Lunar Far-Side Radio Arrays: A Preliminary Site Survey

The intense interest in returning to the Moon by space agencies around the globe is bringing the prospect of developing major scientific instruments there into the planning horizons of the agencies. One major project that is much discussed is a cosmology radio telescope on the lunar farside. The lunar farside is protected against radio interference from human source by 90 dB (9 dex). This makes it a unique location for a low frequency radio telescope to study the cosmic “Dark Ages”, the time before the first stars formed, using the signature of hydrogen. This signature will be found redshifted by the expansion of the Universe to the heavily utilized frequencies of ~20-30 MHz (9 – 18 meters). Initially, small telescopes will provide the first detections of this signal. Such telescopes could be placed almost anywhere on the lunar farside. However, because of this long wavelength, a large diameter, ~200 km, telescope is needed to get the maximum information from this hydrogen signature.

Little attention has been given to where on the farside such an extensive array could be situated. The rugged nature of the lunar farside complicates the deployment of a large diameter radio telescope array. There are only about 8 farside sites that may be smooth enough to be traversable by rovers on a 200 km scale. We have made a preliminary survey of their topography, using Lunar Reconnaissance Orbiter (LRO), Lunar Orbiter Laser Altimeter (LOLA) data, to rank them in order of rover accessibility. Of the 8 sites investigated, only 3 are traversable on the required scale by rovers capable of handling a 22 degree incline. Given that the slopes were measured on a 200 m scale, these are minimum slopes. A more detailed study of the remaining 3 sites down to meter scales using combined LRO/LOLA data with optical imaging from at least two different illumination angles will be the next step.

The rarity of good sites for the ultimate cosmology telescope points to a need for their protection as radio quiet zones to limit harmful interference.