In 1801, when Italian astronomer Giuseppe Piazzi discovered an object orbiting between Mars and Jupiter, some astronomers thought he had found a new planet. But in fact he had found something far more enigmatic: the asteroid belt. Today astronomers are still trying to figure asteroids out. The largest ones are round and regular like miniature planets, but the smaller ones are lumpy and potato-shaped. The assumption has always been that most asteroids are solid rock. Recently, however, astronomer Alan Harris of the Jet Propulsion Laboratory in Pasadena, California, has found evidence that many asteroids may be nothing more than flying clumps of rubble, loosely held together by the mutual gravitational attraction of the pieces.
An isolated asteroid exerts so little gravitational pull on anything else that astronomers can’t be sure how densely packed it is; the only way to measure the mass and thus the density of an object in space is to observe its gravitational effect on other objects. But in 1979, when Harris first looked into the problem, he came up with a clever way of getting an indication of an asteroid’s solidity. If it was not solid, he reasoned, and was thus bound only by its own gravity, it could only spin at a certain maximum rate before loose material would fly off it like water off a shaking dog. The cutoff point, Harris calculated, was one rotation every 2.25 hours: an asteroid rotating faster had to be solid; a slower one might be just a pile of gravel (although it wouldn’t have to be).
Seventeen years ago, rotation rates were known for only a handful of small asteroids, but as telescopes have gotten better, that situation has changed. By soliciting data from researchers around the world, Harris and jpl colleague James Young surveyed the rotation rates of 107 asteroids and found that not a single asteroid in their survey spins faster than once every 2.25 hours; not one, in other words, needs anything stronger than gravity to hold itself together. That doesn’t prove the asteroids aren’t solid, but according to Harris, it makes it extremely unlikely that all or even most of them are. If most were solid, there would be a range of rotation rates, and the sharp cutoff at the two-and-a-quarter-hour rotation mark wouldn’t exist.
Are all asteroids nothing but glorified rubble heaps? Even detailed photos of asteroids, like those taken by the Galileo spacecraft, are open to interpretation. I have been skeptical of the ability of those images to actually define whether they are solid bodies or whether they are multiple chunks, Harris says. It’s really hard to tell the difference between a crater formed in solid material and a crater formed in a powder.
Some asteroids no doubt are solid. People have found meteorites-- asteroid bits that have hit Earth--as large as 66 tons. And just last year astronomers in Australia reported the first asteroid with a rotational period under two hours. Duncan Steel of the University of Adelaide says the rapid brightening and dimming of asteroid 1995 hm suggests it may be a rock half a mile long and 100 yards wide. But even Steel says that this one exception doesn’t invalidate the rubble-pile model.
Harris believes that model applies to most asteroids as large as 60 miles across. Astronomers will get their best chance to test his ideas in 1999, when a recently launched probe called the Near-Earth Asteroid Rendezvous settles into orbit around the asteroid Eros. Unlike Galileo, which whizzed by two asteroids, near will spend enough time orbiting an asteroid to measure its gravitational pull. That will reveal Eros’s mass, which in turn will tell astronomers whether Eros is a slab or a gravel heap.
The latter would be unsettling news for people concerned about the possibility that an asteroid might one day strike Earth. Rubble flying at tens of thousands of miles per hour would do about as much damage as a solid rock--and it would be a lot harder to deflect with, say, a nuclear bomb. If what’s coming your way looks more like a sandbag than a rock, you may have a real problem trying to push it aside, says Harris, who has studied the bomb idea. It reopens the issue of whether nuclear weapons are the right approach to deflect these things.