Asteroids that can pass inside the orbit of Mars are said to be near-Earth asteroids. The near-Earth asteroids are subdivided into several classes. The most distant--those that can cross the orbit of Mars, but that have perihelion distances greater than 1.3 AU's, are dubbed Mars crossers. This group is further subdivided into two groups: shallow Mars crossers (1.58 < D < 1.67 AU) and deep Mars crossers (1.3 < D < 1.58 AU).
The next most distant group of near-Earth asteroids are the Amors (1.0 < D < 1.3 AU). Amor asteroids have perihelion distances greater than the Earth's present aphelion distance of 1.0 AU and therefore do not, at present, cross the planet's orbit. Because of strong gravitational perturbations produced by their close approaches to the Earth, however, the orbital elements of all the Earth-approaching asteroids, except the shallow Mars crossers, change appreciably on a time scale of a few years or tens of years. For this reason about half the known Amors, including 1221 Amor, are part-time Earth crossers. Only asteroids that cross the orbits of planets, such as the Earth-approaching asteroids and objects like 944 Hidalgo and 2060 Chiron, suffer significant changes in their orbital elements on time scales shorter than many millions of years. Hence, the outer-belt asteroid groups (Cybeles, Hildas, Thule, and the Trojans) do not interchange members.
There are two groups of near-Earth asteroids that deeply cross the Earth's orbit on an almost continuous basis. The first of these to be discovered were the Apollo asteroids, 1862 Apollo being detected by the German astronomer Karl Wilhelm Reinmuth in 1932 but lost shortly thereafter and not rediscovered until 1978. Apollo asteroids have semimajor axes (a) that are greater than or equal to 1 AU and perihelion distances that are less than or equal to 1.0 AU; thus, they cross the Earth's orbit when near the perihelia of their orbits. For the other group of Earth-crossing asteroids -- named Atens after 2062 Aten, which was discovered in 1976 by Eleanor F. Helin of the United States, D < 1.0 AU and D > 0.983 AU, the present perihelion distance of the Earth. These asteroids cross the Earth's orbit when near the aphelia of their orbits.
By mid-1991 the number of known Aten, Apollo, and Amor asteroids were 11, 91, and 81, respectively. Most of these were discovered since 1970, when dedicated searches for this type of asteroid were begun: 25 were discovered during the 1970s, 80 during the 1980s, and 49 during the first 20 months of the 1990s. It is estimated that there are roughly 100 Atens, 700 Apollos, and 1,000 Amors that have diameters larger than about one kilometre. Because these asteroids travel in orbits that cross the Earth's orbit, close approaches to the Earth occur, as well as occasional collisions. For example, in January 1991, an Apollo asteroid with an estimated diameter of 10 metres passed by the Earth within less than half the distance to the Moon.
Excerpt from the Encyclopedia Britannica without permission.