The asteroid belt is composed of boulders of rock and metal that orbit in the space between Mars and Jupiter. Asteroids are generally made of clay and silicate rocks (C-type asteroids), silicate materials, and nickel-iron (S-type asteroids), or nickel and iron (M-type asteroids)[1a]. Some may also contain semi-precious stones, such as olivine (peridot).
Asteroids are prevented from becoming gravitationally bound, and forming a single object, because of the strong gravitational pull of Jupiter and, individually, most are not massive enough to become spherical.
Asteroids can also share the orbit of planets, existing in a stable position relative to the planet in a region known as a Lagrangian point. Objects that do this are known as ‘trojans’. There are thought to be as many trojans in the Solar System as there are asteroids in the asteroid belt. The vast majority share an orbit with Jupiter, but they have also been found sharing orbits with Venus, Earth, Mars, Uranus, and Neptune.
The inner Solar System from the Sun to Jupiter. Asteroid belt objects are shown in white. Other asteroids are shown in green and orange. Image credit: Mdf/Public domain.
Over half the mass of the asteroid belt is contained within four objects - Ceres, Vesta, Pallas, and Hygiea. These are all over 400 km in diameter. Italian astronomer Giuseppe Piazzi discovered the largest asteroid, Ceres, in 1801. It's almost 1000 km in diameter and was accepted as a planet for almost 50 years, it is now designated a dwarf planet.
Ceres, mosaic of images taken by the Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Public domain.
Vesta, mosaic of images taken by the Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA/Public domain.
2. Missions through the asteroid belt ↑
NASA's Pioneer 10 probe was the first spacecraft to travel through the asteroid belt in 1972, later followed by Pioneer 11, Voyager 1 and Voyager 2, and Ulysses. Ulysses was a joint mission between NASA and the ESA. It was headed for Jupiter, but the Pioneer and Voyager probes were headed even further, and have now moved beyond the Solar System.
Artist's impression of an asteroid belt. Image credit: NASA/JPL-Caltech/Public domain.
3. Missions to asteroids ↑
Six spacecraft have observed asteroids on their way to other objects:
Five attempts have been made to send spacecraft directly to asteroids:
Finally, NASA's Dawn spacecraft launched in 2007, and began to orbit Vesta in 2011. It left Vesta in September 2012, and is currently in orbit around Ceres.