Giuseppe Piazzi discovered the first asteroid, Ceres, on January 1, 1801. (Picture of Giuseppe Piazzi Image Credit: Palermo Astronomical Observatory)

Three of the largest asteroids are 1 Ceres (diameter about 1,003 km), and 2 Pallas and 4 Vesta (diameters of about 550 km). (Asteroids are usually referred to by both number and name). Only Vesta can be seen with the unaided eye.

The larger asteroids are roughly rounded rocks. The smaller asteroids have irregular shapes. Some asteroids may have satellites of their own.

Around 200 asteroids have diameters of more than 100 km. There are thousands of smaller asteroids. More than 10,000 asteroids have been cataloged and named so far.

All the asteroids in the solar system together weigh much less than the Moon.

Since the asteroid belt is very large and there are not millions of asteroids, the average distance between the asteroids is around 100,000 kilometers. So it is not really tighly packed with asteroids. The Voyager 1 and 2 spacecrafts flew through the asteroid belt without being hit by asteroids. Picture of Voyager spacecraft

Asteroids might be the raw material of a planet that might have formed - if it was not for the massive gravitational forces of Jupiter that prevented this from happening.

Some asteroids are the nuclei (core/head) of comets that are no longer active.

In 1977 the asteroid 2060 Chiron was discovered in an orbit between that of Saturn and Uranus.

The Galileo Spacecraft took close-up pictures of 951 Gaspra in 1991. The photographs showed 951 Gaspra have lots of impact crates.

The Galileo spacecraft also took pictures of the asteroid 243 Ida and its moon, named Dactyl. Several other asteroids have been found to have their own moons.

The spacecraft NEAR Shoemaker studied the asteroid Eros for a year in the year 2000. It was placed in an orbit around Eros. After its mission ended NASA landed it safely on Eros in February 2001. Here is the press release about this event.

Asteroids can be classified (by composition and albedo) into a few distinct types: C type, S type and M-type.

3753 Cruithne is an asteroid that has an unusual orbit near Earth. Cruithne is pronounced 'croo-EEN-ya'. Cruithne shares Earth's orbit, but it does not actually orbit the Earth. Here is detailed information about Cruithne and its unsual orbit.

Most asteroids belong to the C type: dark carbonaceous (made of carbon) chondrites. Astronomers think that these asteroids are the oldest material in the solar system.

S type asteroids are relatively bright stony iron meteorites. The next class, the M-type (irons), asteroids are much rarer. These asteroids are bright too.

Astronomers have calculated that, the chances of a collision between Earth and an asteroid averages out to only one collision about every 300,000 years.

Sources

The Columbia Encyclopedia, Sixth Edition. Columbia University Press
Encarta Encyclopedia