The tracking starts at the Minor Planet Center, which archives data from a global network of professional and amateur astronomers. “We inventory the solar system,” says center director Tim Spahr. “If the world wants to know about an asteroid, we have the catalog.”
The JPL team takes orbit data from the Minor Planet Center and refines it, asteroid by asteroid. A computer program called Sentry projects each NEO’s orbit for 100 years. Once an asteroid starts approaching Earth, it will do so again and again in an orbital waltz driven by gravity as both bodies travel around the Sun. Most NEOs will plunge into the Sun after a million years of this pas de deux.
“It’s a mathematical problem, and a fascinating one at that,” says JPL’s Chodas. “It’s just exhilarating.”
A 900-foot-wide asteroid called Apophis caused a stir in 2004 when JPL calculated there was a 3 percent chance it would slam into Earth in 2029. With the next set of images, JPL’s Steve Chesley forecast a more precise orbit, and he ruled out an impact. However, there’s still a tiny chance it will hit in 2036 or 2068—depending on the exact route the asteroid follows during its next pass near Earth.
If Apophis did drift onto a collision course and was headed for Russia, a Russian military official said last year, his country might prepare a mission to knock it off course. But that would be premature, Yeomans says. “You have to be careful about moving asteroids around in space,” he adds, lest a deflection inadvertently steer Apophis toward Earth. “They should only be moved if they are a real threat.”
Among the groups studying how best to prevent a collision is the B612 Foundation, named for the asteroid in Antoine de Saint-Exupéry’s The Little Prince. Led by Apollo 9 astronaut Rusty Schweickart, the foundation has proposed a mission to a nonthreatening asteroid to test whether gravity from a hovering spacecraft could shift the asteroid’s orbit. “You don’t want to blow them up,” says Schweickart. “All you need to do to protect Earth is to push them gently.”
Exploding an asteroid would require deploying nuclear weapons in space, scientists say. They caution that no one knows how asteroid material would respond to such a blast. Some NEOs are thought to be loosely packed piles of rubble. One recent study suggests that a deliberate explosion would barely disperse the pieces, and they would reassemble under their own gravity.
In Yeomans’ mind, scientists have already demonstrated the best technique: ramming. In 2005, a NASA science mission called Deep Impact crashed an 816-pound copper mass into a comet to learn more about its icy interior. If scientists were to detect a 600-foot-wide asteroid ten years in advance, Yeomans says, it could be deflected with a two-ton projectile traveling six miles per second. He says that’s enough to make it miss the Earth. Barely.
But given the limited number of astronomers and the small telescopes scanning the sky for asteroid threats, says Yeomans, we probably won’t see a small incoming object until it’s just a week or two away from hitting us. “In that case,” he says, “all you can do is evacuate.”
Robert Irion has written about comets, black holes, new planets and other astronomical phenomena for Smithsonian.