What is an asteroid?
An asteroid is a small, rocky or metallic body orbiting the Sun, left over from the formation of the planets. Most are found in the asteroid belt between Mars and Jupiter.
Around 4.6 billion years ago, the Sun was surrounded by a disc of gas and dust. Grains stuck together, grew into bodies kilometres across and eventually built the planets. Not all of that material made it into planets. The leftovers that remain as rocky or metal-rich bodies are asteroids, sometimes called minor planets. They range from boulders a few metres across to Ceres, about 940 kilometres in diameter, which is massive enough to be round and is now classed as a dwarf planet.
Most asteroids orbit in the asteroid belt between Mars and Jupiter. Others share Jupiter's orbit (the Trojans), and some, the near-Earth asteroids, have orbits that bring them into our part of the Solar System. More than a million asteroids are known, and new ones are found every night by survey telescopes.
Asteroids are not all alike. Some, like Vesta, melted early in their history and separated into a metal core and rocky crust, just as Earth did. Others, such as Ryugu and Bennu, are loosely bound piles of rubble rich in carbon and water-bearing minerals, barely changed since the Solar System formed. Spacecraft have visited many, including Eros, Itokawa, Vesta, Ceres, Ryugu, Bennu and the double asteroid Didymos and Dimorphos, which NASA's DART spacecraft struck in 2022.
Asteroids matter to meteorite enthusiasts because they are where most meteorites come from. Collisions in the belt knock off fragments, and gravitational nudges, especially from Jupiter, gradually steer some onto paths that cross Earth's orbit. A meteorite in your hand is therefore usually a small piece of an asteroid, carrying a record of conditions at the birth of the planets.