Back in the early 1990s, Dave Tholen, an astronomer at the University of Hawaii, had an intellectual itch he couldn't quite scratch. His specialty was finding and mapping asteroids?"he had even served on an early "detection committee" that NASA had helped organize. The committee had recommended that Tholen search for asteroids in what's known as the "opposition region" of the sky?"the part opposite the sun, with Earth in between?"because that's where their models had suggested most asteroids might be found. Tholen, however, had other ideas. "It seemed like circular reasoning," he told me. "You're basically saying that the best place to look in the sky is where we have been looking." What if, he wondered, he were to search for asteroids whose orbits lay between the Earth and sun? "Something like Venus or Mercury, but on a more eccentric orbit," he described it to me. If it became an impact hazard, he said, "we'd never see it coming, because it's a