Thousands of small, irregular shaped objects, called asteroids, orbit around the sun between Mars and Jupiter.More than 4000 have been discovered and namedThe largest, Ceres, measuring about 1000 Kilometers across, has now been reclassified as a dwarf planet.
There may be up to a million asteroids measuring up to a kilometer or more across.Many are tiny specks too small to be identified.
Many asteroids are heavily created objects.They probably formed when early planets smashed into one another, leaving a belt of fragments.Most asteroids are rocky, which means that some of them must have come from the upper layers of a planet.But a few are made of metal: they must have come from the cores of old planets.
Asteroids still collide with one another, producing smaller fragments of rock called meteoroids.These sometimes crash to earth as meteorites.
A large asteroid may have been responsible for one of the most devastating events on Earth.Some scientists believe that an asteroid may have crashed into Earth 65 million years ago.The change in climate that followed wiped out the dinosaurs and many other prehistoric species.
Meteorites range in size from minute fragments to boulders measuring many meters across.It is feared that a falling chunk of the asteroid may one day crash into earth, punching an enormous crater in its surface and causing devastation.The explosion would also fill the atmosphere out with dust, blotting out the sun and lowering temperatures worldwide for years on end.
Comets are lumps of ice and rock, only a few kilometers across, that orbit the sun.As they near the sun, their tails start to grow, eventually extending millions of kilometers into space.They always point away from the sun.There is a straight gas tail and a broader, curved dust tail.
The nucleus of the comet is a potato-shaped lump of dust and rock, fused together by frozen gases and water ice.When the comet nears the sun, the ice melts and the outer crust of the nucleus