As of August 2015, the Apollo asteroid group includes a total of 6,923 known asteroids of which 991 are currently numbered. Asteroids are not numbered until they have been observed at two or more oppositions. More than a thousand Apollo asteroids are 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 one thousand people with flying glass from broken windows, was an Apollo class asteroid.