The asteroid Ida as taken by the Galileo spacecraft on its way to Jupiter. August 28, 1993.
Source: Courtesy NASA/JPL-Caltech
Asteroids are huge boulders of metal and rock found mostly in the asteroid belt, between the orbits of Mars and Jupiter. It is here that most asteroids settled after the formation of the solar system.
There are four main asteroid types, based on composition: Carbonaceous (carbon and organics, more common in the outer asteroid belt), Silicaceous ('stony' asteroids, common in the inner asteroid belt), Metallic (mostly iron & nickel alloy) and Enstatite (MgSiO3, quite rare).
To date over 4000 asteroids have been named. The largest are:
Ceres (diameter: 940 km)
Vesta (diameter: 555 km)
Pallas (diameter: 538 km)
Hygeia (diameter: 450 km)
Ceres was the first asteroid discovered, in 1801. The brightest is Vesta, with a magnitude as low as 6.5. It is the only asteroid ever visible to the unaided eye.
There are other groups of asteroids outside the asteroid belt. The Trojans occupy the same orbit as Jupiter, but are found 60° either side of the planet.
Apollo asteroids are also of interest to us. These are asteroids whose orbits approach that of the Earth. Named after the first of this group to be discovered, in 1932, they range in size from hundreds of metres across to some the size of large cities. Astronomers now monitor the positions of these objects. Some have passed extremely close to Earth, such as Icarus at approximately 6 million km away, 1991DA less than 150 million km and 1993KAZ within 145 000 km.
It is now believed that asteroids and comets are closely related as they were formed at about the same time at the beginning of the solar system, around 4.6 billion years ago. Chiron, an object discovered in 1977, was originally designated as an asteroid, but in 1989 suddenly developed a coma and is now considered a huge comet, at least 166 km across.
Collisions between asteroids are quite common, especially in the asteroid belt. The fragments that result may become the meteorites that end up crashing into the Earth. The largest impacts in our solar system occurred between 4.5 and 3.8 billion years ago, when the space between the planets contained many more asteroids than today. After millions of years these rocks were discarded from the inner regions of the solar system and became many of the moons of the outer planets. The moons of Mars, Phobos and Deimos are now also believed to be captured asteroids from long ago.