Exploring the Planets

Discovery of Asteroids

In 1801, while making a star map, Giuseppe Piazzi accidentally discovered a small object 1000 kilometers (600 miles) in diameter between the orbits of Mars and Jupiter. Piazzi named the object Ceres. It was the first asteroid to be discovered. In 1802, two more were discovered. Asteroids are numbered in order of their discovery and many have been named. For example, the first asteroid discovered is 1 Ceres, the second is 2 Pallas, etc. More than 400,000 asteroids have been discovered in the main asteroid belt.

In August, 2006, the International Astronomical Union passed a resolution redefining how celestial bodies are classified. Under this new classification system, Ceres is designated a dwarf planet because it orbits the Sun, has enough mass to form in a spherical shape, has not cleared the area around its orbit, and is not a satellite.

In 2004, the Hubble Space Telescope captured this image of Ceres (right).