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(ala 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 name Ceres, 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 sharp0eyed 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.