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Researchers blow dust off old lunar data

Apollo Lunar Dust Detector Experimant

The Apollo Dust Detector Experiment measured changes in voltage and temperature caused by dust landing on it (Source: NASA)

Sticky problem Dust, not radiation, is the biggest hazard for equipment on the Moon according to an Australian pair of scientists who have analysed data from the Apollo lunar missions.

The finding is the culmination of an experiment that was first sent to the Moon more than forty years ago.

Unlike dust on Earth, lunar dust is made up of 'scratchy' irregular-shaped particles, which cling to everything they touch.

Even before the Apollo missions launched into space, scientists suspected that the dust might damage delicate scientific instruments, as well as affect astronauts walking on the Moon.

To measure how quickly lunar dust builds up on surfaces on the Moon, Professor Brian O'Brien of the University of Western Australia, developed the Apollo Dust Detector Experiment.

"Working on the dust was an accident, because NASA had selected a bigger experiment of mine, a radiation experiment, as one of seven out of 90 proposals that were approved for the astronauts to put on the Moon," says O'Brien.

O'Brien and colleagues included protective covers on their experiment, after concerns that lunar dust kicked up by the lander's rocket exhaust would damage the equipment.

That prompted O'Brien to develop his dust experiment using tiny matchbox-sized detectors with three solar cells and thermometers, to measure changes in voltage and temperature caused by dust landing on it.

"The solar cells give you a voltage which changes with the intensity of the light," says O'Brien. "If there's dust on the cell, then the voltage is decreased by about ten per cent for every half a milligram of dust."

The detectors were added to the flights of Apollo 11, 12, 13 (which didn't land on the Moon), 14 and 15, and continued transmitting data for several years.

"NASA switched off everything in 1977 because it was costing about a million dollars a year," says O'Brien.

Lost data

Decades later NASA realised the data was lost.

"Fortunately I saved my copy, which included about 100 charts made using the SILLIAC computer in Sydney," says O'Brien.

O'Brien, together with research student Monique Hollick, also from the University of Western Australia, recently analysed the data and published their findings in the journal Space Weather.

They found that the lunar dust builds up slowly, forming at a rate of about a millimetre every 1000 years.

"We used a solar proton event in August 1972, as a benchmark because it was the biggest of that solar cycle," says Hollick.

"The solar proton event showed that radiation didn't cause any significant degradation to the solar cells, and the only other known cause of weathering on the Moon is dust," says Hollick.

"Lunar dust is so clingy because it's easily charged by photons from the Sun, and galactic high energy particles like electrons, which ionise the dust, causing it to become electrostatically charged."

Lunar fountains

The electrostatic charging also causes the lunar dust to be levitated above the surface, forming a fountain effect at the lunar terminator, the boundary between night and day.

"During the day, photons hitting dust particles give them a positive charge, and during the night, electrons give them a negative charge," says Hollick.

"So there's this positive-negative electric field set up, which people think causes the dust to be thrown up into fountains along the terminator, and being levitated above the surface in an exosphere."

"NASA's LADEE mission, which is now in lunar orbit, will measure dust in the Moon's exosphere, so its results will be very relevant to our research and could back up our findings."

Tags: the-moon