Seeing on the Far Side of the Moon

Instead of using one very large dish to collect radio waves, data from a number of radio telescopes (called an array) can be stitched together by computers into a coherent single observation. These telescopes can be located at a single site, or they can be separated by oceans. The Event Horizon Telescope (EHT), the instrument that Bouman and colleagues used to image the black hole, is actually a network of telescopes in Europe, North America, South America, Antarctica, and Hawaii. The resolution of the array is proportional not to the diameter of any one instrument, but rather to the distance between those instruments that are farthest apart. The EHT’s black hole measurement was made at a stunning resolution of 25 microarcseconds, roughly the capability from Earth to distinguish a golf ball on the moon. (...)

Space telescopes are incredible instruments. NASA’s most famous, the Hubble Space Telescope, has made numerous significant discoveries since it entered service in 1990, most famously estimating the age of the universe at 13.7 billion years, two orders of magnitude more precisely than the previous scientific estimate of 10 to 20 billion years. But Hubble operates mainly in the optical band, something that is mostly accessible from Earth. NASA’s less famous infrared instrument, the Spitzer Space Telescope, which was deactivated this year after tripling its planned design life, studied bands not observable from the ground. Its replacement, the powerful James Webb Space Telescope, is due to launch next year. It should produce even more stunning observations than Hubble when it comes online, as its sensitivity to infrared light is perfect for capturing optical waves, redshifted by the expansion of the cosmos, from some of the most distant objects in the observable universe.

But the biggest problem with these orbiting telescopes is that they cannot avail themselves of the solution used by terrestrial arrays to increase resolution—adding more telescopes and stitching the data together using computation. James Webb’s aperture is 6.5 meters in diameter, while the Event Horizon Telescope has an effective aperture the size of Earth. Space telescopes lack the power that arrays on the ground can achieve.

Astronomy, then, faces a Catch-22. Terrestrial telescopes can be built with excellent resolution thanks to aperture synthesis, but they have to cope with atmospheric interference that limits access to certain bands, as well as radio interference from human activity. Space telescopes don’t experience atmospheric interference, but they cannot benefit from aperture synthesis to boost resolution. What we need is to develop a telescope array that can marry the benefits of both: a large synthetic aperture like Earth-based arrays that is free from atmospheric and human radio interference like space telescopes.

A telescope array on the surface of the moon is the only solution. The moon has no atmosphere. Its far side is shielded from light and radio chatter coming from Earth. The far side’s ground is stable, with little tectonic activity, an important consideration for the ultra-precise positioning needed for some wavelengths. Turning the moon into a gigantic astronomical observatory would open a floodgate of scientific discoveries. There are small telescopes on the moon today, left behind from Apollo 16 and China’s Chang’e 3 mission. A full-on terrestrial-style far-side telescope array, however, is in a different class of instrument. Putting one (or more) on the moon would have cost exorbitant sums only a few years ago, but thanks to recent advances in launch capabilities and cost-reducing competition in the new commercial space industry, it is now well worth doing—particularly if NASA leverages private-sector innovation.

by Eli Dourado, Works in Progress | Read more:

Image:Antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), on the Chajnantor Plateau. Credit: ESO/C. Malin