Like a sort of homeworld security force, teams of astronomers
are keeping their eyes on the sky for potential deadly invaders
from space. They’ve already found most of the really dangerous
ones, and now they’re looking for those that are the next
threat-level down. The potential invaders are asteroids with
orbits that come close to Earth’s orbit around the Sun. These
giant boulders could someday hit Earth, causing widespread
damage. In the 1990s, Congress instructed NASA to find and plot
the orbits of at least 90 percent of all the near-Earth
asteroids that are one kilometer in diameter or larger — large
enough to cause global damage if they hit us. And the search
teams that NASA’s funded think they’ve done just that. They’ve
found hundreds of objects in that category, but none on a likely
collision course. Now, the search has been expanded to asteroids
that are as small as 140 meters in diameter — about 450 feet.
That’s big enough to cause major regional damage, but not
global. Just last year, searchers foun