Asteroid belt

Some large asteroids, to scale. Ceres contains about 40 % of the mass of the entire asteroid belt.

The asteroid belt was discovered by Astronomers after the invention of the telescope. Ancient Astronomers were aware of seven objects they recognized as planets, that regularly moved against the background of the fixed stars. In the geocentric systems they recognized, Sol, our sun, and Luna, our moon, were considered planets, like Mercury, Venus, Mars, Jupiter and Saturn.

Ancient Astronomers also monitored comets, but did not classify them as planets, because their movements weren't regular.

Telescopic observation enabled the discover of Uranus, and several smaller bodies, like Ceres, Juno and Vesta. Initially, Ceres, Juno and Vesta were also considered planets, until it became obvious that there was a belt of a large number of smaller bodies, all with similar orbits. So Ceres, Juno and Vesta were demoted, and recognized as new object, asteroids.

Ceres is by far the largest asteroid in the asteroid belt, accounting for approximately 40 percent of the mass of the entire belt. It was thought to be the only asteroid with enough mass to be spherical, until it was recognized that Hygiea was also spherical.

As of 2026 over 1.4 million asteroids have been catalogues in the asteroid belt.

The objects in the asteroid belt circle roughly between the orbits of Mars and Jupiter, although exceptional objects, like Icarus, whose orbit approaches Sol closer than Mercuury is also considered an asteroid, as are the Trojan asteroids, which share the same orbit as Jupiter.

Several hundred years after the discovery of the asteroid belt Astronomers realized there was a larger belt of object beyond the orbit of Neptune Neptune, called the Kuiper Belt. It contains many objects larger than Ceres, and is estimated to be 20 to 200 times more massive than the asteroid belt.

Some notable asteroids

name	discovered	discoverer(s)	diameter	orbital distance	notes
1 Ceres	1801	Giuseppe Piazzi	940 kilometres (580 mi)	2.77 au	First asteroid to be discovered, was initially classified as a planet.
2 Pallas	1802-03-28	Heinrich Wilhelm Olbers	512 kilometres (318 mi)	2.77 au
3 Juno	1804-09-01	Karl Ludwig Harding	234 kilometres (145 mi)	2.67 au	Tenth most massive asteroid.
4 Vesta	1807-03-29	Heinrich Wilhelm Olbers	525 kilometres (326 mi)	2.36 au
10 Hygiea	1849-04-12	Annibale de Gasparis	433 kilometres (269 mi)	3.14 au	Believed to be the largest fragment from a destructive collision, that left 7,000 bodies with similar orbits.
16 Psyche	1852-03-17	Annibale de Gasparis	222 kilometres (138 mi)	2.92 au	Psyche is the largest M-type, or metallic asteroid. Commentators speculate over if and when it should be a destination for asteroid mining
433 Eros	1898-08-13	Carl Gustav Witt, Auguste H.P. Charlois	16.8 kilometres (10.4 mi)	1.46 au	The first asteroid to be identified as a near-Earth object.
3200 Phaethon	1983-10-11	Simon F. Green, John K. Davies	5.1 kilometres (3.2 mi)	1.27 au	A near-Earth object.
65803 Didymos	1996-04-11	Joseph L. Montani	0.780 kilometres (0.485 mi)	1.64 au	In 2020 NASA smashed a space probe into Didymos's moon Dimorphis.
99942 Apophis	2004-06-19	Roy A. Tucker, David J. Tholen, Fabrizio Bernardi	0.340 kilometres (0.211 mi)	0.9224 au	When Apophis was first discovered it seemed possible it might strike Planet Earth during a close approach, in 2029.