Concepts for fully-steerable and survey VLSTs at a manned base at

the Moon’s south pole

Roger Angel

University of Arizona

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

Telescopes over past 50 years

• Space– from sounding rockets to great observatories– Each successive mission is uniquely powerful– Typically scientific and failure lifetime ≤10 yr – Hubble exception

• Ground– Not much size increase – Huge increase in power by better detectors,

multiplexing and adaptive optics– Long-lived, rejuvenation on ~ 5-10 year timescale as

science goals and technology change

Lifetimes of current generation space observatories

observatory spectrum dates Life orbit Lifetime set by

Compton gamma 1991-2000 9 LEO Gyro failure

Hubble optical 1990 – 2005? 15? LEO No more maintenance, gyro failure, orbit decay

Chandra x-ray 1999 – 2009? 10? elliptical Gyro failure?

Spitzer infrared 2003 - 2006 2.5 Drift away Cryogen exhaustion

Webb infrared 2011 - 2021 10 L2 Fuel to maintain L2 orbit

Next 50 years on ground

(you must put future space plans in this context)

Several telescopes in 20 – 100 m range• AO should mature to give all sky diffraction

limit at 0.5 m, especially if placed on• Antarctic plateau (L2 of ground)

– 25 m in Antarctica (Giant Magellan Telescope II) would rival JWST for spectroscopy

– 100 m in Antarctic for terrestrial exoplanets• detection at 10 m • spectroscopy in optical

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

10 100 1000 10000

1E-18

1E-17

1E-16

1E-15

1E-14

DCII

DCIII

DCI

CN

2 (

m.s-2

/3)

Height Above Ground Level (m)

MK

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

Thermal good in Antarctica, though 10 micron background 105 higher

GMT and JWST point source sensitivity for >2.5 m, 10, 105 sec (Angel, Lawrence and Storey, 2003 Backaskog conference)

Next 50 years in space, beyond JWST

• Unique space attributes are – No atmospheric absorption in UV, thermal IR– Pristine wavefront– No thermal emission if optics cold

• Large, cold telescopes will outperform ground in UV/ thermal IR

• Very big space telescopes with huge capital investment, like on ground telescopes should have multi decade lifetime, and be refitted every decade

Requirements for location

• Far from Earth, to avoid its thermal radiation (for thermal telescope)

• Accessible by astronauts as well as robots

Possible locations

• LEO– Easiest for astronaut access– Warm, thermal cycling, reduced duty cycle– Re-boosts needed to maintain orbit

• L2– Cold, all sky access with 50% duty cycle– Hardest for astronaut access– Expendable fuel needed to maintain orbit– Pointing gyros subject to failure

• Moon S pole– Cold, 100% duty cycle for 50% of sky– Astronauts nearby if base established– No expendables needed, no gyros– 3 x mass penalty if no established base

Requirements for longevity

• stable orbit – avoid LEO and L2• no gyros• no expendable cryogens• provision for occasional repair and upgrades by

astronauts• long 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 site

• Basics fine– All wavelengths accessible, vacuum of space– Orbit stable on billion year timescale– Telescopes can be “safed” for decade and then

brought back to service

• Moon’s spin axis 1.5 degrees from ecliptic pole– Sun moves around within 1.5 degrees of horizon– Very low temperatures by simple shielding of sunlight

3x mass penalty to descend from lunar orbit

• No air braking, as for Earth and Mars

• Requires rocket

• Apollo vehicle and fuel weighed 2½ times the payload mass delivered to surface

• 18.3 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 base

Why the pole?

• Frozen volatiles in permanently dark, cold craters• Ice can be recovered from regolith in craters• Ice converted to hydrogen/oxygen fuel by locally

produced by solar power• Cryo storage of fuel• reusable 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 regolith

• Install and maintain water extraction, photolysis and fuel storage equipment

• Maintain reusable rocket ferry• Maintain atmospheric conditioning equipment

– Grow 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 pole

Don’t need to be in crater to be cold• Locate on rim far enough from base to avoid dust• sun moves around horizon, simple aluminized

surrounding cylindrical screen will result in cooling to 40K or less– Lower screen to warm up for repairs

• Three UV/O/IR flavors considered here– Fully steerable 16 m– 20 m zenith pointing ultra-deep survey filled aperture– Zenith-pointing wide-field interferometer

Pierre Bely’s 1990 concept for 16 m lunar telescope

• Hexapod mount• 6 variable length legs pre-

manufactured on Earth• low mass• no heavy foundation or

bearing surfaces required• No gyros• Instruments shielded under

regolith igloo• Advantage over L2:

– No gyros, no fuel for orbit correction

– Astronauts nearby

star Ecliptic latitude

type V mag distance parallax notes

Alpha Cen -43 G2 0.0 747 K0 companion

Eps Eri -27.7 K2 3.7 310 Jovian planet?

Lacaille 9352 -27.5 M 1.5 7.3 304 single

Eps Indi -41.4 K5 4.7 276 Two T dwarf companions

Tau Ceti -24.8 G8 3.5 274 single

Kapteyn’s star -67.5 M1.5 8.8 255 single

AX Microscopium -21.9 M0 6.7 253 single

Gj 674 -23.6 M3 9.4 220 single

GJ 832 -32.47 M3 8.7 198 single

S pole lunar telescope sees same southern sky as Antarctic telescopes

Most terrestrial planet candidates < 5 pc visible from lunar S pole!

Zenith-pointing telescopes and interferometers

• Zenith traces out 3º degree diameter circle adjacent to LMC, over 18 year period

• Fixed telescope, optics to steer field 1.5º off-axis, 6 degree field accessible

• Could use optics panels from Earth, or liquid mirror

Ecliptic pole imaged in uv from moon by Apollo astronauts. 6 degree circle around ecliptic pole

20 m liquid mirror telescope on superconducting bearing

• Lunar liquid mirror telescope proposed by Borra ~ 1990

• Unique to Moon – needs gravity

• 2 rpm for 20 m focal length• Use with liquid of low vapor

pressure, vacuum deposited metallic coating (1-butene)

• 20 m aperture on same field 24 hr/day for a year

• This is the ultimate deep field• Imaging and multi-object

spectrograph

Eisenstein and Gillespie estimates for first stars in this deep field

• Lyman alpha flux to be about 1 nJy at z=25 and R=1000 for a 100 M star. 

• Scales linearly with the star's mass• equivalent photons for neutral helium can't pass through

the IGM• Individual stars should be imageable in HeII line at 1640 A• Flux 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 above

Spinning liquid mirrors on Earth limited to ~ 6 m by self-generated wind - no such size limit on Moon

Imaging zenith pointing interferometer

• Fizeau combination (wide field) will work well with no moving parts

• Moon rotation ideal for zenith observation

conclusions• For 20 m class space telescopes, longevity of

decades highly desirable• Possibility of astronaut access for assembly and

robot back-up also highly desirable• If a long term polar moon base is established,

then an observatory nearby makes a lot of sense• Locally fueled ferry to lunar orbit will remove

mass penalty• Long term base only tolerated if operating cost

<1/4 NASA budget (< $4B/yr), say 1 visit/year