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The powdery lunar soil was great for making footprints, but was a problem for astronauts like Charlie Duke, shown here during the Apollo 16 mission in 1972. It got in their eyes and throats, and clung stubbornly to every surface. (NASA)

Stronger Than Dirt

Lunar explorers will have to battle an insidious enemy—dust.

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(Continued from page 1)

It wasn’t until last year that Timothy Stubbs and his colleagues at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, came up with an explanation, which they called the “dynamic fountain” model. In a drinking fountain, the arc of water from the spout appears suspended in one position, but the water molecules are constantly in motion. Similarly, according to Stubbs, microscopic grains of lunar dust are constantly leaping from the surface and falling back again due to a weird phenomenon unknown on Earth: electrostatic lofting.

Just as rubbing a balloon against your shirt creates a static charge that can levitate the hair on your head, lunar dust particles with opposite charges will attract each other, and like-charged particles will repel. How does the dust become charged? On the moon’s sunlit side, solar ultraviolet and X-ray radiation beats down relentlessly, knocking electrons from atoms in the lunar soil. The electrons escape into space, and positive charges build up on the lunar surface. The tiniest motes of dust—just a few hundred-thousandths of an inch in size—are repelled from the rest and launched upward, some reaching miles above the surface. Lunar gravity eventually pulls them back down, but electrostatic repulsion kicks them off again. The process is repeated over and over to form a tenuous “atmosphere” of moving dust particles.

The same happens on the lunar far side, which is bombarded by solar wind particles flowing around the moon—except that the net charge is negative since the solar wind is mostly electrons. Data from the 1998 Lunar Prospector mission suggests that the electrical potential might amount to hundreds of volts on the night side, even higher than on the day side, possibly launching dust particles to higher velocities and altitudes.
Evidence supporting the dynamic fountain model may be buried in old data from the Lunar Ejecta and Meteorites experiment, left on the moon by Apollo 17 in 1972. LEAM had three sensors that could record the speed, energy, and direction of tiny particles. The experiment was designed to look for fallout from lunar meteorite impacts, as well as material raining down from comets or interstellar space. But in a classic case of serendipity, “LEAM recorded a high number of particles every lunar sunrise,” recounts Gary Olhoeft, professor of geophysics at the Colorado School of Mines, “mostly from east or west rather than from above, and mostly much slower than expected.” Even stranger, a few hours after every lunar sunrise, LEAM’s temperature rocketed up so high—near that of boiling water—that the instrument had to be turned off because it was overheating. Olhoeft and others now suspect that dust lofted from the moon covered the LEAM, darkening its surface so the experiment package absorbed sunlight rather than reflected it. But nobody knows for sure. LEAM operated only briefly before the Apollo program ended.

Then there are the puzzling rays seen by the Apollo astronauts. Because the specks of dust bouncing around on the moon would be too small to see with the naked eye, explorers on the surface wouldn’t likely notice them. But astronauts on the night side around sunrise might see the moving dust causing the sunlight to scatter, looking like “a weird, shifting glow extending along the horizon, almost like a dancing curtain of light,” according to Stubbs. And at certain times during the lunar cycle, when the moon passes through an active part of Earth’s magnetosphere, Stubbs speculates that “dust would start flying at high velocities”—not at densities that could be seen from Earth, but perhaps in large enough amounts to get into unprotected machinery on the moon.

Last year, in a 77-page report listing 20 risks that required further study before we should commit to a human Mars expedition, NASA’s Mars Exploration Program Analysis Group ranked dust number one. The report urged study of its mechanical properties, corrosiveness, grittiness, and effect on electrical systems. Most scientists think the only way to answer the questions definitively is by returning samples of Martian soil and rock to Earth well before launching any astronauts.

Many also believe a lunar sample return will be necessary. True, the Apollo astronauts brought back some 800 pounds of lunar rocks from six landing sites. But the dust played a dirty trick: The gritty particles deteriorated the knife-edge indium seals of the bottles that were intended to isolate the rocks in a lunar-like vacuum. Air has slowly leaked in over the past 35 years. “Every sample brought back from the moon has been contaminated by Earth’s air and humidity,” Olhoeft says. The dust has acquired a patina of rust, and, as a result of bonding with terrestrial water and oxygen molecules, its chemical reactivity is long gone. The chemical and electrostatic properties of the soil no longer match what future astronauts will encounter on the moon.

To better understand lunar dust, Olhoeft is trying to undo the damage. During the Apollo program six steel vacuum chambers were built, each 10 feet long, that could be pumped down to 10-12 torr—one- trillionth the atmospheric pressure of sea level on Earth—to duplicate the vacuum on the moon. After the Apollo program shut down, five of the giant tanks were scrapped. The remaining chamber, recently refurbished, is in Olhoeft’s laboratory at the Colorado School of Mines. He plans to insert a sample of real lunar dust, pump the pressure down to lunar vacuum, cycle the chamber’s temperature to duplicate the harsh lunar day and night, and bombard the contents with radiation and electrons to try to resuscitate some of its original properties.

At NASA’s Marshall Space Flight Center in Huntsville, Alabama, in a smaller basketball-size vacuum chamber located inside the Dusty Plasma Laboratory, researcher Mian Abbas is running a positively Zen-like experiment. Each morning, he enters the lab and sits down to examine a single speck of lunar dust. For as long as 10 or 12 days at a stretch, he shines an ultraviolet laser onto the particle and painstakingly controls the strength of electric fields until the speck levitates. “Experiments on single grains are helping us understand how lunar dust on the moon can be given an electric charge and lofted to high altitudes,” Abbas explains.

Olhoeft, Stubbs, and others are also mining original Apollo data, such as that from LEAM, in the hope that the unread tapes might yield information useful in designing lunar spacesuits and equipment. It’s easier said than done: Many original computer tapes from Apollo experiments, including ones that were never analyzed, can no longer be read. Not only are some of the data formats obsolete, many of the tapes have degraded due to less-than-optimum storage. Some of the data may be permanently lost. So the dust researchers do what they can. They pore over frame after frame of footage taken by the Apollo astronauts, measuring the trajectory of dust particles kicked up by boots and rover wheels, hoping to better understand the physics. Others, like Bruce Damer of Digital Space in Santa Cruz, California, are building computer models of the dust so that design engineers can test-drive hypothetical digging machines and see what gets clogged.

While these scientists study the dust itself, engineers are coming up with prototype systems for combating it. At the Kennedy Space Center in Cape Canaveral, Florida, Carlos Calle and colleagues in the Electrostatics and Surface Physics Laboratory have demonstrated a device they think can be embedded in spacesuit fabrics to create oscillating electric fields. The rapid shifting of the fields would cause dust particles to hop from electrode to electrode until they get thrown off the suit altogether. An even more imaginative dust-busting concept comes from Lawrence Taylor, a planetary scientist at the University of Tennessee at Knoxville, who describes himself as “one of those weird people who like to stick things in kitchen microwave ovens to see what happens.” When he tried it with a small pile of lunar soil, he found that it melted “lickety split”—within 30 seconds—at only 250 watts of power. The nanophase iron in the dirt concentrated the microwave energy to sinter, or fuse, the loose soil into large clumps. Taylor’s experiment has inspired him to propose machinery for turning bothersome lunar dust into useful solids: rocket landing pads, bricks for habitats, radiation shielding, even roads and radio antenna dishes.

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