FORTY years after NASA ditched the idea of landing Apollo 17 on the far side of the moon, the forbidden fruit is being sought once again. Not by astronauts this time, but by astronomers seeking a quiet spot from which to observe the universe's " dark ages".
This was an epoch in the development of the cosmos, which lasted for a few hundred million years after the big bang, before stars and galaxies began to form. The only way to observe the dark ages is to look for faint radio signals from neutral hydrogen - single protons orbited by single electrons - which filled the early universe.
Telescopes on Earth, such as the Murchison Widefield Array in Western Australia, are searching for such signals, at frequencies above 100 megahertz. This can probe the universe back to 400 million years after the big bang.
To explore even earlier times, telescopes need to receive radio waves at frequencies below 100 megahertz. Interference from radio sources on Earth such as FM radio and the planet's ionosphere can mess up these signals. "You get to the point where the ionosphere is just a hopeless barrier," says Dayton Jones of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "You have got to go to space, and the most promising location by far is the far side of the moon."
This is why astronomers were discussing it at an American Astronomical Society meeting in Anchorage, Alaska, this month. Telescopes behind the moon would not have to contend with Earth's ionosphere, and they would also be shielded from our planet's radio chatter. "It is a very pristine environment for low-frequency observation," says Jones.
The first shot at radio astronomy from the moon's far side will probably be a mission called the Dark Ages Radio Explorer, being designed by Jack Burns at the University of Colorado at Boulder, and colleagues.
If selected as a mission by NASA in its review next year, DARE will orbit the moon at an altitude of 200 kilometres. It will collect neutral-hydrogen signals between 40 and 120 megahertz. That corresponds to 80 million to 420 million years after the big bang. Its antenna is designed to pick up signals from the entire sky. The craft will be a little toughie, with parts made from an Astroquartz/Kevlar fibre, which is very thermally stable - particularly handy when moving in and out of sunlight as it orbits the moon.
The DARE team has begun testing the probe's antenna at remote locations on Earth, starting with the National Radio Quiet Zone surrounding the Green Bank telescope in West Virginia. "It may be a radio quiet zone, but it's not quite," says DARE team member Abhirup Datta. "You can still see the FM bands coming in, and of course the ionosphere is a problem."
Not everyone reckons a space-based solution is needed to study the universe's dark ages. "Existing ground-based experiments will yield good progress on this problem at a tiny fraction of the cost of a space mission," says Steven Tingay of Curtin University in Bentley, Western Australia, who headed the construction of the Murchison array.
Burns disagrees. Preliminary tests reveal that the Earth's ionosphere is absorbing signals from space and re-emitting them as noise in frequencies below 80 megahertz. "If we can verify and characterise that, that slams the lid on any attempts to do this kind of experiment from the ground," says Burns.
Once DARE has done its job, his team want to deploy bigger telescopes on the lunar far side to image the first stars and galaxies. These antennas would be made of conducting material imprinted on extremely lightweight films of polyamide, micrometres thick.
In one design, three 100-metre-long arms of such films are attached to a central box of electronics. The arms would be rolled up tight for launch and, once on the moon, a rover sent along with the unit will move it to its required spot and help unfurl the arms. The rover would likely have to be controlled by astronauts orbiting a Lagrange point over the lunar far side.
To test this scenario, Burns's team will work with astronauts based on the International Space Station next year. The astronauts will remotely operate a Mars rover called K-10. It is being outfitted to unwind films of polyamide on a simulated Martian landscape at NASA's Ames Research Center in Moffett Field, California.
"The ultimate experiment we'd like to do for cosmology on the far side would involve thousands of these antennas," says Burns.
But what if the basic idea proves unfeasible, in terms of cost or in overcoming obstacles in the terrain? At JPL, Jones and his team are working on another solution: rolled-up antennas that inflate like party blowers seconds before they touch the lunar surface. "They are essentially immune to whatever irregularities there are at the surface," says Jones.
Astronomers have their sights set on at least one site for such telescopes: the flat bed of the 180-kilometre-wide Tsiolkovskiy crater, exactly where the Apollo 17 astronauts first wanted to land.