­­ Q: How did a NASA spacecraft land on an asteroid? A: On Feb. 12, 2001, the Near Earth Asteroid Rendezvous spacecraft made a controlled descent to the surface of asteroid 433 Eros, after orbiting it for a year. But it wasn't ever supposed to land. Instead, the plan was to crash it into the asteroid. After the year's orbit was up, NEAR already met all of it’s objectives and was quickly running out of fuel, so scientists decided to try to land it.

­ 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.