But NASA scientists are clearly eager to speed up the rate of discovery of small asteroids, and thus expand the pool of candidate rocks for the ARM mission.
The Earth coexists with a swarm of asteroids of varying sizes. Thanks to a number of asteroid searches in the past 15 years, some funded by NASA, about 95 percent of the near-Earth objects (NEOs) larger than 1 kilometer (about three-fifths of a mile) in diameter have already been detected, and their trajectories calculated. None poses a significant threat of striking the Earth in the foreseeable future.
The science is clear: Catastrophic impacts, such as the one implicated in the extinction of the dinosaurs about 65 million years ago, are very rare, and no one needs to panic about killer rocks.
But as one goes down the size scale, these objects become more numerous and harder to detect. Congress in 2005 charged NASA with finding all the asteroids greater than 140 meters (459 feet) in diameter. Asteroids that size are generally regarded as large enough to take out a city.
According to NASA, there are also probably about 25,000 near-Earth asteroids that are 100 meters (328 feet) or larger. Only 25 percent of those have been detected, many through NASA's Near Earth Object Program. The administration is asking Congress to double the budget for asteroid detection, to $40 million, Garver said.
But the Grand Challenge would elicit help from academics, international partners and backyard astronomers. The search for NEOs took on greater urgency on Feb. 15, when, on the very day that a previously detected asteroid was about to make a close pass of the Earth, an unknown 50-foot-diameter rock came out of the glare of the sun and fireballed through the atmosphere above the Russian city of Chelyabinsk.
The asteroid's disintegration caused a shock wave that shattered windows and caused hundreds of injuries and major property damage. It was the first recorded instance of an asteroid causing human casualties. (In 1908 an asteroid exploded over Siberia and flattened trees in a vast, unpopulated area.)