Sun-Stirred Lunar Dust Could Wear Down Moon Machines
This lunar laser ranging retroreflector array left by Apollo astronauts might be getting slowly worn down by a natural, dust-driven decay process.
A recent study has revealed that light-reflecting scientific instruments placed on the moon by astronauts 40 years ago have mysteriously degraded. The findings suggest that equipment placed on the barren, weatherless lunar surface can in fact suffer performance problems in the long term.
Avoiding this gradual, natural breakdown – probably caused by lunar dust – will be "important not just for [the next generation of reflector] devices but for almost anything you'd design to put on the moon," said lead author Tom Murphy, an associate professor of physics at the University of California, San Diego.
The threat of lunar dust to space-age technology is not new, of course, and has been known about since it caused equipment failures during the Apollo missions. Fine particles of the pulverized soil, or regolith, also clung to astronauts' suits and were a general nuisance on the expeditions.
In those instances, though, it was human activity that kicked up the offending dust. In contrast, three reflector arrays deployed by astronauts, along with two reflector-equipped Soviet rovers, have been all alone on the moon since the late 1960s and early 1970s.
Given such lengthy solitude in a desolate environment, researchers had presumed the reflectors would remain in fine working order.
"The fact that these reflectors are still being used has often been seen as anecdotal evidence that they have suffered no degradation," said Murphy. "I think this was a premature conclusion because it doesn’t mean that [the reflectors] are in the best of health."
In essence, the aging reflectors might serve as an example of how lunar conditions might slowly but surely wear down future manmade objects destined for prolonged service on the moon .
Earth-bound researchers bounce laser beams off of these lunar retroreflectors to precisely calculate the distance to the moon. This so-called lunar laser ranging is used to perform tests of gravitational theory as well as infer the contents of the moon and trace its slow spiral away from our planet.
The technique received a boost in 2006 when the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) instrument opened in New Mexico with about 100 times the sensitivity of other such setups in Texas and France.
Using APOLLO, Murphy and his colleagues calculated that the number of photons, or particles of light, returning to Earth from the reflectors has fallen 10-fold since the experiment began. The decline began in the late 1970s, or within a decade of original reflector placement. That timing ruled out that departing lunar spacecraft with astronauts aboard had simply sprayed the nearby devices with dust.
Murphy's lunar rangers also saw that during a full moon, the reflectors' efficiency plummeted another factor of 10. "We started calling it the 'full moon curse,' and we started laughing about it, but then after a while it wasn’t funny anymore," Murphy said.
After ruling out APOLLO errors and local atmospheric effects, Murphy and his team were left with only one possibility: The reflectors on the moon themselves were the source of the hiccup.
The researchers also found hints of the full moon curse preserved in the data previously collected by other less precise lunar laser ranging instruments. Professional experience and other instrument improvements over the years on the Earth-side of things, however, had largely masked the underlying diminishing return from the reflectors, Murphy said.
A lunar dustup
In a study published in this month's issue of the journal Icarus, Murphy and his collaborators proposed several mechanisms that could explain the reflector decay. The deposition of dust ultimately shook out as a more likely troublemaker than, say, damage to the reflectors' glass cubes from sunlight or the impact abrasions of micrometeorites.
Settling dust would block light into and out of the reflectors, Murphy said, and during a full moon when the reflectors are most exposed to solar illumination, the dust could warm them and change how light moves through their carefully calibrated glass.
As for how the dust got on the reflectors, though there is virtually no atmosphere on the moon, let alone wind that could stir up this powder, researchers have long suspected that dust moves about due to so-called electrostatic effects.
The idea is that sunlight striking the lunar soil strips some molecules of their electrons, "ionizing" them and imparting a positive electrical charge. Since like charges repeal, the dust particles push away from each other, with some lofting as far as tens of kilometers above the moon's surface.
On the dark side of the moon, electrons get dumped by the solar wind and likewise generate such lunar "dust fountains." At the terminator, or the line between night and day, these opposite charges should also cause a flow of dust, Murphy said, affording dust another avenue to foul equipment.
Despite this sneaky, dusty dynamism, the biggest danger to future moon gear from regolith will still be grains dislodged by human activity.
"We figure we will do most of it ourselves," said Mark Hyatt, dust management project manager at NASA's Glenn Research Center in Ohio and who was not involved in the study. Historically, "most of what see in systems performance loss we contaminate ourselves. The natural phenomena . . . are many orders of magnitude slower," Hyatt said.
Nonetheless, this slow but steady accumulation of dust could block communications signals from remote, stationary antennas , for example, or coat lenses on moon-based telescopes. Shielding over sensitive areas or tilting the orientation of devices to resist build-up are two basic solutions, Murphy said.
Next-generation robotic rovers planned for the moon in several years will also need to incorporate mitigation strategies to thwart naturally depositing dust.
"Future rover designs can stow solar arrays, reflectors, cameras and instrument optics when not in active use, and during peak dust transport," said William "Red" Whittaker, chairman and chief technical officer of Astrobotic, a firm competing in the Google Lunar X prize, and a Carnegie Mellon professor of robotics.
In addition, dark lunar dust could prevent excess heat from radiating off of delicate electronic components, causing them to overheat, Murphy said, so engineers will need to be mindful.
Moreover, the electrostatic forces that animate the dust could be cause for concern, according to William Farrell of NASA’s Goddard Space Flight Center.
Dust fountains might serve as a "poor man's indicator," Farrell said, of electrically unstable areas. "The natural dust itself is not necessarily a showstopper, but its source might be an indicator of an electrostatically active region, and you might have to worry about that for sensitive electronics," Farrell said.
Learning to live with it
In years ahead, a greater understanding of the dust-driven conditions on the moon should help technologists properly prepare for mechanized exploration and perhaps human habitations one day.
Ryan Kobrick, a recent PhD graduate from the University of Colorado at Boulder whose thesis explored lunar dust considerations for spacecraft design, noted that dealing with dust will be a fact of lunar life.
"All systems on the moon are going to get dirty," Kobrick said.