Earthbound on rocky, chaotic course.

The origin of stones that fall from the sky and are recovered as meteorites is a long-standing puzzle. Although many people suspect that the most common type of meteorite comes from the jumble of asteroids found in a wide belt between the planets Jupiter and Mars, until recently no one could adequately explain how any of these rock fragments manage to get into orbits that cross the Earth's path.

The missing ingredient may be the peculiar behavior of trajectories within a "chaotic zone" that appears to coincide with a gap in the distribution of asteroids, says Jack Wisdom of the Massachusetts Institute of Technology. Asteroids that find themselves in this chaotic region may spend as much as a million years within nearly circular orbits. Then suddenly their orbits stretch to become so elliptical that they cross the paths of Mars and Earth and, over the years, get swept up by the planets. This process leaves a gap in the asteroid belt.

In his research, Wisdom focused on the absence of asteroids in a gap about 2.5 times the earth's distance from the sun. Particles found at this distance would complete three orbits around the sun in the time that it takes Jupiter to circle once. Wisdom discovered that Jupiter induces a chaotic Zone that accounts for the precise size and shape of the gap. This zone is defined by a range of initial positions and velocities, which commit particles that start with those characteristics to an erratic path.

In the roller derby of the asteroid belt, collisions between chunks of rock continually spray debris in all directions. If any one of these fragments happens to be in the right place and has the right velocity, then its future course become essentially unpredictable. In this chaotic regime, two particles with almost identical initial positions and velocities can end up in very different orbits. In the June 27 NATURE, Wisdom calculates that one out of five of these particles ends up on an earth-crossing orbit within 50,000 years.

When Wisdom first presented his idea two years ago, most astronomers were skeptical. "Many people thought that if you had such unusual behavior," says Wisdom, "it must be an artifact of the method rather than its true behavior." For the results described in his NATURE paper, Wisdom found enough computer time to do the necessary calculations using an accepted mathematical technique instead of the unconventional method that initially led to his discovery.

One person who took Wisdom's idea seriously was George W. Wetherill of the Carnegie Institution of Washington (D.C.). Wetherill calculated what the orbits of these meteorites would look like when they hit the earth. The results matched earth-based observations almost exactly. "The most abundant type of meteorite, the ordinary chondrite," says Wetherill, "is indeed derived from the mechanism discovered by Wisdom."

Comments Carl D. Murray of the University of London in England, "The beauty of the mechanisim is that it does not involve any complicated procedures that require more than one close approach to another planet." Other gaps in the asteroid belt also appear to be associated with chaotic behavior. Wisdom is now trying to trace out the boundaries of these chaotic zones more carefully. "If you understand the dynamics," he says, "then you can see to what extent the distribution of the asteroids can be explained. But the real goal is to understand the dynamics well enough to begin looking at the primitive formation of the asteroids."

Wisdom, who has already explored the chaotic behavior of Saturn's satellite Hyperion (SN: 7/23/83, p.59), is also looking into the possibility of a chaotic zone around Pluto. Says Wisdom, "We're trying to see if we can explain some of why the solar system is the way it is."