When the Sun and planets first formed there were leftovers.
These scraps from the early Solar System cooled to form asteroids and comets.
Comets are made of rock and ice.
Asteroids are predominately composed of rock, metal or carbon.
The size of asteroids varies greatly, from as small as a pebble, to hundreds of miles across.
Millions of them can be found in the asteroid belt, a vast donut-shaped ring between the orbits of Mars and Jupiter.
They travel in elliptical paths, taking between three to six years to circuit the Sun.
They can also be found outside of the asteroid belt when their orbits are altered by collisions with other asteroids, or the gravitational pull of larger bodies, like planets.
An asteroid can even be captured by a planet, to become a moon.
Moons of Mars: Phobos, Deimos
Or meet a more violent end, smashing into it with huge force.
Asteroids that come close to our planet are called near-Earth objects.
Meteoroids are the tiny asteroids that are constantly bombarding our atmosphere, but due to friction, most burn up and never reach the ground, causing a flash of light that we call a meteor.
Meteors – light from burning meteoroids
Much rarer and potentially more dangerous are meteorites; these are meteoroids that make it through the Earth's atmosphere.
Meteorites – survive entry and hit Earth
The majority are small and relatively harmless by the time they reach the ground.
But if an asteroid or comet larger than 5km across collided with Earth, it could cause colossal destruction.
A major theory of evolution is that the impact of an asteroid of this size led to the extinction of the dinosaurs, 65 million years ago.
More recently, in 1908, their destructive powers were clear when one caused an explosion that destroyed a huge area of land in Siberia.
2000 km2 were destroyed
Craters on Earth show that we have repeatedly been struck by asteroids and comets, and it seems inevitable that we will be hit again.
But fortunately, the chances of another large asteroid collision, like the one which caused mass extinction some 65 million years ago, are extremely slim.