Comets and Asteroids
Planets are not the only things to orbit the Sun in our Solar System. There are much smaller objects that also travel about the Sun. Two of these types of object are asteroids and comets. They are similar in many ways - but the main difference between an asteroid and a comet is their appearence (what they look like). In fact, some scientists claim that comets may become asteroids once they lose the ice and gases associated with them.
The first person to discover an asteroid was an Italian astronomer called Giuseppe Piazzi in 1801. He called the asteroid Ceres after a Roman goddess. Ceres is found in a region of the Solar System called the Asteroid Belt. There are many asteroids to be found here.
The diagram shows that the asteroids in the Asteroid Belt orbit the Sun roughly in between the orbits of Mars (of the inner planets) and Jupiter (an outer planet). In the diagram the asteroids are shown in white.
The asteroid called Ceres is the biggest asteroid in the Asteroid Belt and is also known as a dwarf planet (Pluto is also a dwarf planet).
Although there are hundreds of thousands of asteroids in the belt they are spread so thinly over a vast area that space vehicles (only unmanned ones travel this far) are able to pass through the Asteroid Belt relatively easily. The first spacecraft to travel through the Asteroid Belt was Pioneer 10. It entered the Asteroid Belt on July 16th 1972. At the time, scientists were worried that the spacecraft could be damaged - but it passed through the region safely.
However, since there are so many asteroids there, collisions do happen - sometimes causing the asteroids to break into fragments - forming new asteroid families. Fragments of asteroid collisions can be the source of meteorites which are attracted by the gravity of other planets and can crash onto a planet's surface.
On the other hand, sometimes, slower moving asteroids can collide and join together to make a new, larger asteroid.
The word comet comes from a Greek word meaning 'the hair of the head'. In fact, the Greek philosopher Aristotle called comets 'stars with hair' over two thousand years ago. These names refer to the 'fuzzy' appearence of comets.
Comets have a rocky core, or nucleus, which may have other substances (such as water ice and frozen gases) stored within it.
As a comet travels nearer to the Sun the ice, gases and dust are released from the nucleus and the comet starts to form a 'tail'. A cloud (or coma) also begins to form around the nucleus.
The animation shows a comet orbiting the Sun. You can see that the tail of gases always points directly away from the Sun. The diagram also highlights the fact that comets have a highly elliptical (a very squashed circle) orbit. This is why we only see certain comets quite rarely. For example, Halley's Comet passes near Earth every 75 or 76 years. The rest of the time the comet is a long way from Earth's orbit (Halley's Comet is due to return in 2061).