Asteroids are small rocky objects that move in elliptical orbits in the asteroid belt between the orbits of Mars and Jupiter. Their average distance form the Sun is about 2.9 astronomical units (A.U.). Asteroids are also called minor planets.
The largest asteroids are 1 Ceres (diameter about 940 km), 2 Pallas (diameter about 580 km), and 3 Vesta (diameter about 540 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 asteroids. More than 10,000 asteroids have been cataloged and named.
All asteroids in the solar system together way much less than the moon.
Since the asteroid belt is very large and there are not millions of asteroids, the average distance between asteroids is around 100,000 kilometers. The Voyager 1 and 2 spacecrafts flew through the asteroid belt and were hit by asteroids.
Asteroids might be the raw material of a planet that might have formed. Perhaps the massive gravitational forces of Jupiter 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Some asteroids are the nuclei (core/head) of comets that are no longer active.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 photo shows 951 Gaspra with lots of impact craters.
The Galileo Spacecraft took pictures of the asteroid 243 Ida and its moon. Several other asteroids have been found to have moon(s).
The spacecraft NEAR Shoemaker studied the asteroid Eros for a year. After its mission ended, NASA landed it safely on Eros. February 2001.