A group of asteroids that cross the Earth's orbit, but whose average distances from the Sun are greater than that of Earth; also known as Earth-crossing asteroids. Their perihelion distances are 1.017 AU (Earth's aphelion) or less. Most Apollo asteroids are small (up to 5 km diameter) and highly irregular in shape. They are named after (1862) Apollo, the first of the group to be discovered, by the German astronomer Karl Reinmuth (1892–1979) in 1932. Apollo is a Q-class asteroid of diameter 1.5 km. It came within 0.07 AU (10.5 million km) of Earth in 1932, but was then lost until 1973. Apollo's orbit has a semimajor axis of 1.470 AU, period 1.78 years, perihelion 0.65 AU, aphelion 2.30 AU, and inclination 6°.4. Apollo can approach to within 0.028 AU (4.2 million km) of Earth's orbit and can also make close approaches to Venus and Mars. Some Apollo asteroids such as Phaethon and Icarus approach closer to the Sun than Mercury. The largest Apollo asteroid is (1866) Sisyphus, diameter 8 km. Other notable named Apollos include Hephaistos, Toro, and Toutatis. The Apollo asteroids were once thought to be extinct cometary nuclei, but in fact Jupiter may perturb asteroids from near the 3:1 Kirkwood gap in the main asteroid belt into Apollo-type orbits. See also Near-Earth Asteroid.
Subjects: Astronomy and Astrophysics.