A near-Earth asteroid is an asteroid whose orbit occasionally brings
it close to Earth. Most, if not all, such asteroids appear to have come
from the asteroid belt and have been
given new orbits through collisions with other asteroids or the gravitational
influence of Jupiter. There are three main
groups: the Aten group (which stays mainly
inside Earth's orbit), the Apollo group (which crosses Earth's orbit), and the Amor
group (which orbits mostly between Earth and Mars).
Estimates suggest there are roughly 100 Atens, 700 Apollos, and 1,000 Amors
with diameters greater than about one km. Because these asteroids travel
in orbits that cross or approach Earth's orbit, some of them can, occasionally,
pass relatively close to our planet or even threaten to collide with it. Potentially hazardous
asteroids are coming under increasing scrutiny.
TYPES AND GROUPS