Minor Planets - The Discovery Of Asteroids, Main-belt Asteroids, Beyond The Main Belt, The Collision Threat
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There are many thousands of minor planets, also termed miniplanets or asteroids, within our solar system. They vary in size from a foot or so in diameter to hundreds of miles in diameter. The majority of asteroids, the main-belt asteroids, circle the Sun in a donutshaped region between the orbits of the planets Jupiter and Mars. Several other asteroid families have been identified, including a large population of objects beyond the orbit of Neptune termed the Kuiper belt and approximately 70 objects, the Plutinos, that circle the Sun on remote, elliptical orbits resembling Pluto's. Controversy has raged among planetary astronomers in recent years over whether Pluto itself should be counted as a large asteroid rather than as a true planet. There is also a significant population of asteroids having orbits which cross, or come near to crossing, that of Earth; these are termed near-Earth asteroids (NEAs).
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The first asteroid was discovered serendipitously by the Italian astronomer Giuseppe Piazzi (1746–1826) on the night of January 1, 1801. This asteroid, subsequently called Ceres after the Roman goddess of corn and harvests, has a diameter of 584 mi (940 km) and is the largest asteroid in our solar system. The next three largest asteroids, Pallas, Juno, and Vesta, were discovered in 1802, 18…
Main-belt asteroids revolve around the Sun on nearly circular orbits. It takes a main-belt asteroid 2.8–8.0 years to complete one circuit around the Sun. Images obtained by NASA's space probe Galileo, presently in orbit around Jupiter, have confirmed the long-held belief that asteroids are irregularly shaped objects. Galileo imaged the asteroid Gaspra in October 1991 and found it to …
Not all asteroids reside in the main belt. The asteroid Hidalgo, discovered by American astronomer Walter Baade in 1920, for example, travels along an orbit which takes it from the inner edge of the main belt (2.0 AU from the Sun) to beyond the orbit of Saturn (9.7 AU). Likewise, the strange asteroid Chiron, discovered by American astronomer Charles Kowal (1940–) in 1977, moves along an orb…
Three main NEA groups have been identified: the Aten group, the Apollo group, and the Amor group. An NEA is assigned to one of these groups depending on how nearly it approaches Earth's orbit and whether it makes its
closest approach near perihelion (the asteroid's point of nearest approach to the Sun) or aphelion (its point of greatest distance from the Sun). The Aten asteroid…
In February 2000, asteroid studies took a remarkable jump forward when the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft, first of NASA's "Discovery" series of relatively low-cost space probes, went into orbit around the asteroid 433 Eros, an S-class asteroid about 8 × 8 × 21 mi (13 × 13 × 33 km) in size. NEAR-Shoemaker, the first s…
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