Asteroids The word "asteroid", which means "starlike", indicates that these small solar system objects are not visible to the naked eye and look like bright points through a medium-sized telescope. Numerous small bodies of rock and metal, which could not aggregate into planets because of the gravitational pull of Jupiter, have been trapped between the orbits of Mars and Jupiter, in the so-called asteroid belt. Ceres, the biggest asteroid in this region, was discovered in 1801; it is about 930 km across and orbits the Sun at about 2.8 times the distance of the Earth from the Sun. Asteroid evolution Asteroids have not remained intact during the 4.6 billion years since the formation of the solar system. All of them have suffered numerous collisions and fragmentation processes. The biggest ones have melted, with the metals sinking to the core and the lighter rocky materials forming a crust (like this happened on the Earth). Finally, some of them have happened, under chaotic perturbations caused by the strong gravitational pull of the large planets, to be injected into the inner Solar System on orbits crossing the Earth's orbit. Images of asteroids Asteroids on such "Earth-crossing" orbits can then induce catastrophic impacts which are extremely rare, but may nevertheless play a role in the evolution of Life on Earth. In recent years, several asteroids have been visited by spacecraft, e.g. Gaspra, Ida (and its moon Dactyl) and Eros. The images show that their surfaces are heavily cratered by impacts.