Dark side of the moon ideal for radio telescope
12:06 08 June 2007
NewScientist.com news service
The idea of building a radio observatory on the Moon has been bolstered by a high-level report on lunar science. Situated on the far side, it would be able to look back to the "dark ages" of the early universe, and map the magnetic fields of planets around other stars.
NASA asked the US National Research Council (NRC) in 2006 to advise the agency on what kinds of science could be tackled from the Moon, and what projects should be given top priority.
On Tuesday, an NRC committee headed by George Paulikas, a former scientist with The Aerospace Corporation in El Segunda, California, US, reported its findings. The report is entitled The Scientific Context for the Exploration of the Moon.
One of the astronomical ideas considered most promising by the committee was a radio observatory on the Moon's surface. A paper on the idea was submitted by a group of scientists led by Joseph Lazio of the Naval Research Laboratory in Washington, DC, US.
They envisage plating metallic antennas onto a flexible plastic film that can be rolled up for transport to the Moon, then unrolled on the lunar surface like a carpet, creating an instant array of radio receivers.
Such an observatory could investigate very low frequency sources in the 1 to 10 megahertz range, which would normally be drowned out by the constant din of radio traffic from Earth. The Moon’s bulk prevents these Earthly radio signals from reaching its far side.
"The far side of the Moon is one of the most radio quiet places in the solar system, because it's the one place that doesn't really have any time in view of the Earth", says David Lawrence of the Los Alamos National Laboratory in New Mexico, US, a member of the NRC committee that prepared the report.
By observing at low frequencies, the observatory could map out structures prevalent during the period of reionisation, an era when radiation from the first stars and galaxies dramatically transformed the universe, altering the omnipresent hydrogen clouds to make them transparent to radiation.
It has so far been impossible to directly observe anything but the very end of this period, but scientists are keen to know what was happening in the universe at that time, as the formation pattern of galaxies could cast light on the nature of dark matter and dark energy.
It would even be possible for such an observatory to probe planets orbiting other stars. The interaction of charged particles such as electrons with the magnetic fields of extrasolar planets should produce low-frequency radio waves.
They could provide information on the interiors of extrasolar planets, as the internal structure and composition governs the strength of the magnetic field.
Another aim would be to shed light on the mysterious sources of cosmic rays – speeding charged particles that appear to come from all directions in the sky. Wherever these particles are accelerated, they should also give off low-frequency radio waves that a suitable observatory could detect.
Lazio's team has proposed an initial radio observatory on the near side of the Moon consisting of three 500-metre-long antenna-bearing strips that would meet in a Y shape. It would be mostly limited to investigating outbursts of the Sun, but it would also serve as a testing ground for a more ambitious detector, spread over a few kilometres on the quiet, far side of the Moon.
Bigger is better
The more modest radio observatory design is called Radio Observatory for Lunar Sortie Science (ROLSS). Each strip could be rolled up into a cylinder just a metre long and 25 centimetres across. They could be deployed by astronauts on a return to the Moon, but it is simple enough that robots could also do the job.
Even ROLSS might see some distant objects, but only with blurry vision. "For the best astrophysics you want something that's much bigger, let's say 10 kilometres or 100 kilometres across," says team member Robert MacDowall of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.
The committee's report casts doubt on the suitability of the moon for telescopes working in visible light, infrared and ultraviolet wavelengths, partly because the accumulation of lunar dust on their optics could interfere with their vision. Dust does not affect low frequency radio waves, however.
Other promising ideas highlighted in the report include taking cores of lunar soil to look for evidence of ancient solar flares in the isotopes laid down; looking for hardy bits of Earth rock, which could have been blasted off our planet as a result of meteorite impacts and could contain information about the planet's early history; and a wide variety of geological investigations that could shed light on the formation and evolution of the Moon itself.
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