Stronger Than Dirt
Lunar explorers will have to battle an insidious enemy—dust.
- By Trudy E. Bell
- Air & Space magazine, September 2006
(Page 3 of 4)
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.
That’s the other thing about dust: It isn’t always bad. On Mars, dust might actually help clean up its own mess. The solar panels on the twin rovers currently on Mars were expected to have been coated long ago with dust that would degrade their power output and bring the mission to a halt. More than once, though, tornado-like dust devils have scoured the panels clean and given the rovers new life.
But that’s the rare bit of good news. Dust storms on Mars will be a serious, continual worry. During Martian summer, daytime highs peak at 68 degrees Fahrenheit, and on these balmy afternoons the planet’s dust devils come alive. These are no little Arizona desert whirlwinds, a few yards across, that pass by in seconds. Martian dust devils are monster columns reaching miles into the sky and nearly half a mile across, 10 times larger than any tornado on Earth. When they pass by, the reddish sand and dust whips around faster than 70 mph, dropping the local visibility to zero for minutes at a time. And they’re everywhere. The Mars rover cameras have filmed them in action, and orbiting spacecraft have spotted their dark tracks all over the planet. “If you were standing next to the Spirit rover midday during Martian summer, you’d see half a dozen [dust devils] at any instant,” says Mark Lemmon, a Mars atmosphere specialist at Texas A & M University in College Station.
The sand in the lower part of a Martian dust devil would be a major hazard. Because the atmospheric pressure on Mars is only one percent of that at sea level on Earth, astronauts won’t feel much wind. But their spacesuits and faceplates would be pinged by high-speed material that would collect in every fold and crevice. Worse, the swirling dust and sand may be electrically charged, to the point of “possibly inducing arcing to a spacesuit or vehicle, and creating electromagnetic interference,” according to William Farrell, one of Stubbs’ colleagues at NASA’s Goddard center.
Farrell has chased dust devils across Arizona deserts and measured their electrical currents. Like levitating lunar dust, the grains of sand and dust become charged as a result of constantly banging into one another. On Mars as well as on Earth, the dust, which can blow in from anywhere, may be quite different from the local sand. When unlike materials rub together (like party balloon and shirt sleeve), one material gives up some of its electrons to the other in what’s known as triboelectric charging (“tribo” means “rubbing”). The smaller dust particles tend to take on a negative charge, having robbed electrons from the larger sand grains.
Triboelectric charging is known to occur on Mars. It came to the attention of NASA engineers building the Sojourner rover for the Mars Pathfinder even before the spacecraft left Earth. When the engineers ran the wheels for a prototype Sojourner over simulated Martian dust in a simulated Martian atmosphere, the model built up a charge of hundreds of volts. That discovery inspired the scientists to add ultrathin half-inch-long tungsten needles at the base of the rover’s radio antennas, to drain any excess charge into the thin Martian air.
Dust devils could even lead to lightning on Mars. All dust devils are powered by a rising central column of hot air, which carries the negatively charged dust upward and leaves the heavier, positively charged sand swirling near the base. The charges become separated, and the separation creates an electric field. In terrestrial dust devils, Farrell has measured electric fields of up to 20,000 volts per meter—peanuts compared to the fields in thunderstorms, where lightning doesn’t flash until the fields become 100 times stronger, enough to break apart air molecules. But 20,000 volts per meter “is very close to the breakdown of the thin Martian atmosphere,” Farrell points out. And because Martian dust devils are so tall, their stored electrical energy can be greater, possibly strong enough to unleash lightning.