Design of a Space-Based Laser Guide Star Mission to Enable Ground and Space Telescope Observations of Faint Objects
Jim Clark ([email protected]), Greg Allan, Kerri Cahoy, Yinzi Xin Massachusetts Institute of Technology Ewan Douglas, Jennifer Lumbres, Jared Males
University of Arizona 1 Overview
● Motivation: Imaging faint objects
● Background: Why satellite LGS?
● Approach and results: ○ Spacecraft design and L2 mission operations ○ Earth-orbiting pathfinder ○ Optical testbed
● Summary and future work
2 Motivation: imaging faint objects
● Seeing dim things near bright things: ○ Exoplanets ○ Asteroids ○ Space situational awareness
● Enabling technologies: coronography and adaptive optics ○ DeMi will demonstrate the https://en.wikipedia.org/wiki/File:Deformable_mirror_correction.svg AO actuators, but still need a very bright reference star for faint objects 3 Background: why satellite LGS?
● LUVOIR segment stability must be ~picometers for exoplanet imaging ○ Hard/expensive/impossible(?) to build
https://asd.gsfc.nasa.gov/luvoir/design/ 4 Background: Why satellite LGS?
● LUVOIR segment stability must be ~picometers for exoplanet imaging ○ Hard/expensive/impossible(?) to build
● Not many natural guide stars are bright enough to support segment control ○ And are not necessarily near targets
5 Background: Why satellite LGS?
● LUVOIR segment stability must be ~picometers for exoplanet imaging ○ Hard/expensive/impossible(?) to build
● Not many natural guide stars are bright enough to support segment control ○ And are not necessarily near targets
● A formation-flying CubeSat with laser transmitter is bright enough…optical requirements established in Douglas et al. 2019 [1]. 6 One possible architecture [1]
7 Approach: Spacecraft design fits in 12U
Propulsion Power
34 cm Laser
Avionics
ADCS
W Kammerer and 22 cm 22 cm J Clark (MIT) 8 Results: How many LGSs?
● Electric propulsion minimizes the needed LGS constellation size.
● Best performer so far: Busek BIT-3
● Still trading: fly close or far from observatory?
9 Results: How precise?
10 Results: Negative impact mitigation
● Propellant plume will move aside ● No direct sun reflections ● Stay behind the mask ● Just in case, L2 is unstable...
11 Approach: Pathfinder mission
● GEO to ground lasercom is TRL 9
● But, few telescopes track LEO
● Vmag of lasercom terminals is -5 12 Results: Pathfinder coverage
13 Approach: Testbed
● Will test impact of control loop (bandwidth, accuracy, jitter, etc.) on coronagraph contrast.
J. Lumbres (U. of A.)
14 Results: Testbed
● Characterizing the ZWFS:
J. Lumbres (U. of A.)
15 Ongoing and future work
● Testbed: model validation ● Engineering: how LGS complements onboard metrology ● Refine costs and risks ○ Improved science yield per dollar ○ Cost value of relaxing requirements, complexity
16 Summary
● The space science community can leverage lasercom technology for planetary exploration, astrophysics, and SSA
● Lasercom is TRL-9 on bigsats and CubeSats; we strongly encourage Earth- orbit technology demonstrations (also useful for Earth) and then incorporation into future solar system exploration missions
17 References
1. Douglas, E. S., Males, J. R., Clark, J., Guyon, O., Lumbres, J., Marlow, W., and Cahoy, K. L., “Laser Guide Star for Large Segmented-aperture Space Telescopes. I. Implications for Terrestrial Exoplanet Detection and Observatory Stability,” The Astronomical Journal, vol. 157, Jan. 2019, p. 36. 2. Stark, C. C., Roberge, A., Mandell, A., Clampin, M., Domagal-Goldman, S. D., McElwain, M. W., and Stapelfeldt, K. R., “LOWER LIMITS ON APERTURE SIZE FOR AN EXOEARTH DETECTING CORONAGRAPHIC MISSION,” The Astrophysical Journal, vol. 808, Jul. 2015, p. 149. 3. Overbye, D., “NASA Again Delays Launch of Troubled Webb Telescope; Cost Estimate Rises to $9.7 Billion,” The New York Times, Jun. 2018.
18 Backup: Abstract as Submitted
Large segmented aperture telescopes are planned for future space observatories such as LUVOIR (Large UV Optical Infrared Surveyor) to enable the resolution and contrast necessary to measure faint astrophysical objects, such as directly imaging Earth-like exoplanets. Precision surface control of these complex large optical systems to ~10 picometers required for high contrast coronagraphy is a challenge as the telescopes are planned with over one hundred meter-sized segments. Our simulations show that imaging a star of -1 magnitude or brighter with a Zernike wavefront sensor should relax the segment stability requirements by factors between 10 and 50 depending on the wavefront control strategy. Because stars this bright are rare, fielding a laser guide star (LGS) on spacecraft flying in formation with the large observatory will allow the telescope to be built to relaxed tolerances while still achieving the required contrast during scientific observations.
In this work, we present the detailed design of a small Laser Guide Star satellite for use paired with either a large space observatory or ground telescope. Using the CubeSat form factor, while the laser guidestar payload can be accommodated in 1.5U, we develop a 2U system to comfortably accommodate the propulsion systems necessary to enable the orbital motion required for the guide star to formation fly synchronously along with the telescope boresight for as many targets as possible, and with the precision required to enable the wavefront sensing and control during observation. We describe a design reference mission (DRM) for deploying 18 Laser Guide Stars to L2 to assist LUVOIR. The L2 DRM covers over 250 exoplanet target systems with 5 or more revisits to each system over a 5 year mission using sixteen 12U CubeSats. We also present a design reference mission for three laser guide star satellites to geostationary orbit for use with 6.5+ meter ground telescopes to look at HD 50281, HD 180617, and other near-equatorial targets. We also present simulations on the maximum level of thruster noise permitted during the observations to maintain precision formation flying with the observatories.
Using CubeSat laser guide stars can decrease the complexity and cost of future large segmented aperture space telescopes, as well as greatly increase the science yield of these missions. 19 Backup: Satellite Laser Guide Star Example: Use existing GEO ComSat Terminal TDP1 on Alphasat
Parameter Nominal Power Max. Power
Transmit power (W) 0.1 5
Wavelength (nm) 1064
Beam divergence (urad) 7.1 (half-angle) (1.5 arcsec)
Range (km) 35,786
Spot diameter (m) 508
Throughput 10%
Received flux (W/m2) 4.9 x 10-8 2.5 x 10-6
Received flux (phot/s/cm2) 2.6 x 107 1.3 x 109 Most sodium guide star systems are Received magnitude -4.6 -8.9 mag +9 or +10 Required mag < 8
Margin factor 120,000 5,900,000 Backup: GEO Commsats visible from US telescopes
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