A recent study has revealedthat light-reflecting scientific instruments placed on the moon by astronauts40 years ago have mysteriously degraded. The findings suggest that equipmentplaced on the barren, weatherless lunar surface can in fact suffer performanceproblems in the long term.
Avoiding this gradual,natural breakdown — probably caused by lunar dust — will be "important notjust for [the next generation of reflector] devices but for almost anythingyou'd design to put on the moon," said lead author Tom Murphy, anassociate professor of physics at the University of California, San Diego.
In those instances, though,it was human activity that kicked up the offending dust. In contrast, threereflector arrays deployed by astronauts, along with two reflector-equippedSoviet rovers, have been all alone on the moon since the late 1960s and early1970s.
Given such lengthy solitudein a desolate environment, researchers had presumed the reflectors would remainin fine working order.
"The fact that thesereflectors are still being used has often been seen as anecdotal evidence thatthey have suffered no degradation," said Murphy. "I think this was apremature conclusion because it doesn’t mean that [the reflectors] are in thebest of health."
In essence, the agingreflectors might serve as an example of how lunar conditions might slowly butsurely wear down future manmade objects destined for prolonged service on themoon.
Earth-bound researchersbounce laser beams off of these lunar retroreflectors to precisely calculatethe distance to the moon. This so-called lunar laser ranging is used to performtests of gravitational theory as well as infer the contents of the moon andtrace its slow spiral away from our planet.
The technique received aboost in 2006 when the Apache Point Observatory Lunar Laser-ranging Operation(APOLLO) instrument opened in New Mexico with about 100 times the sensitivityof other such setups in Texas and France.
Using APOLLO, Murphy and hiscolleagues calculated that the number of photons, or particles of light,returning to Earth from the reflectors has fallen 10-fold since
Murphy's lunar rangers alsosaw that during a full moon, the reflectors' efficiency plummeted anotherfactor of 10. "We started calling it the 'full moon curse,' and we startedlaughing about it, but then after a while it wasn’t funny anymore," Murphysaid.
After ruling out APOLLOerrors and local atmospheric effects, Murphy and his team were left with onlyone possibility: The reflectors on the moon themselves were the source of thehiccup.
The researchers also foundhints of the full moon curse preserved in the data previously collected byother less precise lunar laser ranging instruments. Professional experience andother instrument improvements over the years on the Earth-side of things,however, had largely masked the underlying diminishing return from thereflectors, Murphy said.
In a study published in thismonth's issue of the journal Icarus, Murphy and his collaborators proposedseveral mechanisms that could explain the reflector decay. The deposition ofdust ultimately shook out as a more likely troublemaker than, say, damage tothe reflectors' glass cubes from sunlight or the impact abrasions ofmicrometeorites.
Settling dust would blocklight into and out of the reflectors, Murphy said, and during a full moon whenthe reflectors are most exposed to solar illumination, the dust could warm themand change how light moves through their carefully calibrated glass.
As for how the dust got onthe reflectors, though there is virtually no atmosphere on the moon, let alonewind that could stir up this powder, researchers have long suspected that dustmoves about due to so-called electrostatic effects.
The idea is that sunlightstriking the lunar soil strips some molecules of their electrons,"ionizing" them and imparting a positive electrical charge. Sincelike charges repeal, the dust particles push away from each other, with somelofting 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 "dustfountains." At the terminator, or the line between night and day,these opposite charges should also cause a flow of dust, Murphy said, affordingdust another avenue to foul equipment.
Despite this sneaky, dustydynamism, the biggest danger to future moon gear from regolith will still begrains dislodged by human activity.
"We figure we will domost of it ourselves," said Mark Hyatt, dust management project manager atNASA's Glenn Research Center in Ohio and who was not involved in the study.Historically, "most of what see in systems performance loss we contaminateourselves. The natural phenomena . . . are many orders of magnitudeslower," Hyatt said.
Nonetheless, this slow butsteady accumulation of dust could block communications signals from remote, stationaryantennas, for example, or coat lenses on moon-based telescopes. Shieldingover sensitive areas or tilting the orientation of devices to resist build-upare two basic solutions, Murphy said.
Next-generation roboticrovers planned for the moon in several years will also need to incorporatemitigation strategies to thwart naturally depositing dust.
"Future rover designscan stow solar arrays, reflectors, cameras and instrument optics when not inactive use, and during peak dust transport," said William "Red"Whittaker, chairman and chief technical officer of Astrobotic, a firm competingin the Google Lunar X prize, and a Carnegie Mellon professor of robotics.
In addition, dark lunar dustcould 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 electrostaticforces that animate the dust could be cause for concern, according to WilliamFarrell of NASA’s Goddard Space Flight Center.
Dust fountains might serve asa "poor man's indicator," Farrell said, of electrically unstableareas. "The natural dust itself is not necessarily a showstopper, but itssource might be an indicator of an electrostatically active region, and youmight have to worry about that for sensitive electronics," Farrell said.
Learning to live with it
In years ahead, a greaterunderstanding of the dust-driven conditions on the moon should helptechnologists properly prepare for mechanized exploration and perhaps humanhabitations one day.
Ryan Kobrick, a recent PhDgraduate from the University of Colorado at Boulder whose thesis explored lunardust considerations for spacecraft design, noted that dealing with dust will bea fact of lunar life.
"All systems on the moonare going to get dirty," Kobrick said.