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Most of the asteroids in our solar system lie between 2.2 and 3.3 AU, in the region know as the asteroid belt. The asteroid belt is located between the orbits of Mars and Jupiter. This happens for a reason; Mars and Jupiter are so far apart that stable orbits of small bodies (a.k.a. asteroids) can exist in the region between them.
These asteroids are too small to be seen without a telescope; hence they were not discovered until the beginning of the 19th century. At that time, astronomers were searching for another planet they thought should be in the large gap between Mars and Jupiter.
In January of 1801, the Sicilian astronomer Giovanni Piazzi thought he had found the missing planet when he discovered the first asteroid, which he named Ceres (which is about the size of the state of Texas), orbiting at 2.8 AU from the Sun. However, his discovery was followed by 3 more discoveries of mini-planets within the next 3 years. Clearly, there was not a single missing planet between Mars and Jupiter, but rather a whole bunch of objects. By 1890 more than 300 had been discovered by sharp-eyed observers. In that year, Max Wolf of Heidelberg introduced astronomical photography to the search for asteroids, which greatly sped up the discovery of additional objects. More than 10,000 asteroids now have well-determined orbits.
Major Asteroids in the Asteroid Belt
Name (km) (10^6km) Albedo AU Discoverer Date
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1 Ceres 457 413.9 0.10 2.77 G. Piazzi 1801
511 Davida 168 475.4 0.05 3.18 R. Dugan 1903
15 Eunomia 136 395.5 0.19 De Gasparis 1851
52 Europa 156 463.3 0.06 3.10 Goldschmidt 1858
10 Hygiea 215 470.3 0.08 3.14 De Gasparis 1849
704 Interamnia 167 458.1 0.06 3.06 V. Cerulli 1910
2 Pallas 261 414.5 0.14 2.77 H. Olbers 1802
16 Psyche 132 437.1 0.10 2.92 De Gasparis 1852
87 Sylvia 136 521.5 0.04 3.48 N. Pogson 1866
4 Vesta 262.5 353.4 0.38 2.36 H. Olbers 1807
951 Gaspra 17x10 205.0 0.20 Neujmin 1916
243 Ida 58x23 270.0 ? J. Palisa 9-11-1884
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Asteroid Belt