i n a t e a l l command i n f o r m a t i o n a nd re-
ce ive and d i s p l a y t e l e m e t r y d a t a r e q u i r e d f o r m i s s io n c on-t r o l . A l t h o u g h a l l commands a re o r i g i n a te d i n t h e PMOC,emergency p roced ures in c l ud e backup command gen er a t io n a tt h e D S N s t a t i o n s , i f n e c es sa ry .
The PMOC h a s c om pu ti ng c a p a b i l i t y b o t h f o r commandingt h e two s p a c e c r a f t and f o r i n t e r p r e t i n g t h e d a t a s t r e ama s it i s r e c e i v e d f r o m t h e D S N s t a t i o n s f o r u se by f l i g h tc o n t r o l l e r s m o n i t or i n g s p a c e c r a f t p er fo rm an ce .
S e v e r a l g r o u p s of s p e c i a l i s t s d i r e c t an d s u p p o r t l a u n chi n t e r p l a n e t a r y , o r b i t a l a nd at m os p he r ic e n t r y o p e r a t i o n s .
T h e P i o n e e r M i ss i o n O p e r a t i o n s t ea m c o n s i s t s o f p e r -s o n n e l f rom g ov er nm en t an d c o n t r a c t o r o r g a n i z a t i o n s , ando p e r a t e s u nd er c o n t r o l of t h e P r o j e c t Manager and Mi ssi onOperat ions System Manager.
B ecause P ion ee r V enus i nc lu de s two m i ss io ns , t w of l i g h t o p e r a t i o n s g r ou p s h av e be en named fo r each- -an O r -
b i t e r group and a Mult ipro be group. Both groups have t h e
same e l e m e n t s . The Sc i en ce A n a ly s i s Team i n each groupi s composed o f s c i e n c e o p e r a t i o n s p e o p l e f ro m t h e p r o j e c t
and t h e p r i n c i p a l i n v e s t i g a t o r s (o r t h e i r r e p r e s e n t a t i v e s )f o r each exper im en t on boa rd t h e O r b i t e r and M ul t ip robe .They determine t h e s t a t u s o f ea ch s c i e n t i f i c i n st r u me n t ,and fo rm u la t e command sequences f o r th e ins t ru m e nt s .
Both groups a l s o have Sp ac ec ra f t Pe r form ance A na ly -s i s teams, made up of e n g i n e e r in g s p e c i a l i s t s on s pa ce -c r a f t s y st em s s uc h as : communicat ions , t h e r m a l c o n t r o la nd p ow er. T he se t ea ms a n a l y z e a nd e v a l u a t e s p a c e c r a f tp e r fo r m a n ce a nd p r e d i c t s p a c e c r a f t r e s p o n s e s t o commands.
A t h i r d o r g a n i z a t i o n s e r v es b ot h s p a c e c r a f t . T h i s i st h e N av ig a t ion and Maneuver s g roup , which h a n d l e s space-
c r a f t n a v ig a t io n and o r i e n t a t i o n i n s pa ce ; o r b i t a l i n j e c -t i o n , t r i m , and c h an g es a nd p r o b e - t a r g e t i n g a nd l a u n c h .T h i s group is made up o f e n g in e e r i n g s p e c i a l i s t s i nsp a c e c r a f t o r i e n t a t i o n g eo m et ry , t r a j e c t o r i e s , and maneu-v e r s . The J e t P r o p u l s i o n L a b o r a t o r y , u nd er c o n t r a c t t oAmes, does c om p ut er a n a l y s i s o f D S N t r a c k i n g i n f o r m a t i o nt o d et er mi ne s p a c e c r a f t t r a j e c t o r i e s .
The M iss ion O p era t io ns Team a l s o i n c l u d e s a l a u n chs p e c i a l i s t , a h ar dw ar e e x p e r t and a computer sys tems de-
v e lo p me n t an d o p e r a t i o n s g r o u p .
S u p p o r t g r o u ps a t Ames a n d o t h e r 3ASA f a c i l i t i e s a s-
s i s t t h e m i s s i o n o p e r a t i o n s t ea m t o per fo rm com pute r so f tya red ev el op m en t, m i s s i o n c o n t r o l and o f f - l i n e d a t a p r o c e s s i n g .
N A S A ' s Deep S pac e Network ( D S N ) w i l l t r a c k a n d re -
c e i v e d a t a d i r e c t l y from a l l s i x P i o n e e r Venus s p a c e c r a f t( t h e O r b i t e r , t h e Bus a nd t h e f o u r p ro b e s ) . Commands a r e
t r a n s m i t t e d t o s p a c e c r a f t f ro m t h e P i o n e e r M i s si o n Ope ra-t i o n s C o n t r o l C e nt e r th r o u g h t h e D S N s t a t i o n s .
T r a c k i n g w i l l b e by t h e DSN's g l o b a l n e tw or k o f 2 6 - m( 8 5 - f t . ) and h i g h l y s e n s i t i v e 64-m ( 2 1 0 - f t . ) a n t e n n a s . The 6 4 s
w i l l be u s e d d u r i n g c r i t i c a l p h a s e s of t h e m i s s io n s u c h a sr e o r i e n t a t i o n , v e l o c i t y c o r r e c t i o n s , o r b i t i n s e r t i o n , and
e n t r y of t h e f o u r p ro b e s i n t o Ve nu s' a tm o sp h er e -- as w e l la s f o r s p e c i a l s c i e n c e e v e n t s s uc h a s o c c u l t a t i o n . A t t h ee nd o f t h e O r b i t e r p r im a r y m i s s i o n , Venus w i l l be 2 0 3
m i l l i o n km ( 1 2 6 m i l l i o n m i . ) f a r t h e r from E a r t h t h an a tO r b i t e r a r r i v a l .
D u r i n g t h e c r i t i c a l two-hour p e r io d o f a tm osphe r i ce n t r y by t h e Bus a nd f l i g h t s down t o t h e s u r f a c e by t h ef o u r p r o b e s , b o t h t h e 6 4 - m ( 2 1 0 - f t . ) a n t e n n a s a t G o l d s t o n e ,C a l i f . , and a t C a nb e rr a , A u s t r a l i a , w i l l be used t o re-
ce iv e and reco rd Venus a tm osphere da ta , com ing i n s im ul -t a n e o u s l y f r o m a l l f i v e p ro be c r a f t .
The Deep Space N e t w o r k w i t h f a c i l i t i e s l o c a t e d a ta p p r o x im a t e l y 1 2 0- de g re e i n t e r v a l s a r ou n d t h e E a r t h , w i l ls u p p o r t t h e P i o n e e r Venus s p a c e c r a f t . The p r i m a r y m i s s i o n
o f t h e O r b i t e r i s 1 5 , m o n t h s : s i x months i n t r a n s i t and e i g h tmonths i n o r b i t . A s t h e O r b i t e r and M ul t ip robe " s e t " a tone s t a t i o n due t o t h e E a r t h ' s r o t a t i o n , t he y w i l l r i s e
a t t h e n e x t o ne .
The D S N , o p e r a t e d by t h e J e t P r o p u l s i o n L a b o r a to r y( J P L ) , P as ad en a, C a l i f . , h a s s i x 26-ml ( 8 5 - f t . ) p a r a b o l i c - r e -f l e c t o r d i s h a n t en n as , t w o a t G old sto ne , i n C a l i f o r n i a ' sMojave D e s e r t ; two a t Madrid , S pai n and t w o a t C anber ra .
T h e r e a re a l s o t h r e e 64-m ( 2 1 0 - f t . ) a n t e n n a s , o ne e a c h a t Gold-s tone , M adr id and C anber ra .
R ad io s c i e n c e e x p e r i m e n t e r s w i l l e s t im a te wind speeds
a nd d i r e c t i o n s i n t h e Venus a t m os p h er e by c om pu ti nq t h et h e e x a c t f l i g h t pa t h s of t h e f o u r p ro be s u s in g D S N d a t a .I n a d d i t i o n t o t h e G ol ds to n e and C an b er ra s t a t i o n s , two
NASA S T D N s t a t i o n s a t Guam an d S a n t i a g o , C h i l e , w i l l sup-p o r t t h i s e f f o r t . Radio i n t e r f e r o m e t r y i n a t r i a n g u l a t i o np r o c e s s w i l l be u se d i n t h i s c o mp u ta ti on . (See M u l t i p r o b eExper im en t s - R a d i o S c i e n c e . )
At all times, incoming telemetry data from the spacecraftis formatted at DSN stations for high-speed transmission toAmes computers. These computers will check for unexpected or
critical changes in data and provide information for analysisby specialists in the spacecraft, experiments and ground sys-
tem. Their analyses will be used for spacecraft control. Out-
going commands are verified by Ames computers and sent to DSN
stations where they are reverified by computer and then trans-mitted. Navigation data and trajectory computations for the
Pioneer spacecraft are furnished by JPL's Navigation System
Section under contract to Ames. They do computer analysis ofDSN Doppler and range tracking information to provide spacecraft
trajectories for calculation of Venus orbit and planetary
targeting.
For Pioneer Venus, the DSN has made a number of special
modifications. Added receivers are needed to handle the fivedifferent data streams at once of the four probes and Bus.
Special wideband recorders are required to cope with the largefrequency shifts which will happen with the changes in velocity
at entry -- and atmospheric effects on signal propagation asthe probes descend to Venus' surface. To save all of the one-
change-only data, due to variances outside the predicted range
of frequency changes, the DSN has provided special equipment toautomatically tune the receivers to the signal transmitted by
each probe.
Incoming telemetry is formatted at DSN stations for trans-
mission via NASA Communications System (NASCOM) high-speed cir-cuits to the pioneer Mission Computing Center (PMCC). There it
is processed to supply various types of real time display infor-
mation on spacecraft and instrument status.
In addition to use of telemetry for providing missionoperations and quick-look data, all telemetry will be processed
at the PMCC to provide data records for the individual experi-menters in the form of Experimenter Data Records. Provided to
principal investigators, it becomes the raw material for use by
them in producing mission findings.
For all of NASA's unmanned missions in deep space, the DSN
provides tracking information on course and direction of theflight, velocity and range from Earth. Its global stations
also receive engineering and science telemetry and send com-
mands. All communications links are in S-band frequency (though
Venus Orbiter occultation experiments are X-band carrier only).
No telemetry data are sent.
DSN stations relay spacecraft Doppler tracking to JPL. High
speed data links allow real time transmission of all data from
spacecraft directly to the PMCC at Ames. Throughout the mission,
scientific data recorded on magnetic tape will be sent from DSN