Minor Planets

Three main NEA groups have been identified: the Aten group, the Apollo group, and the Amor group. An NEA is assigned to one of these groups depending on how nearly it approaches Earth's orbit and whether it makes its

closest approach near perihelion (the asteroid's point of nearest approach to the Sun) or aphelion (its point of greatest distance from the Sun). The Aten asteroids, for example, make their closest approach to Earth's orbit when they are at aphelion; the Amors when they are near perihelion.

The NEAs have received considerable attention in recent times—including several popular science-fiction books and movies—because such asteroids occasionally collide with Earth. A truly catastrophic collision with an asteroid larger than 6.2 mi (10 km) in diameter occurs on average once every 100 million years; asteroids on the order of 0.62 mi (1 km) in diameter strike the Earth every few hundred thousand years; and asteroids in the 200–300-m range strike the Earth about every 60,000 years. Following a mandate of Congress, NASA has been performing a systematic sky-search for NEAs with diameters 1 km and greater since 1998. The goal is to find 90% of all NEAs 1 km and larger by 2008; over 635 such NEAs had been discovered as of late 2002. Only one of these objects seems to have any chance of actually striking Earth, and the probability of a collision (in the year 2880) is about presently estimated at about 1 in 300. However, many NEAs probably remain to be discovered, and a body much smaller than 1 km in diameter could still destroy much of human civilization.

A number of schemes to divert any asteroid that might be found to be on a collision course with Earth are being discussed, including the detonation of nuclear weapons near the asteroid, the focusing of sunlight on it using large, space-based mirrors, and the attachment of rockets or "mass drivers" to its surface; however, no general agreement has yet been reached on the best method. Planning is complicated by the fact that some asteroids may turn out to be loose assemblages of floating rubble rather than large, solid rocks; if so, diverting them would be more difficult.