Asteroids are solid pieces of rock left over when the planets were formed, 4,600 million years ago. At that time the Solar System was a huge cloud of gas and dust. The center of the cloud came together to form the Sun, It grew big and hot. Other parts of the cloud formed the planets and their moons, or satellites. The pieces left over formed the asteroid belt. Meteorites and comets still whiz through space, some on a set orbit, others randomly

Several hundred thousand asteroids have been discovered and given provisional designations so far. Thousands more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth.

Icarus, Adonis, Eros and Hidalgo are all wandering asteroids. Instead of orbiting the Sun in the asteroid belt they go both closer to and farther away from the Sun. The path of Eros, Adonis and Icarus cross the path of the Earth. Eros is 23km (14 miles) in diameter. The others are smaller.

The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak bright light in the sky (a "shooting star" or a "falling star") produced by the entry of a small meteoroid into the Earth's atmosphere. If you have a dark clear sky you will probably see a few per hour on an average night; during one of the annual meteor showers you may see as many as 100/hour. Very bright meteors are known as fireballs.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in 300,000 years on average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences.

The impact of a comet or asteroid hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago.

Comets have been described as dirty snowballs. Most comets are frozen ice, rock and other materials left over from the formation of the Solar System. Most are less than 10 km's across.

Comets have in the past shaped how life on this planet has developed, it is thought that some have hit the planet so causing most life to be killed off, just like the dinosaurs. It is suspected that the dinosaurs were killed off by an asteroid rather than a comet. But the comet have helped to bring vast amounts of water to Earth, allowing it to evolve into a life bearing planet.

There are many comets that are seen over a set period. Halleys comet is just one of the well known comets. Towards the end of this millennium two comets were seen, these were comet Hale-Bopp and comet Hyakutake.