When the Near Earth Asteroid (NEAR) spacecraft eases into orbit around the asteroid 433 Eros next year, dust may be one of its first discoveries. Earlier this year, the Mars Observer spacecraft found that the Red Planet's larger moon, Phobos, is coated with dust up to a meter (3.3 ft) deep.
"That was a surprise," said Dr. James Spann of NASA's Marshall Space Flight Center. "I'm not sure that anyone had an idea of what things look like on these small bodies. I don't imagine that Phobos is so unique that it would have dust and the rest of the small bodies like it won't."
Spann is the director of the Dusty Plasmas Laboratory that was established at NASA's Marshall Space Flight Center earlier this year. It was designed to let scientists study the tiniest motes of dust in space-like conditions. Understanding how dust grains interact with each other, and how they react when exposed to sunlight will provide insight into how planets and stars form.
The Mars Global Surveyor did not see the dust directly, but inferred its presence by measuring how quickly Phobos loses its body heat when the sun sets. The rapid loss is best explained by a huge surface area - much greater than if Phobos were a solid, relatively smooth body. A coating of dust would provide the increased surface area - the surfaces of all the miniscule grains added together - to radiate heat into space. This is why, for example, a desert cools so quickly after sunset.
So why did dust collect on Phobos, and are Eros and other asteroids also are coated with dust?
NEAR, one of NASA's Discovery-class missions, was launched on Feb. 17, 1996, and has already explored one asteroid, 253 Mathilde, in a quick flyby on June 27, 1997. NEAR will rendezvous with 433 Eros on January 10 and settle into an orbit that will be gradually tightened until it lands on the asteroid. Although it does not carry a thermal emission spectrometer like that on the Mars spacecraft, it may return some information about dust on Eros.
"It may be that electrostatic forces keep those grains on there," Spann said. Because of the low mass of such bodies, "I don't think that gravity would do it."
Spann noted that we tend to think of gravity as the only dominant force in the universe. Indeed, it holds solar systems and galaxies together, and makes leaving this planet an expensive undertaking. But other forces work at smaller scales.
"When you get really tiny grains of material, the surface area relative to the mass becomes more important," Spann explained. "The way small grains respond to electrostatic forces is different from our perceptions of billiard balls colliding."
Asteroids and dust grains alike probably subject to the same surface charging effect that manmade satellites experience. Ultraviolet light and X-rays from the sun knock electrons off the satellite's surface material, thus building a slight electrical charge. Asteroids, over the billions of years in deep space, would encounter a lot of dust.
Grains charged by sunlight will be attracted to or repelled by each other. Studying that effect is one of the tasks of the Dusty Plasmas Lab.
"When small bodies come in contact with each other or a larger body, the van der Waals force begins to dominate and things happen," Spann said. The Van der Waals force is a weak attraction between very tiny bodies. "It's like dust on your cabinets. It gets there and sticks."
This opens the possibility that Eros might have a coating of dust. Does it also suggest that some asteroids are giant dust bunnies?
"Some of the models for planetary formation are nothing more than giant dust bunnies that accrete enough mass to compact and become solid," Spann said. "I don't think that's the case now because we have some ideas of the masses of the known asteroids."
But it may turn out that in addition to wiping out dinosaurs and other species every hundred million years or so, that asteroids have also been sweeping up much of the dust in the solar system. And that could make them a storehouse of primordial material for exploration.