The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s. They are Earth crossing asteroids that have an orbital semi-major axis greater than that of the Earth (> 1 AU) but perihelion distances less than the Earth's aphelion distance (q < 1.017 AU).
As of November 2016[update], the steadily growing number of known Apollo asteroids has reached a total of 8,180 members. It is by far the largest group of near-Earth objects, compared to the Aten, Amor and Atira asteroids. Currently, there are 1,133 numbered Apollos. Asteroids are not numbered until they have been observed at two or more oppositions. There are also 1,472 Apollo asteroids large enough, and may get close enough to Earth to be known as potentially hazardous asteroids.
The closer their semi-major axis is to Earth's, the less eccentricity is needed for the orbits to cross. The February 15, 2013 Chelyabinsk meteor that exploded over the city of Chelyabinsk in the southern Urals region of Russia, injuring an estimated 1000 people with flying glass from broken windows, was an Apollo class asteroid.
The largest known Apollo asteroid is 1866 Sisyphus, with a diameter of about 8.5 km. Examples of known Apollo asteroids include:
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