Concepts for fully-steerable and survey VLSTs at a manned base at the Moons south pole

Roger Angel

University of ArizonaThe Science potential of a 10-30 m UV/Optical Space Telescope, STScI Feb 2004

Telescopes over past 50 years Spacefrom sounding rockets to great observatoriesEach successive mission is uniquely powerfulTypically scientific and failure lifetime 10 yr Hubble exceptionGroundNot much size increase Huge increase in power by better detectors, multiplexing and adaptive opticsLong-lived, rejuvenation on ~ 5-10 year timescale as science goals and technology change

Lifetimes of current generation space observatories

Next 50 years on ground(you must put future space plans in this context) Several telescopes in 20 100 m rangeAO should mature to give all sky diffraction limit at 0.5 mm, especially if placed onAntarctic plateau (L2 of ground)25 m in Antarctica (Giant Magellan Telescope II) would rival JWST for spectroscopy100 m in Antarctic for terrestrial exoplanets detection at 10 mm spectroscopy in optical

Figure 1. Atmospheric turbulence profiles projected for Dome C by Lawrence et al (2003).

AO will work well in Antarcticaatmospheric turbulence mostly at low altitude, unlike temperate sites

Thermal good in Antarctica, though 10 micron background 105 higherGMT and JWST point source sensitivity for l>2.5 mm, 10s, 105 sec (Angel, Lawrence and Storey, 2003 Backaskog conference)

Next 50 years in space, beyond JWSTUnique space attributes are No atmospheric absorption in UV, thermal IRPristine wavefrontNo thermal emission if optics cold Large, cold telescopes will outperform ground in UV/ thermal IRVery big space telescopes with huge capital investment, like on ground telescopes should have multi decade lifetime, and be refitted every decade

Requirements for locationFar from Earth, to avoid its thermal radiation (for thermal telescope)Accessible by astronauts as well as robots

Possible locationsLEOEasiest for astronaut accessWarm, thermal cycling, reduced duty cycleRe-boosts needed to maintain orbitL2Cold, all sky access with 50% duty cycleHardest for astronaut accessExpendable fuel needed to maintain orbitPointing gyros subject to failureMoon S poleCold, 100% duty cycle for 50% of skyAstronauts nearby if base establishedNo expendables needed, no gyros3 x mass penalty if no established base

Requirements for longevitystable orbit avoid LEO and L2no gyrosno expendable cryogensprovision for occasional repair and upgrades by astronautslong life against radiation damage

If our telescope is a million miles away, we may have trouble getting astronauts to visit. But if they are at a long term moon base, we should think about locating our telescope nearby

The moon as a telescope siteBasics fineAll wavelengths accessible, vacuum of spaceOrbit stable on billion year timescaleTelescopes can be safed for decade and then brought back to serviceMoons spin axis 1.5 degrees from ecliptic poleSun moves around within 1.5 degrees of horizonVery low temperatures by simple shielding of sunlight

3x mass penalty to descend from lunar orbitNo air braking, as for Earth and MarsRequires rocketApollo vehicle and fuel weighed 2 times the payload mass delivered to surface18.3 tons rocket carried a payload of 7 tons (the fueled ascent stage and crew)

Telescope on moon makes sense if there is a long term, manned polar baseWhy the pole? Frozen volatiles in permanently dark, cold cratersIce can be recovered from regolith in cratersIce converted to hydrogen/oxygen fuel by locally produced by solar powerCryo storage of fuelreusable ferry vehicle from surface to lunar orbit powered by local fuel - removes mass penalty

Polar base astronauts will need range of skills:Install and maintain mining gear. Need to get > 0.5 km down 45 degree slope below crater rim to get permanently-shadowed ice-containing regolithInstall and maintain water extraction, photolysis and fuel storage equipmentMaintain reusable rocket ferryMaintain atmospheric conditioning equipmentGrow plants using local water

Given these capabilities, assembling and maintaining 20 m telescopes would be present little additional challenge

Ice at south pole as measured by neutron flux (Lunar prospector)

Lunar Surveyor 1967 image of Shackleton crater at south pole (18 km diameter)

Sunshine available for nearly continuous solar power. Clementine map for lunar winter

Surrogate crater Dionysius * typical18 km crater but illuminated

* sharp rim

Dionysius rim close-up(for Shackleton ice >500 m from rim)

Typical cross section of 18 km crater

Telescopes at S poleDont need to be in crater to be coldLocate on rim far enough from base to avoid dustsun moves around horizon, simple aluminized surrounding cylindrical screen will result in cooling to 40K or lessLower screen to warm up for repairs

Three UV/O/IR flavors considered hereFully steerable 16 m20 m zenith pointing ultra-deep survey filled apertureZenith-pointing wide-field interferometer

Pierre Belys 1990 concept for 16 m lunar telescopeHexapod mount6 variable length legs pre-manufactured on Earthlow massno heavy foundation or bearing surfaces requiredNo gyrosInstruments shielded under regolith iglooAdvantage over L2:No gyros, no fuel for orbit correctionAstronauts nearby

S pole lunar telescope sees same southern sky as Antarctic telescopes Most terrestrial planet candidates < 5 pc visible from lunar S pole!

starEcliptic latitudetypeV magdistanceparallaxnotesAlpha Cen-43G20.0747K0 companionEps Eri-27.7K23.7310Jovian planet?Lacaille 9352-27.5M 1.57.3304singleEps Indi-41.4K54.7276Two T dwarf companionsTau Ceti-24.8G83.5274singleKapteyns star-67.5M1.58.8255singleAX Microscopium-21.9M06.7253singleGj 674-23.6M39.4220singleGJ 832-32.47M38.7198single

Zenith-pointing telescopes and interferometersZenith traces out 3 degree diameter circle adjacent to LMC, over 18 year periodFixed telescope, optics to steer field 1.5 off-axis, 6 degree field accessibleCould use optics panels from Earth, or liquid mirrorEcliptic pole imaged in uv from moon by Apollo astronauts. 6 degree circle around ecliptic pole

20 m liquid mirror telescope on superconducting bearingLunar liquid mirror telescope proposed by Borra ~ 1990Unique to Moon needs gravity2 rpm for 20 m focal lengthUse with liquid of low vapor pressure, vacuum deposited metallic coating (1-butene)

20 m aperture on same field 24 hr/day for a yearThis is the ultimate deep fieldImaging and multi-object spectrograph

Eisenstein and Gillespie estimates for first stars in this deep fieldLyman alpha flux to be about 1 nJy at z=25 and R=1000 for a 100 M star. Scales linearly with the star's massequivalent photons for neutral helium can't pass through the IGMIndividual stars should be imageable in HeII line at 1640 AFlux about 10% of the Lyman alpha line (i.e. 0.1 nJy)

6 m, zenith pointing spinning mercury mirror, by Paul Hickson. Comet image from earlier prototype aboveSpinning liquid mirrors on Earth limited to ~ 6 m by self-generated wind - no such size limit on Moon

Imaging zenith pointing interferometerFizeau combination (wide field) will work well with no moving partsMoon rotation ideal for zenith observation

conclusionsFor 20 m class space telescopes, longevity of decades highly desirablePossibility of astronaut access for assembly and robot back-up also highly desirableIf a long term polar moon base is established, then an observatory nearby makes a lot of senseLocally fueled ferry to lunar orbit will remove mass penaltyLong term base only tolerated if operating cost