ASTEROIDS

There are minor bodies of the Solar system, for the most part consisted of silicates and metals. Most of them are small, of some meters until tens o'clock of kilometers, and of very irregular forms. Few ones reach several hundreds or up to thousand kilometers in diameter. This is the case of Ceres, the first asteroid, discovered in 1801 for Giusseppe Piazzi.

Asteroid (433) Eros. Reconstruction of the images taken the spatial probe NEAR-Shoemaker in February, 2000. Credits: NEAR Project, NLR, JHUAPL, Goddard SVS, NASA.

Almost all the asteroids are in the region between Mars and Jupiter known like principal belt. This has been the first bodies ring minor known (the second one was the transneptuniano). In the first stages of the evolution of the Solar system million bodies formed of up to some hundreds of km of diameter, from the aggregation of the silicates and metals that were abounding in the region of the terrestrial planets. While those who formed in the interior region Mars joined giving place to the terrestrial planets, those that formed a little further on of Mars could not join to form another planet. The closeness of Jupiter modified his orbits in such a way that, on having hit, between themselves they were doing it at speeds so high that, instead of joining to form a major object (as it happened to him to the most interior objects), more children were breaking the sliced objects.
Not all the asteroids are in the principal belt; some of them have been ejected of this one due to gravitational perturbations and mutual collisions. The orbits of some of these ejected asteroids approach to the Earth and all those ones whose minimal distance to the Sun is minor than 1,3 times the distance of the Earth are considered to be Nearby Asteroids (or NEA, of English near earth asteroids). Some NEA are potentially dangerous since they can collide with the Earth.

Glossary: "100 basic concepts of Astronomy”