Asteroids are small objects which orbit around the Sun and which show no cometary activity nor have the potential for it. The word ‘small’ is meant to exclude planets from the definition. All known asteroids have diameters less than 1000 km down to a few centimeters. 1 Ceres is the largest known asteroid, discovered on the 1 January 1801 by Giuseppe Piazzi in Palermo, Italy with a diameter of 950 km. Asteroids are also called minor planets. The newly discovered minor body known as 2001 KX76, whose diameter makes it the largest-known minor body in the solar system is a Trans-Neptunian (TNO). Most asteroids orbit within a confined region between Mars and Jupiter, called the main Asteroid Belt.
Out of about 30 000 known objects, only 8500 asteroids have precisely determined orbits. Each of these asteroids has a definitive identification, consisting of a number that indicates its order of entry and a name proposed by the discoverer (e.g. ‘3752 Camillo’ identifies the 3752nd asteroid discovered). Each is marked by its year of discovery and two letters indicating the date of first observation as temporary identification (e.g. ‘1998 BC’ identifies the third asteroid (C) discovered during the Second half of January (B) 1998). A thousand asteroids have been detected by the Infrared Astronomy Satellite (IRAS) which provided a radiometric diameter and the albedo determination of most of them. About 700 aster- oids have the rotational period determined. Less than 1000 asteroids are larger than 30 km across, and of these, about 200 asteroids are larger than 100 km. Extrapolating the asteroid size distribution down to 1 km, an estimation of about one million asteroids whose diameter is larger than or equal to 1 km is obtained. The global mass of the population has been estimated to be of the order of one-thousandth of the Earth’s mass.
Asteroids were considered as uninteresting ‘vermin of the skies’ until it was understood that they store a rich variety of information. In fact, asteroids are believed to be the remnants, either fragments or ‘survivors’ of the swarm of Planetsimals from which the terrestrial planets were formed: the records of primordial chemical and physical processes can still be found frozen in the actual characters of asteroids. The study and analysis of these objects provides information on the nature of these primordial swarms even if differentiation and/or internal activity affected in a different way their evolution.
The current knowledge on the asteroid population has been obtained mainly from the ground-based observations. Few large asteroids are included in the observational programme of the Hubble Space Telescope (HST).NASA’s Galileo Mission approached for the first time two asteroids, 951 GASPRA and 243 IDA, offering to the planetary science community the opportunity to have a close look at two S-type main-belt asteroids.
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