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Lunar SNAP Observatory

Lunar SNAP Observatory. Overview. What trade studies and modifications required to place SNAP on the moon? Trade Studies Location Northern latitudes South Pole Aitken basin crater Detectors require cooling ~30W Deployable radiator Cryopump

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Lunar SNAP Observatory

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  1. Lunar SNAP Observatory

  2. Overview What trade studies and modifications required to place SNAP on the moon? • Trade Studies • Location • Northern latitudes • South Pole Aitken basin crater • Detectors require cooling • ~30W • Deployable radiator • Cryopump • Nuclear –vs- solar power (batteries for 14-day hibernation) • Observatory modifications • Lander • Moving outer baffle and dish • Telescope (1/6g mirror mount) • Hexapod and rotational mount • Augers to dig into lunar surface • Dust considerations (power, stray light, contamination) • Mass and launch vehicle

  3. Communications • Distance 384,000km –vs- ~1,500,000km for L2 • Better link margin than L2 • Smaller dish • Communications at poles not continuous (LOS to Earth) • Increased memory (onboard storage for comm. Blackouts) • Located in crater at pole • No visibility to Earth • Possibility of Communication Relay Satellite in polar orbit • Antenna mounted above rim of crater (robotic or manned deployment) could have continuous view of Earth • Maximum latitude for continuous view of Earth: 60º • Recommendations • Land on Earth-facing side of moon • Do not land inside crater

  4. Power • Assume no nearby colony with solar farm or nuclear power, design like a robotic space mission. • Solar power, arrays currently sized for 5-600W mission at L2 • Dust contamination? • Huge (100,000WHr/week) batteries needed for shadow operations (Hibernation a better option) • Hibernation (14-day) does not allow mission requirement of 4-day cadence without 2nd lander • Nuclear RTG (radioisotope thermoelectric generators) • Need two 300W RTGs of the Cassini/PNH/Galileo/Ulysses class (56kg each) • Treaty issues? • Conclusion: RTG Power necessary

  5. Detector Cooling • Detectors require cooling • ~30W at 140K • Cryopump • Additional power required • ~6W cryopumps available • ~50W (at 140K) less common (Creare, flown to Hubble, requires 900W) • Very large and massive • Introduces vibration into optical system (would require isolation) • Cryopumps are limited lifetime items • Liquid helium dewar: limited lifetime item • Deployable radiator (see next slide) • Flexible cooling links • Deployed thermal insulation to shield radiator from lunar thermal radiation • Observatory body-mounted mirror to increase effective solid angle of cold sky • Must orient radiator closely anti-sun to avoid sunlight (precludes “mowing the lawn”), could possibly increase size of body-mounted mirror • Prefer deployable radiator

  6. Lunar SNAP Observatory

  7. Location • Northern or Southern landing zones preclude opposite hemisphere survey • Option to place observatory at lunar South Pole Aitken Basin crater • Pros: • Nearly continuous sunlight at crater rim • Continuous darkness inside crater • Cons: • Power supply and communication dish (or relay satellite) external to crater • Difficult to land observatory on sloped crater • Options for Northern latitude placement • Pros: • Axial tilt of moon: 1.543º to Ecliptic, therefore >52º latitude allows continuous view of circular cap of radius 20º around North Ecliptic Pole, 30º lunar limb avoidance (stray light) • RTG power unaffected • Con: • 14-day monthly shadow complicates thermal design • Latitude <60 For continuous view of Earth (com-link) • Recommendation: Locate on Earth-facing longitudes, in Northern hemisphere between latitudes 52º and 60º

  8. Additional Considerations • Augers required to dig into regolith • Firm foundation for observatory • May not be necessary with tip-tilt pointing control system • Dust • Photoelectric effect charges dust during day, suspended by Coulomb repulsion, reverse at night (bias voltage considered on mirror coatings) • Prospector 3 camera cut off by Apollo 12 astronauts and returned to earth. It was found to be functional despite dust. • Dust seen from orbit with unaided eye by Apollo astronauts • Conclusion: • Dust probably more of a stray light issue than functional • Drift scan using fixed observatory and rotation of moon does not meet 4-day cadence requirements for SNe survey Prospector 3

  9. Additional Equipment • Rotating outer baffle sunshade: 100kg extra • Telescope • 18pt. wiffletree mount, good to 30º tilt relative to surface normal: 50kg • Used on GBOs and SOFIA, where variable orientation mirror is required • Gravity sag 6x smaller than on Earth to begin with • Trade on “Mowing the Lawn” observation scheme: • Eliminate entirely and use filter wheel: 20kg • Larger mirror for radiator would allow observatory rotation similar to currently planned L2 “Mowing the Lawn” operations: 50kg • Hexapod rotates telescope, rotation mount rotates baffle/radiator (long flexible links to radiator) • Hexapod and rotational mount: 150kg • 0.1-0.2um accuracy for hexapod mounts, roughly 30º pitch/yaw motion • Inboard hexapod mounts, ±250mm stroke (very large) • Would require hybrid long-stroke, precision legs • Caged during trip to moon • Hinged cover: 45kg • Deployable radiator: 100kg • 2x RTG (56kg*2=112kg) • Descent stage equipment, structure and avionics (dry): 800kg • Lunar mount/augers: 150kg • Total extra dry mass for lunar observatory: 1427kg

  10. Delta-V/Fuel Requirements • Observatory landing on moon mass: 3427kg • Bi-prop fuel required to land: 2000kg • Mass of Descent Module: • 5400kg • Orbital insertion delta-V=854m/s: 60% additional fuel/propulsion • 3300kg • system mass launched toward moon by launch vehicle • 8700kg total • Only one current LV option, see next slide…

  11. Launch Vehicle Capacity to Moon 2860 kg 4765 kg 9600 kg 6550 kg 4240 kg

  12. Conclusions • Location • Land on Earth side of moon, between 52º & 60º latitude • Observatory modification • Power: two 300W RTGs • Lander, injection stage and fuel • Rotating outer baffle • Gimbaled dish • Telescope modification: wiffletree • Hexapod and rotational mount for telescope • Detector cooling via deployable radiator • Hinged door • Open issues • Dust (stray light from suspended dust, and mirror contamination) • Stable mounting on surface requires study (are augers required?) • Political implications of RTG use on lunar surface • Implementation of “Mowing the Lawn” scheme (filter wheel, larger radiator mirror, flex links) • Significant mass increase required for lunar orbit injection and descent to surface (2000kg observatory increases in mass to 8700kg) • Mass requirements necessitate EELV (Delta-IV Heavy)

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