DFL 11/29/2006Astrophysics & Return to Moon0 Dirt, Gravity, and Lunar- Based Telescopes: Dan Lester University of Texas Astrophysics Enabled by The Return

DFL 11/29/2006 Astrophysics & Return to Moon 1

Dirt, Gravity, and Lunar-Based Telescopes:

Dan LesterUniversity of Texas

Astrophysics Enabled byThe Return to the Moon

Space Telescope Science Center 29 November 2006

The Value Proposition for Astronomy

?


Genesis of the lunar astronomy vision

“So many factors favor the Moon as a site for future large-scale space astronomy that planning an observatory there deserves the closest attention in the years ahead.”

William Tifft, Steward ObservatoryAeronautics and Astronautics December 1966

The world in 1966: Earth-based sites (1” seeing) emulsions , photomultipliers post-Gemini, pre-Apollo OAO-2 (point/track 1’/1”)

and also …

we were actively headed to the Moon

OAO-2

Hale 5m


Advantages of the Moon for astronomy c.1966

• Vacuum (compared to Earth)multiwavelength not seeing-limited

• Radiation isolation (compared to Earth orbit)no damage to sensitive emulsions

• Stable surface (compared to free space) proven tracking technologies no human perturbations

• Thermal control (compared to low Earth orbit)long diurnal cycle & lunar polar craters

• Accessibility (if near an outpost)service, maintenance

This vision was smart, both scientifically and technologically!


Lunar telescopes were a bold answer to our needs!

Innovative optical, mechanical, thermal, and civil engineering.


But then something changed …

… we came to understand that telescopes in free-spacecould meet our needs, offering advantages previously seen only for the lunar surface.


HST gave technology leap - free-space potential

• demonstrated precise pointing and tracking (0.003”)

• demonstrated widefield diffraction-limited performance

• demonstrated precise thermal control in tough environment

• demonstrated high observational efficiency

• demonstrated long timescale survivability in space

and, in particular

• demonstrated accessibility for servicing and maintenance

Space performance with ground-based reconfigurability

All this with what is now 25 year old technology …


Strategic Planning 101 (part 1)

• Can we deploy and service telescopes on the lunar surface?

Yes, but possible doesn’t make optimal.

• Does lunar surface offer uniquely enablingopportunities to priority astronomical research?

• Does Exploration architecture offer uniquelyenabling opportunities to this research?

• Cost offsets to sweeten the deal?e.g “50% performance for 10% of the cost”?

Relationship of astronomy and lunar exploration needs to be couched in a strategic context.

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Strategic Planning 101 (part 2)

What the lunar surface offers:

• gravity (and reaction mass)• rocks (grit, dust & regolith)

• possibly sustained human presence• possibly useful ISRU materials

Are either of these conspicuously enabling to astronomy?

In many respects, no.

NO QUESTION that lunar siting is VASTLY better for astronomy than terrestrial siting. But free space is too!

In a full-cost picture, NO QUESTION that lunarsiting is more expensive than free-space siting.


What astronomy needs

• Precise alignment• Precision acquisition and tracking• Large field of regard• Low natural background • Large baselines/collecting areas• Low temperatures• Quality optical surfaces• Assured comm & power• Upgrade/repair opportunities

Astronomical needs based on real mission conceptsmade to meet established science priorities.

Of course, these UVOIR needs not developed with expectation of lunar surface capabilities.


Potential problems for lunar surface siting - 1

• Precise alignment: thermal stability, low flexure

1/6 Earth gravity - bending modes.

Changing illumination - temperature changes. TPF-C needs 10mK temperature stability!

• Precision acquisition and tracking

Slow-moving moving coordinate system, but need very high precision.

Natural seismic activity low, but induced activity may be a risk.

• Large field of regard

One hemisphere FOR, significantly less if in crater. Supernovae and NEOs monitoring?



• Large baselines and collecting areas

Gravity disadvantageous for assembly. Delivery to surface adds risk.

Non-uniform surface complicates optical linkage and UV plane-filling.

• Low natural background emission

“Horizon glow” may add to background emission.

IR shielding challenging. Sun & Earth not blockable simultaneously.

• Low temperatures

Cosmic background-limited IR telescopes will need T <10K. Probably unachievable passively in sunlit parts of the Moon.



• Low scatter, high reflectivity optical surfaces

Dust: natural – electrostatically levitated and meteoritic, and activity-driven – surface ops, etc., can compromise performance.

• Assured communications and power

Solar power assured only 1/2 time except in very limited areas (Malapert, etc.). Direct comm convenient only on nearside.

• Upgrade/repair opportunities by humans

Risk and propulsion requirements to both agents and their tools.

Mitigation of problems like these translates to COST.


Where we find humans in space

Surprising that accessibility by humans is often cited as

an advantage somehow unique to the lunar surface!


A digression: advantages of free-space

• Panchromatic operation• Zero-g; mass advantage• Contamination free• Low latency for Earth orbits

and in particular for Earth-Sun L1&2 …

• Extraordinary thermal stability• Very low temps at L2 w/shields• Extraordinarily low torques• Continuous communication link• Continuous solar power• Low energy paths to Earth-Moon L1&2

Not a big surprise, that Earth-Sun Lagrange points are prime

destinations for future telescopes!

WMAP

JWST


Exploration and astronomy - a new vision

Exploration architecture - CEV, etc. can make free space an astronomy-enabling place! Build on HST legacy.

Construction, maintenance of BIG telescopes. Access at Earth-Moon L1 as part of lunar program, ops at Earth-Sun L2.

Teams are actively looking at opportunities here. Future In-Space Operations (FISO)


Build big things in free space … we already do this


The bottom line …

Creativity and innovation assure us lunar surface telescopes can be deployed/built/maintained/serviced.

We can do it! (But complexity translates into cost.)

Lunar surface is vastly better for telescopes than the Earth’s surface, but compared with new free-space opportunities, is no longer as compelling as it used to be.

Not at all clear that we need to do it!


The dirt on lunar dust … view from the experts

“… one of the most aggravating, restricting facets of lunar surface exploration is the dust and its adherence to everything no matter what kind of material, whether it be skin, suit material, metal, no matter what it be and it’s restrictive friction-like action to everything it gets on.”

“There's got to be a point where the dust just overtakes you, and everything mechanical quits moving.“

Gene Cernan; Apollo 17

“The LM was filthy dirty and it has so much dust and debris floating around in it that I took my helmet off and almost blinded myself. I immediately got my eyes full of junk, and I had to put my helmet back on. I told Al to leave his on.” “We tried to vacuum clean each other down, which was a complete farce. In the first place, the vacuum didn’t knock anything off that was already on the suits. It didn’t suck up anything, but we went through the exercise.”

Pete Conrad; Apollo 12

“Dust is going to be the environmental problem for future missions, both inside and outside habitats.”

Harrison “Jack” Schmitt, Apollo 17


Dust clouds on the Moon -- not a subtle effect

scattered light by naked eye from lunar orbit

scattered light by primitive TV camera


Is lunar dust a risk for UVOIR optics?

• For ops-driven dust … almost certainly.

• For naturally levitated dust … we just don’t know yet.

Lessons from lunar laser retroreflectors are neither clear nor clearly applicable.


So what’s so bad about a little dust on optics?

• reflectivity (goes as 1-absorptivity)gold coating is 97% reflective (not a really big deal)

• scattering (goes as absorptivity)TPF needs CL<100 ; <103/m2 of 10µm particles! (hard!)

• emissivity (goes as absorptivity)background emission proportional to emissivity (hard!)SAFIR needs <1% from contamination

Performance of low scatter, high MTF, and thermal IR systems strongly dependent on surface cleanliness.


But there could be niches -- using gravity

Very large aperture with arotationally shaped liquid mirror.(Angel et al.)

NEED gravity to make it work.

Technical feasibility? Cost??

Priority science drivers for deep small-field operation?

Trade against (pointable!) large telescopes in free-space.

Liquid Mirror Telescope for the Moon


But there could be niches -- using rock (for shielding)

Quiet zone of the Moon (QZM) offerspotentially low radio background.

Is QZM protected?

Depends on farside development?

Trade against active interference rejection techniques.Trade against free-space telescope at larger distance from Earth.

Deep Radio Surveys from QZM


But there could be niches -- using rock (for platform)

Interferometric Array on the

Lunar Surface

The Moon offers a platform to deploy an optically linked array of interferometric telescopes.

• Penalty of redeployment to fill UV plane, and limited FOR?

• Impact of monthly temp swings, power availability?

• Trade against requirements in free space. (As baselined for LISA, TPF-I, DARWIN, etc.)


But there could be niches -- using rock (as a detector)

• High energy particle detector

• Gravitational wave detector

• High energy photon (gamma ray)

etc. etc.

This is where we should be focusing our attention.


Summary

Don’t reject Moon for astronomy, but approach it in a way that is cognizant of science

priorities and Exploration capabilities elsewhere in space.

Lunar surface astronomy conceived by visionaries

reaching for enabling opportunities.

Niche astronomy opportunities should be evaluatedwith regard to science priorities and other approaches.

W. Tifft H. Smith


Reexamination of lunar astronomy

in the recent literature.

A sample of papers.


Space Policy vol 20, p 99-107, 2004astro-ph/0401274


Physics Today November 2006


StarDate Magazine July/August 2006

Documents

DFL 11/29/2006Astrophysics & Return to Moon0 Dirt, Gravity, and Lunar- Based Telescopes: Dan Lester University of Texas Astrophysics Enabled by The Return