Bits and Pieces. Spacecraft Systems Propulsion –Already discussed Communications Science instruments Power.

Bits and Pieces

Spacecraft Systems• Propulsion

– Already discussed

• Communications• Science instruments• Power

Communications

• A number of issues:– Limited power– Large distances– Reception– Multiplexing

Communications

• Typical spacecraft transmission power ~20 W

• Limited power solved in two ways– Large receiving stations– Directional microwaves

• Round-the-clock communication possible by the DSN

Communications

• How about at the spacecraft?– Cannot have huge antennae - typically 5m

diameter– High transmission power from Earth– Highly sensitive amplifiers, narrow band-

pass, phase locking, low data rates all used

– See “Basics of Space Flight Chs. 10, 11

Communications

• Two principal types of spacecraft antennae:– High gain antennae provide primary

communications• Highly directional, high data rates possible

– Low gain antennae provide wide angle coverage at the expense of gain

• Low pointing accuracy needed, hence can be used for initial contact or in the event of problems. Low data rates only.

Communications

• Spacecraft receivers/transmitters– Many spacecraft use the “S” or “X” bands (~ 2

and 5 GHz respectively)• See “Basics of Spaceflight” p. 101

– DS1 testing a “Ka” band transponder (~20 GHz)

• Advantages; smaller, more directional (hence less power), less suceptible to poor ground station conditions - e.g., bad weather

Instrumentation

• Detectors

• Remote sensing

• Other systems– Basics of Space Flight Chapters 11, 12, 13

Detectors

• Charged particle detectors– Measure composition and distribution of

interplanetary medium

• Plasma detectors– Measure interactions of solar wind with

planetary magnetic fields

• Dust detectors

• Magnetometers

Remote Sensing

• Imagers

• Spectrometers– Remotely measure compositions

• Polarimeters– Determine the size, composition and

structures of particles in, for example, planetary rings

Other Systems

• Data recording– Record data for later playback– Tape recorders used, now being replaced by

high capacity solid-state memories

• Fault protection– “Default” procedures to re-establish contact

with Earth, etc. if something goes wrong– Redundancy - Duplication of important

systems

Power• Typical spacecraft (e.g., Voyager, Galileo etc.) require 0.3-2.5 kW, over possibly decades!• Two currently available methods for long-term power

– Photovoltaic cells (solar panels)– Radioisotope Thermal Generators (RTGs)

Power

• Solar Panels– Utilise photovoltaic effect across a

semiconductor junction– Usually gallium arsenide or silicon

http://www.iclei.org/efacts/photovol.htm

n-type

p-type

Power

• At 1 AU, silicon solar panels can provide 0.04 A/cm2 at 0.25 V per cell. GaAs is more efficient.

• Solar power can, in practice, be used out to the orbit of Mars.

• Output degrades by about 2% per year due to radiation damage - faster if there is high solar activity!

Power

• Radioisotope Thermal Generators (RTGs)– Use thermoelectric effect– Heat provided by decay of radioactive isotopes, usually Pu-238

Pu-238R

adiator

n

p

Power

• Special issues relating to RTGs– Safety

• They cannot “explode”• Design ensures RTG units remain intact, even after re-entry and impact in the event of an accident

• PuO2 in insoluable, ceramic form

• Get the launch right!• http://www.jpl.nasa.gov/cassini/rtg/

Power• Typical RTG contains 11 kg PuO2 fuel, producing about 300 W of electricity from about 400 W of heat.

• Total mass about 60 kg.• Decay rate of about 1-2% per year

– e.g., Voyager RTGs provided 470 W at launch (1977), now provide 330 W• Probably still good for at least another 20 years

What Next?

• “DS 1”– Launched October 1998– Test of new technologies, for example...

• ion engine• autonomous navigation and operations• Ka band transponder

– Asteroid and comet encounters– Solar wind studies– http://nmp.jpl.nasa.gov/ds1/

What Next?

• Continued Mars campaign– Launches at each opportunity

• 2001 (orbiter)• 2003 (orbiter and rover)• 2005… (orbiters, landers, rovers, sample

return…)

– Failure of the 1998/99 missions raised a few questions

– http://mars.jpl.nasa.gov/

What Next?

• “Stardust” comet and interplanetary material sample return– Launched Feb. 1999– Encounter with comet Wild 2 in Jan. 2004– Sample return 2006– http://stardust.jpl.nasa.gov/