ExoplanetSat: A Nanosatellite Space Telescope for Detecting Transiting Exoplanets CubeSat Developers’ Workshop April 20-22, 2011 San Luis Obispo, CA Matthew W. Smith 1 ([email protected]), Sara Seager 1 , Christopher M. Pong 1 , Sungyung Lim 2 , Matthew W. Knutson 1 , Timothy C. Henderson 2 , Joel N. Villaseñor 1 , Nicholas K. Borer 2 , David W. Miller 1 , Shawn Murphy 2 1 2 GODDARD SPACE FLIGHT CENTER
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ExoplanetSat:A Nanosatellite Space Telescope for Detecting Transiting Exoplanets
Sara Seager1, Christopher M. Pong1, Sungyung Lim2,
Matthew W. Knutson1, Timothy C. Henderson2, Joel N. Villaseñor1,
Nicholas K. Borer2, David W. Miller1, Shawn Murphy2
1 2
GODDARD
SPACE FLIGHT CENTER
Outline
• Science
• Concept of operations
• Long term vision
• Spacecraft design
– Ongoing trades
– Payload
– Attitude determination and control
• Hardware test results
• Future Work
22011 CubeSat Developers' Workshop
Exoplanet Science & Motivation
• High-level goal: Search the brightest
Sun-like stars for transiting Earth-size
planets
– Constellation of satellites
– Bright star search enables follow up
characterization studies (vs. Kepler)
• Prototype goal: 3U CubeSat capable
of 10 ppm phototometry (7σ detection
of Earth-sized planets) for bright
(0 ≤ V ≤ 6) Sun-like stars
• Why CubeSat form factor for transit searches of bright stars?
2011 CubeSat Developers' Workshop 3
Bright stars are
spread across
the sky
Need many,
dedicated
telescopes
Low cost per
spacecraft,
frequent
launches 3U CubeSat
form factor
Concept of Operation
2011 CubeSat Developers' Workshop 4
Orbit Night:
Hold attitude
Observe target star
Orbit Day:
Hold attitude
Charge batteries
Acquisition:
Detumble satellite
Initialize attitude estimate
Orbit Insertion:
Deployment
from P-POD
Slew:
Point solar arrays to sun
Measurement:
Time series of stellar fluxSlew:
Point optics to target star
Bro
wn
et
al. 2
00
1
Long-Term Vision
• Fleet of small satellites (3U CubeSats, 6U CubeSats, EPSA-class)
in low-Earth orbit, collectively monitoring hundreds of Sun-like stars
2011 CubeSat Developers' Workshop 5
Phase 1:
• Single prototype
• Tech demonstration
(arsecond-level pointing)
• Observe alpha centauri
(brightest Sun-like star)
• Search for transits of
known super Earth
exoplanets
Phase 3:
• Full planet detection survey
• Seek 95% confidence of 3+
planet detections
• Observe bright stars to V = 8
• Observe 250 stars
• Expanded fleet
Phase 2:
• Add 3U models + 6U
models with 120 mm
apertures
• Observe 20 brightest
stars for Earth-sized
transits
• 10-15 spacecraft needed
Spacecraft Design
6
Reaction wheels
+ Torque coils
Avionics
• MAI-200
Not shown: patch antennas, wiring
2011 CubeSat Developers' Workshop
• Lens
• Piezo stage
• CCD
• CMOS
imagers
• Baffle
(not shown)
• Flight processor
• CCD drive electronics
• Piezo stage controllers
• Comm. transceiver
• MEMS gyros
• Electrical power subsystem (EPS) + batteries
Payload
Solar array
(35 W, peak BOL)
Ongoing Trades
• Mass
– Currently at approximately 5.5 kg
• Volume
– Off-the-shelf vs. custom lens
– Evaluating board layout (PC-104 cards vs. custom PCBs)
• Detector architecture
– Number and placement of CMOS imagers for star tracking
– Science detector selection
2011 CubeSat Developers' Workshop 7
vs.
Payload
• Variation within CCD pixel requires arcsecond-level optical pointing
• Combined star tracker (CMOS imagers) and science telescope (CCD)
– CCD: Defocused, ≥1 s integration time to collect many photons
– CMOS: In focus, ≤100 ms integration time to provide frequent updates to estimator
2011 CubeSat Developers' Workshop 8
CCD
CMOS
(x6)
+Y
+X
Lens
Lens mount
±50 µm XY
Piezoelectric stage
Piterman & Ninkov, 2002
Single pixel sensitivity map
Required optical
pointing:
arcsecond -level
Focal
plane
Focal plane
Attitude Determination & Control
• Two-stage pointing control
1. Coarse pointing:
Reaction wheels (< 120 arcsec 3σ)
2. Fine pointing:
Piezoelectric stage (5-10 arcsec 3σ)
• Simulation
results
2011 CubeSat Developers' Workshop 9
Satellite
Piezo.Stage
CMOS
Cap. Sensor
RW
RWControl
PiezoControl
Extended Kalman
Filter
CMOSProcessing
CCD
12 Hz Sampling
> 100 Hz Sampling
4 Hz SamplingActuators
Sensors
Software< 1 Hz Sampling
-80 -60 -40 -20 0 20 40 60 80-80
-60
-40
-20
0
20
40
60
80
X Position [arcsec]
Y P
ositio
n [
arc
sec]
-80 -60 -40 -20 0 20 40 60 80-80
-60
-40
-20
0
20
40
60
80
X Position [arcsec]
Y P
ositio
n [
arc
sec]
Star Tracking
Coarse
pointing
(no stage)
Fine
pointing
(with stage)
ADCS Testing
• Hardware in-the-loop test
– Successful proof-of-concept demonstration of fine
pointing stage: 2.3 arcseconds (3σ)
– Inject simulated residual pointing errors from
reaction wheels using star field emulator
– Correct for pointing errors on spacecraft emulator
with lens, detector, piezoelectric stage
2011 CubeSat Developers' Workshop 10
Star field emulator
Spacecraft emulator
Computer
-40 -30 -20 -10 0 10 20 30 40-40
-30
-20
-10
0
10
20
30
40
X Position [arcsec]
Y P
ositio
n [
arc
sec]
-40 -30 -20 -10 0 10 20 30 40-40
-30
-20
-10
0
10
20
30
40
X Position [arcsec]Y
Positio
n [
arc
sec]
Future Path
• ADCS hardware-in-the-loop test bed
– Spherical air bearing
– Two-stage control functional demo
(piezo stage + reaction wheels in the loop)
• Payload
– Intrapixel sensitivity measurements
– Mature science data processing pipeline
• Avionics development
– FPGA + Microcontroller architecture
• “Bus” subsystems currently at varying levels
of maturity
– Power – Comm
– Structure – Thermal
• Environmental testing at Draper, MIT, NASA GSFC
• Goal: launch in 2012-13 time frame
– Selected under NASA CubeSat Launch Initiative in January, 2011
2011 CubeSat Developers' Workshop 11
MIT 3DOF
spherical
air bearing
test stand
Conclusion
• ExoplanetSat will combine the low-cost CubeSat
platform with two-stage attitude control to detect Earth-
sized planets around the brightest stars
• The 3U prototype is under development with a potential
launch date through the NASA CubeSat Launch Initiative
• Key engineering breakthrough: very high precision
pointing (arcsecond-level) in a CubeSat
• ExoplanetSat initiates the graduated growth of a
modular, extensible constellation, with the final phase
being many satellites surveying bright stars for other
Earths
2011 CubeSat Developers' Workshop 12
Acknowledgements
• NASA Jet Propulsion Laboratory
– Dr. Wes Traub
– Strategic University Research Partnerships Program (SURP)
• Lincoln Laboratory, Advanced Imaging Technology Group
– Dr. Vyshi Suntharalingham
– Dr. Barry Burke
• NASA Goddard Space Flight Center
– Dr. Stephen Rinehart
• MIT
– Students of 16.83x / 12.43x
– Department of Aeronautics and Astronautics
– Dr. George Ricker
2011 CubeSat Developers' Workshop 13
Some Relevant Literature
M. W. Smith, et al., “ExoplanetSat: detecting transiting exoplanets using a low-cost CubeSat platform,” Proc. SPIE, Vol. 7731 (2010).
C. M. Pong, et al., “Achieving high-precision pointing on ExoplanetSat: Initial feasibility analysis,” Proc. SPIE, Vol. 7731 (2010).
C. M. Pong et al., “One-arcsecond line-of-sight pointing control on ExoplanetSat, a three-unit CubeSat,” Proc. Am. Astron. Soc. GNC Conference, 11-035 (2011)
A. Piterman & Z. Ninkov, “Subpixel sensitivity maps for a back-illuminated charge-coupled device and the effects of nonuniform response on measurement accuracy,” Opt. Eng. 41(6) 1192-1202 (2002).
D. G. Koch, et al., “Kepler Mission Design, Realized Photometric Performance, and Early Science”, ApJ L. 713:L79-L86 (2010).
G. Walker, et al., “The MOST Astroseismology Mission: Ultraprecise Photometry from Space”, Pub. Astron. Soc. Pac. 115:1023-1035 (2003).
N. C. Deschamps, et al., “The BRITE space telescope: Using a nanosatellite constellation to measure stellar variability in the most luminous stars”, Acta Astronautica 65:643-650 (2009).
T. M. Brown, et al., “Hubble Space Telescope Time-Series Photometry of the Transiting Planet of HD 20945”, ApJ 552: 699-709 (2001).