What is an Asteroid?

A mathematical sequence in the distances of the planets from the sun was discovered by a mathematician named Johann Titus and an astronomer named Johann Bode in 1772. This sequence predicted that a planet should be orbiting between Mars and Jupiter 2.8AU (2.6x108 mi / 4.2x108 km) from the sun. In 1801, Giuseppi Piazzi, an Italian astronomer discovered a faint object at that distance which he then named Ceres. Since Ceres was fainter than Mars or Jupiter, Piazzi concluded that it was much smaller. Many other small bodies were later discovered around that area. These objects were named asteroids (meaning star-like) or minor planets.

Asteroids are irregularly shaped, varying 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). There are 20,000 known asteroids and they orbit the sun in between the orbits of Mars and Jupiter. Asteroids rotate in periods of 3 to 30 days based on the fluctuations in their brightness.

Little is known about asteroids besides size, shape and rotation. Their mass is difficult to approximate as they are not large enough to affect the gravity of Mars or Jupiter. Their densities are about 2 to 4 g/cm3, which is typical of rocky bodies.

Asteroids are classified according to the spectra of light they reflect:

• C - Dark, probably carbon-containing (carbonaceous)
• S - Twice as bright as C, probably made of stony iron
• M - Similar to iron meteorites
• P and D - Low brightness, reddish

Asteroids appear to be of two different origins:

• Primitive, essentially unchanged pieces of the early solar system (C)
• Smashed remnants of differentiated pieces of the solar system

It is believed that asteroids are the remainders of planetismals, which are early pieces of the solar system formed between Mars and Jupiter. Some of the planetismals began to form planets but were smashed due to Jupiter’s gravity, while others did not even begin to form planets.