Credit NASA with another milestone in space exploration: On February 12, 2001, a spacecraft landed on the surface of an asteroid for the first time in history.

After a year spent orbiting the asteroid 433 Eros, the Near Earth Asteroid Rendezvous (NEAR) spacecraft made a controlled descent to the surface. But what exactly is an asteroid? And what was the NEAR Shoemaker mission about?

In 1772, a mathematician named Johann Titus and an astronomer named Johann Bode discovered a mathematical sequence in the distances of the planets from the sun -- this sequence predicted the possibility of a planet orbiting in between Mars and Jupiter at 2.8 AU (2.6x108 mi / 4.2x108 km) from the sun. So astronomers began to search for this possible planet, and in 1801, an Italian astronomer named Giuseppi Piazzi found a faint body at that distance that he named Ceres. However, Ceres was fainter than Mars or Jupiter, so Piazzi concluded that it was much smaller. Other small bodies were later found in this same vicinity. These objects were named asteroids (meaning star-like) or minor planets.

Asteroids are small, rocky bodies that orbit the sun in between the orbits of Mars and Jupiter, which is anywhere from 2.1 AU (1.95x108 mi / 3.15x108 km) to 3.2 AU (3.0x108 mi / 4.8x108 km) from the sun. There are more than 20,000 known asteroids. They are irregularly shaped and vary in size from a radius of 1 km (0.62 mi) to several hundred kilometers (Ceres is the largest, with a radius of 284 miles / 457 km). By measuring fluctuations in their brightness, we know that many asteroids rotate in periods of three to 30 days.­