Asteroids are rocky objects in the Solar System that are much smaller than planets. Most of them are gathered in a belt that lies a third the way between the orbits of Mars and Jupiter. Largest amongst them is Ceres, which is today classified as a dwarf planet.
It used to be thought that the countless asteroids were fragments of a former planet that broke up. However, today it is accepted that they are debris left over from the formation of the Solar System. They were prevented from collecting together to form a planet by the powerful disruptive pull of mighty Jupiter.
Mystery of the missing planet
Before any asteroids were known, astronomers had measured the distances of the planets, and found a puzzlingly larger gap between Mars and Jupiter. Some in the 18th century became convinced that an as yet undiscovered planet must be lying in the region.
Hungarian nobleman Baron Franz von Zach organised a hunt for it in 1800, allocating different observers a region of sky to search. They became known as the Celestial Police.
On New Year’s Eve, 1800, a previously unrecorded object was identified in the constellation of Taurus, though not by one of the Baron’s team. A Sicilian monk, Giuseppe Piazzi, spotted it from Palermo Observatory and followed its movement for several weeks. He had discovered Ceres.
If Ceres was the missing planet, however, it was not alone. In 1802, Heinrich Olbers, of the Celestial Police, located a second object which was named Pallas. Karl Harding found a third, Juno, in 1804, and Olbers discovered his second, Vesta, in 1807. Ceres and Vesta were both orbited and studied in great detail in recent years by NASA’s Dawn space probe.
The asteroids were called planets at the time they were discovered, but, it became clear that they must be a lot smaller than traditional planets. So instead, leading astronomer of the time William Herschel came up with the term asteroid, meaning starlike, to describe them. They are also often referred to as minor planets.
More than 500,000 asteroids have been detected since those first discoveries, with 200 or so bigger than 100 km across and 750,000 bigger than 1 km. Though the vast majority are Main Belt asteroids between Mars and Jupiter, some are grouped together and locked in locations relative to the orbits of various planets.
Others have been steered into orbits that bring them across the orbit of Earth, and they are known as Near Earth Asteroids, or Potentially Hazardous Objects because of the low risk that one could collide.
Asteroids come in three main types – carbonaceous (rocky clay), silicaceous (a stony-iron mix), and metallic. A number have been found to have smaller asteroids orbiting around them, and one even has its own set of rings. If you added all the asteroids together, you still wouldn’t have enough material to make an object the size of our Moon.