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The Scientific Context for Exploration of the Moon
such a telescope would not be affected by dust deposition. As a result of the high astronomical priority of this work and the uniquely enabling character of the radio-quiet farside lunar surface, such efforts deserve cultivation.
With this in mind, near-term studies should be started to improve the understanding of the requirements and possible limitations of such a low-frequency radio interferometer effort, perhaps defining near-term site survey experiments that would help clarify the potential. What are the optimal sites for such an installation? There is also need to extend the original 40-year-old work that identified the “quiet zone of the Moon” and which is the only source of data on this subject to this day. What are the temporal characteristics of the frequency environment? To the extent that wide access to the lunar surface is not provided in the current VSE architecture, how close can such a facility be to the planned polar outpost and still have optimal performance? Given that farside human landings are not currently planned, how would such a facility be deployed and operated? Can a credible design for a major installation be developed that does not assume pre-existing infrastructure (communications, power, and so on)? With the recent predictions that sunlit/shaded edges of the Moon will develop substantial electrical potential gradients, are electrostatic discharges a possible natural noise source?
Currently, conventional single-aperture, single-spacecraft telescopes in free space that do not require precision constellation management now use proven pointing and tracking technology. The platform of the Moon is by no means as enabling for astronomy as it was once thought to be. Lunar gravity, while small compared with that on Earth, nevertheless requires that telescopes on the Moon be more massive than those based in free space in order to achieve a requisite stiffness that can preserve optical alignment as the telescope moves across the sky. However, a large telescope can be expected to have its surface shape maintained by active optics, and the issues of maintaining the surface by inertial reaction to the structure and the problem of free vibrations of that structure become larger as the telescope becomes larger. In the range of telescopes as large as the James Webb Space Telescope (JWST), such considerations seem to favor free-space locations. For larger apertures these issues will need to be re-examined. The frigid conditions in lunar polar craters might serve the needs of future thermal infrared telescopes, but passive cooling strategies now being designed into JWST for Earth-Sun Lagrange point L2 provide such low temperatures at modest fractional cost.
For ultraviolet astronomy and for astronomy involving the precise control of scattered background emission (e.g., planet detection), tolerance for dust contamination is very small. For thermal infrared astronomy, the observational background (which determines the sensitivity) is proportional to the absorbtivity (and re-radiation), not the reflectivity, so small dust deposits on the optics can seriously compromise the performance. As a result, the lunar surface appears to be minimally suitable for large UV/optical/IR telescopes.
Accessibility by humans is not clearly a unique advantage offered by the lunar surface. There is a large base of experience in deploying, maintaining, and servicing large telescopes in free space from the Hubble Space Telescope, as well as substantial expertise in in-space construction of large facilities from the International Space Station experience. The lunar-return implementation architecture for the Vision for Space Exploration, now in development, does not provide explicitly for in-space opportunities. Nevertheless, it appears likely that telescopes in free space, whether in LEO or at the dynamically convenient and thermally highly optimal Earth-Sun Lagrange points, may eventually be serviceable by humans and robots using augmentations to this architecture. In the case of remote Earth-Sun Lagrange points, such access could most conveniently be ensured after returning these telescopes by known pathways that require only small changes in spacecraft velocity to get to closer sites, such as the Earth-Moon Lagrange points. In this context, however, the lunar surface may well play a key role in operational support for such relocation efforts.2
The lunar surface gravity and solid surface do offer potential advantages for particular lunar observatory architectures and deserve further consideration by way of trade-off studies against functionally comparable free-space facilities. Several concepts may be viable, at least in the long term. A large liquid mirror telescope on the lunar surface near a pole that could offer extremely deep observations into very limited regions of the sky near the local
The development of a heavy-lift (Ares 5) launcher could offer major advantages to astronomy, providing the ability to lift large telescopes, fully assembled, into free space. In these respects, the astronomy community is particularly excited about the space transportation capabilities that will arise as the results of the lunar exploration architecture. See, for example, the proceedings of the Astrophysics from the Moon conference, Space Telescope Science Institute, 2007, at www.stsci.edu/institute/conference/moon, and the Lunar Exploration Science Workshop, Tempe, Arizona, February 27-March 2, 2007, at https://www.infonetic.com/tis/lea/, accessed May 29, 2007.