asteroids

Asteroids, sometimes called minor planets or planetoids, are small Solar system bodies in orbit arond the sun, especially in the inner Solar system. For almost two centries, from dicovering the first asteroid, Ceres, in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs. The innermost of these are the Kuiper belt objects are belived to be predominantly comet-like in composition, though some may be more akin to asteroids. The minor planets beyond Jupiter's orbit are rarely called "asteroids", but all are commonly lumped together under the term "asteroid" in popular presentations. We include Trojans (bodies captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more correctly be called "minor planets" instead of asteroids.Because it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System, this article will restrict itself for the most part to the classical asteroids: objects of the main asteroid belt, Jupiter trojans, and near-Earth objects.It is believed that planetesimals in the main asteroid belt evolved much like the rest of the Solar Nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. At least two asteroids, Ceres and Vesta, grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.Objects in the main asteroid belt vary greatly in size, from a diameter of 950 kilometres for the dwarf planet Ceres and over 500 kilometres for the asteroids 2 Pallas and 4 Vesta down to rocks just tens of metres across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The orbits of asteroids are often influenced by the gravity of other bodies in the solar system or the Yarkovsky effect. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families have only been recognized within the main asteroid belt. About 30% to 35% of the bodies in the main belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. 
 * Asteroid **


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