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Planetary science. A windfall for defenders of the planet.

If U.S. astronauts hopscotch from one near-Earth asteroid practicing for their trip to Mars (see main text), it will be a big help to astronomers campaigning to find and fend off the rare NEAs that threaten to collide with Earth. NASA-led astronomers have now found about 90% of the estimated 1000 NEAs a kilometer across and larger that could have been on a collision course with Earth. It turns out that none actually are, so the risk in this century of a civilization-ending impact is essentially gone. But that leaves the other part of the threat, the one from objects 140 meters in diameter and larger—so-called city killers. Only about 5% of them have been found under NASA9s search program. And at current discovery rates, most of the city killers would remain undetected for many decades.