These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. Several thousands of asteroids are known to exist but it is thought that there must be around 100,000 in all, most of which are too small or too faint to be detected from the Earth at present.
The first asteroid, Ceres, was discovered by Giuseppe Piazzi in 1801. The smallest detected asteroids have diameters of several hundred feet. Together with comets and meteroids, asteroids make up the minor bodies of the solar system. They are the left over parts from the formation of the solar system. The vast gravitational pull of Jupiter is thought to have stopped the members of the asteroid belt from forming a planet.
Some asteroids travel in eccentric, elliptical orbits which cross the orbit of the Earth. These are known as Earth crossing asteroids. For purposes of cataloging they are numbered sequentially as they are discovered.
Some asteroids travel in pairs, spinning around a common centre.
- Most asteroids fall into three classes:
- C-types: dark, carbonaceous rocks, silicates - roughly 72 percent of all asteroids.
- S-types: grayish stony asteroids, nickel + iron + silicates - roughly 13 percent of all asteroids.
- M-types: metallic, nickel + iron - roughly 10 percent of all asteroids.
- Near Earth asteroids, ie those that come close to Earth are categorized as:
- Aten Asteroids - have orbits that lie inside the Earth′s orbit.
- Amor Asteroids - have orbits that cross the orbit of Mars and approach Earth′s orbit.
- Apollo Asteroids - have orbits that cross the Earth′s orbit.
- Arjuna Asteroids - orbit the Sun in a near circular path and are no more than 100m in diameter.