Near-Earth Asteroid

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Asteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt by a combination of asteroid collisions and the gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. Approximately 250 NEAs have been found to date, probably only a few percent of their total population. The largest presently known is 1036 Ganymed, with a diameter of ~25.5 miles (41 kilometers). The NEA population appears to represent most or all asteroid types found in the main belt. NEAs are grouped into three categories, named for famous members of each: 1221 Amor, 1862 Apollo, and 2062 Aten. The Amors cross Mars’ orbit but do not quite reach the orbit of Earth. The Apollos cross Earth’s orbit and have periods >1 year.  The Atens cross Earth’s orbit and have periods <1 year. With expected lifetimes of around 10 million years, the ultimate fate of NEAs may be gravitational ejection from the Solar System or collision with one of the terrestrial planets.

Some or all content above used with permission from J. H. Wittke.

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