Astronomers know surprisingly little about the Trojan asteroids, which travel along with Jupiter in two separate clumps, one preceding the planet in its orbit and the other following. Only recently, thanks to years of painstaking research, have astronomers realized that the Trojan asteroids are probably as numerous as those in the main asteroid belt between Mars and Jupiter. The reason they aren’t recognized as an important part of the asteroid population is that they are so far away. So we see only the bigger ones, says astronomer Hal Levison of the Southwest Research Institute in Boulder, Colorado.
One of the things astronomers would like to know about the Trojans is the stability of their orbits. Do any drift from the swarm and travel to the inner solar system--and perhaps intersect Earth’s orbit? For most of the Trojans, Levison says, this is not really a consideration.
Each of the two swarms clusters about a Lagrange point--regions of gravitational stability where the centrifugal force of the asteroids’ orbits counterbalances the gravitational pull of Jupiter and the sun. But some Trojans travel in orbits quite a distance from these stable points, and Levison wondered if over time these asteroids could begin to wander. He selected 36 of these atypical Trojans and projected their orbits over the next 4 billion years. Twenty-one, he found, wound up leaving the swarm.
His computer model showed that once the asteroids move far enough away from the Lagrange points, the subtle gravitational influence of the outer planets eventually provides enough pull to send the Trojans into completely different orbits. Some 1,200 Trojans have probably left their parent swarms in the last 100,000 years and are at large in the solar system, says Levison. When you do a back-of-the-envelope calculation of how often one of these guys should hit Earth, you get something like once every 500 million years, Levison says, adding that this hazard is minor compared with the threat from comets and other asteroids.
Do astronomers know of any objects that might be wandering Trojans? Not yet, says Levison, and it would be hard to distinguish them from comets, icy objects that the Trojans probably resemble and that far outnumber everything else in the solar system. There are something like 10 million comets that cross the paths of planets in the solar system, Levison says, and a million of them are inside the orbit of Neptune.