Graphic and research by Paul Chodas, NASA Jet Propulsion Laboratory

Researchers have identified more than 2,300 asteroids and comets that are big enough to cause considerable damage on Earth and could possibly hit us. These “potentially hazardous objects” look ominous on the flat plot here, but because they travel in three-dimensional orbits, the perfect timing needed to intersect Earth makes the likelihood of collision remote. The symbol sizes shown also deceive; each object is many thousands of times as small as Earth.

NASA is concerned none­theless. Scientists estimate that they have found fewer than 1 percent of the projectiles. “We are discovering them at a rapid clip, but the population is very large,” says Donald K. Yeomans, manager of the Near Earth Object Program Office at NASA’s Jet Propulsion Laboratory. A NASA advisory group says that for $250 million to $300 million annually over 10 years, the space agency could inventory the objects and develop and test technologies that could alter a worrisome asteroid’s trajectory. One option: ram it with a massive space­craft to knock it off course. —Mark Fischetti