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Telescopes on the Moon?

Telescopes on the Moon
A telescope on the Moon? CREDIT: NASA / USGS / LPI / ASU / Google / Megan
The idea of building a telescope on the Moon has been around for a while. There are many early references to papers and conference contributions on the topic by (amongst others) Stanisław Gorgolewski, a man who is, I suspect, well-known to anyone who has visited the radio telescope at Toruń. One of his more recent conference abstracts discusses the idea from a modern perspective including using lunar telescopes for SETI searches.

It's a topic that does seem to keep coming back. A paper on this subject caught my eye while I was browsing the daily update of papers on astro-ph arXiv on Wednesday. Written by Sebastian Jester (MPIA Heidelberg) and Heino Falcke (ASTRON / Radboud Universiteit Nijmegen), the paper describes (in some detail) how radio telescopes on the Moon could be useful for a whole range of experiments, from searching for extra-terrestrial intelligence to trying to detect the signiture of the epoch of reionisation in the early universe.

Why the Moon? Well, two obvious advantages are that a) there are no people there, and b) there is no atmosphere. No people means no mobile phones, pagers, or other radio noisy devices to interfere with sensitive observations. This is why most new radio telescopes such as the MWA are being built in remote places like the Murchison here in Western Australia. What about the atmosphere? Well, one advantage of radio astronomy is that usually you don't have to worry too much about the atmosphere. This doesn't apply at very low frequencies, however, where the ionosphere starts to cause real problems below about 50 MHz, so it seems logical to build a very low frequency telescope somewhere with no atmosphere.

One reason for renewed interest in lunar telescopes seems to be the interest in finding the signature of the epoch of reionization by observing redshifted neutral hydrogen. This is an important spectral line in radio astronomy, occuring at a frequency of 1.4GHz (a wavelength of 21cm) in the local universe. The further out into space you look, the further back in time you are looking, and the faster stuff is moving away from you. Due to the Doppler effect, the faster it's moving away, the more red-shifted the emission becomes and that 1.4GHz spectral line gets shifted to lower and lower frequencies. The reionization signature comes from a time in the history of the universe when the neutral gas that filled most of the universe was first ionized by the high energy uv radiation from the first stars at a redshift high enough that the 1.4GHz emission from hydrogen gets shifted down to somewhere below a few hundred MHz.

These kinds of experiments use dipole arrays like the MWA to observe the sky. This has advantages for working on the Moon. Unlike with optical telescopes, there are no sensitive, easily scratched optics to worry about - that lunar dust would cause all sorts of problems for an optical mirror. These arrays also have no moving parts, the beams are steered electronically, so there are no mechanical problems to worry about meaning easy deployment and minimal maintenance. We know how to build electronics that can survive in the extreme environments both in space and on the Moon, as well as survive the stress of launch, because we've been doing it for fifty years.

So, while an Arecibo on the Moon is somewhat fanciful, it is not inconceivable that we could have a radio telescope up there at some point.

Posted by Megan on Monday 09th Feb 2009 (01:49 UTC) | Add a comment | Permalink


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