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Apollo 17 astronaut Harrison Schmitt called it the biggest environmental problem on the moon. His crewmate Eugene Cernan said it was "probably one of our greatest inhibitors" to lunar operations. What could pose such a dire threat? The pervasive, abrasive culprit: lunar dust.
The tiny grains cling to spacesuits and scientific instruments, causing myriad problems—clogging, abrasion, inhalation, obfuscation—for lunar visitors and the experiments they leave behind. In a new study using data from instruments installed on the moon in 1969, Brian O'Brien, a now-independent researcher in Floreat, Western Australia, and a former professor of space science at Rice University in Houston, determined that the angle of the sun in the lunar sky modulates the clinginess of the lunar dust. O'Brien's paper is set to be published in Geophysical Research Letters.
O'Brien's data comes from matchbox-size Dust Detector Experiments delivered to the lunar surface by astronauts on Apollo 11 and Apollo 12. Each device had multiple solar cells, whose electrical output tracked the amount of dust shrouding the device over time.
Without the benefit of a robust atmosphere, the lunar surface is bombarded with energetic solar radiation, which lends a clingy electrostatic charge to the dust grains, O'Brien found. "Analyses imply this adhesive force weakens as solar angle of incidence decreases," his study says. Once the sun reached about 45 degrees, the moon's weak gravity was sufficient to overcome the dust's clinginess and pull it from the surface a smooth vertical solar cell.
If his theory proves correct, O'Brien writes, "future lunar astronauts may have greater problems with dust adhesion in the middle half of the [lunar] day than faced by Apollo missions," which took place during the morning of the lunar day (roughly equivalent to a month on Earth). But O'Brien has a low-tech suggestion for fending off the sun's aggravating influence: "A sun-proof shed may provide dust-free working environments on the moon."
Electron micrograph of astronaut Alan Bean's Apollo 12 suit, showing embedded dust particles and abrasions, courtesy of NASA