Monday, April 20, 2009
Discovered After 40 Years: Moon Dust Hazard Influenced by Sun's
In the 1960s and 1970s, the Apollo Moon Program struggled with a
minuscule, yet formidable enemy: sticky lunar dust. Four decades
later, a new study reveals that forces compelling lunar dust to cling
to surfaces -- ruining scientific experiments and endangering
astronauts' health -- change during the lunar day with the elevation
of the Sun.
The study analyzes the interactions on the Moon among electrostatic
adhesive forces, the angle of incidence of the Sun's rays, and lunar
gravity. It concludes that the stickiness of lunar dust on a vertical
surface changes as the Sun moves higher in the sky, eventually
allowing the very weak lunar gravity to pull the dust off.
The study has been accepted for publication in Geophysical Research
Letters, a publication of the American Geophysical Union (AGU.)
"Before you can manage the dust, you have to understand what makes it
sticky," says Brian O’Brien, the sole author of the paper. His
analysis is the first to measure the strength of lunar dust's adhesive
forces, how they change during the lunar day -- which lasts 710 hours
-- and differ on vertical and horizontal surfaces. O'Brien used data
from the matchbox-sized Dust Detector Experiments deployed on the
Moon's surface in 1969 during the Apollo 11 and Apollo 12 missions.
Lunar dust has long been described as the No. 1 environmental hazard
on the Moon. It causes miscellaneous havoc: from destroying scientific
equipment deployed on the lunar surface -- dusty surfaces absorb more
sunlight and make devices overheat -- to creating blinding dust clouds
that interfere with lunar landings. It also may be a health hazard to
space travelers, since dust clinging to space suits detaches when
astronauts reenter their lunar module. It then floats free in zero
gravity, ready to be inhaled, during the 3-day journey back to Earth.
Lunar dust particles are minuscule, with an average size of 70
micrometers, the thickness of a human hair. The particles get
positively charged by photoelectric effects caused by powerful solar
ultraviolet radiation and X-rays -- the thin lunar atmosphere does not
attenuate solar radiation -- generating strong electrostatic adhesive
forces which compel the specks of dust to cling to surfaces of
scientific instruments and space suits.
In his new study, O'Brien analyzed the behavior of dust on horizontal
and vertical solar cells in one of the Apollo dust-detecting
experiments. On the first morning of the experiment, the lunar module
-- 130 meters (426 feet) away from the dust detector -- took off from
the Moon’s surface. The blast of exhaust gases completely cleansed a
dusty horizontal solar cell, because it was illuminated only by weak
early-morning light and thus the adhesive force of dust was faint. But
only half the dust covering the vertical cell was removed by the
blast, because its surface faced east -- into more intense sunlight --
and thus the sticky forces were stronger.
O'Brien found that later, as the Sun rose and the angle of incidence
of the Sun's rays on the dusty vertical surface facing east decreased,
the electrostatic forces on the vertical cell weakened. The tipping
point was reached when the Sun was at an angle of about 45 degrees:
then the pull of lunar gravity counteracted the adhesive forces and
made the dust start falling off. All dust had fallen by lunar night.
"These are the first measurements of the collapse of the cohesive
forces that make lunar dust so sticky" O’Brien says.
In 1965, NASA selected O'Brien, an Australian physicist who was then a
professor of Space Science at Rice University in Houston, Texas, to be
the principal investigator in one of seven lunar experiments designed
for the Apollo Program. O'Brien started researching lunar dust in 1966
because he feared for an instrument he developed that was to be left
behind on the Moon by the Apollo 14 mission. He worried that the
device, which measured the flux of charged particles, would end up
covered in dust and ruined. Lunar dust is "a bloody nuisance," he
In 1970, O'Brien published a paper which he says proved that rocket
exhaust gases from the Apollo 11 Lunar Module had lofted dust and
debris which then coated the surface of a lunar seismometer -- the
first instrument deployed by human hands on a celestial body. The
seismometer then overheated by 50 degrees and failed after three
weeks' operation. An official 1969 NASA report was incorrect in
stating that no contamination had occurred, O'Brien says.
But it wasn’t until late 2006, when O'Brien learned from NASA’s
website that the space agency had misplaced data tapes from its
dust-detecting experiments, that he decided to revisit his own set of
173 tapes. NASA had sent him these tapes one by one in 1969 and 1970,
when he was working at the Department of Physics at University of
Sydney. He took them with him when, in 1971, he moved to Perth for a
new job. O'Brien's tapes are now the only known record of data from
those vintage experiments.
Working alone and self-funded, the 75-year-old scientist dedicated two
years to analyzing paper charts printed out in 1969 and 1970 from the
magnetic tapes, which contain 6 million measurements, most of them yet
to be analyzed.
For future Moon and Mars missions, O'Brien offers a practical solution
to the dust hazard: Use a wide Sun-proof shed, to block the rays that
enhance dust's adhesive forces.
"Getting closer to understanding the physics of the lunar dust
problems means moving one giant step towards management of the
hazards," O’Brien says.
The Apollo astronauts knew that moon
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
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.
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.
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
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
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.