The Hale-Bopp comet is 40km long and passes Earth every 4026 years.

Amongst the planets and moons, thousands of asteroids and comets that have been catalogued and their orbits calculated - thousands more remain unknown.

Comets
Comets, are small ‘dirty snowballs’ made from a mixture of ice, frozen gases, and dust. All comets are left over from the formation of the Solar System.

Comets travel three times faster than asteroids and are only visible when near the Sun. Haley’s comet is 16km long and passes the Sun every 76 years. 

Hale-Bopp, at 40km long, swings by only once every 4026 years. It’s believed that half of the near-Earth asteroids may be ‘dead’ comets.

Meteors
Approximately twice a week, a meteor about the size of a pillow will hurtle towards the Earth and detonate with the force of an atomic bomb.  Fortunately, our atmosphere means they vaporize around five miles above the ground. If a piece of meteor survives to reach the Earth's surface, it is known as a meteorite.

Millions of meteors strike the Earth every day - most of them the size of a grain of sand. HOwever, from time-to-time, something larger crashes through the Earth’s natural defence shield - sometimes with catastrophic effect. 

It was one of these fireballs that some scientists now believe caused the extinction of the dinosaurs when it smashed into the ground at the Chicxulub in the Yucatan province of Mexico, 65 million years ago.  That one was believed to have been 8km in diameter.

Asteroids
Most asteroids behave in an orderly fashion, orbiting the Sun in the asteroid belt between Mars and Jupiter.  Some escape their orbit and pose a threat to us. 

Asteroids are believed to be debris left over from the formation of the Solar System 4.6 billion years ago.

Thought to be made from rock and metal, asteroids range in size from pebbles to 934km across.