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Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres.
Asteroids number in the hundreds of thousands. For longer lists, see list of exceptional asteroids, list of asteroids, or list of Solar System objects by size. Asteroid moons; A number of smaller groups distinct from the asteroid belt; The outer Solar System with the giant planets, their satellites, trojan asteroids and some minor planets. Jupiter
Outside the top four, the ranking of all the asteroids is uncertain, as there is a great deal of overlap among the estimates. The largest asteroids with an accurately measured mass, because they have been studied by the probe Dawn, are 1 Ceres with a mass of (939.3 ± 0.5) × 10 18 kg, and 4 Vesta at (259.076 ± 0.001) × 10 18 kg.
253 Mathilde, a C-type asteroid. C-type (carbonaceous / ˌ k ɑːr b ə ˈ n eɪ ʃ ə s /) asteroids are the most common variety, forming around 75% of known asteroids. [1] They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks and minerals.
Barring detailed mass determinations, [4] the mass can be estimated from the diameter and assumed density values worked out as below. = Besides these estimations, masses can be obtained for the larger asteroids by solving for the perturbations they cause in each other's orbits, [5] or when the asteroid has an orbiting companion of known orbital radius.
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff.
These lists contain the Sun, the planets, dwarf planets, many of the larger small Solar System bodies (which includes the asteroids), all named natural satellites, and a number of smaller objects of historical or scientific interest, such as comets and near-Earth objects.
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 (a > 1 AU ) but perihelion distances less than the Earth's aphelion distance (q < 1.017 AU).