Something is getting in the way (Image: Tom Zagwodzki/Goddard Space Flight Center)

The Apollo astronauts knew that moon dust was troublesome stuff. Now that dust could limit our ability to find cracks in Einstein’s general theory of relativity.

Many of our best tests of relativity come from lunar ranging experiments. Several times a month, teams of astronomers from three observatories blast the moon with pulses of light from a powerful laser and wait for the reflections from a network of mirrors placed on the lunar surface by the Apollo 11, 14 and 15 missions, as well as two Soviet Lunokhod landers. By timing the light’s round trip, they can pinpoint the distance to the moon with an accuracy of around a millimetre – a measurement so precise that it has the potential to reveal problems with general relativity.

But now Tom Murphy from the University of California, San Diego, who leads one of the teams at the Apache Point Observatory in Sunspot, New Mexico, thinks the mirrors have become coated in moon dust. “The lunar reflectors are not as good as they used to be by a factor of 10,” he says.


Photons gone missing

The fainter light is a problem for lunar ranging experiments. Out of every 100 million billion (1017) photons Murphy’s team fires at the moon, only a handful make it back to Earth. Most of are absorbed by Earth’s atmosphere on the way to the moon and back, or miss the mirrors altogether.

Murphy first suspected two years ago that the dust problem was cutting the light down even further. He was puzzled to detect far fewer photons than he expected, even when the atmospheric conditions were perfect. His team also saw a further drop when the moon was full and used to joke about the full moon curse. This gave Murphy some clues.

He suspects that moon dust is either coating the surface of the mirrors or has scratched them. Both scenarios would increase the amount of heat the mirrors absorb, and so during a full moon, sunlight falling on the mirrors would heat them up and change their optical properties. As a result, the mirrors would not reflect light as efficiently.

Even though the moon has no atmosphere, dust can be stirred up from the surface by the impact of micrometeorites.

Traces of dust

Murphy has scoured measurements stretching back to the 1970s and found that the problem first appeared between 1979 and 1984, and has been getting worse. However he is unwilling to predict if the mirrors will deteriorate further.

The Apache Point experiment can still make measurements, but the degradation is a bigger problem for other lunar ranging experiments that use less powerful lasers. More measurements from different sites would improve the limits on general relativity.

Murphy’s findings also highlight problems that astronomers might face if they ever build a telescope on the moon.

The results were reported at the American Physical Society meeting in Washington DC and have been submitted to the journal Icarus.