A comet is an icy body that orbits the sun in a long, oval shaped orbit. A comet is made up of a solid nucleus, or the centre of the comet, an atmosphere called the coma and one or two tails. The nucleus resembles a dirty snowball. This effect is caused by the various types of ices and dust particles. As the comet approches the sun, the ice vapourizes The gases and particles that fly off the comet fly away from the sun, forming the tail or tails. Depending on the amount of dust in the nucleus, the comet can have a dust tail, a gas tail or possibly both.
Most comets have a nucleus of 16 kilometres or less across. Some comets comas have been know to reach 1.6 million kilometres. Some tails may extend to a length of 160 million kilometres. The direction of these tails depend where the Comet is in relation to the Sun. The Sun's solar wind forces the dust particles and gas away from the Sun, so the tail or tails are always facing away from the Sun.
The composition of a Comet can be determined by the light that a comet emits. Scientits learnt alot about the composition of comets by studying Halley's Comet in 1986. The comet crossed the Earths orbit, and 5 spacecraft recorded information on the apperance and chemical composition. From studying the comet, scientist believe that composition of comets are similar to Halley's Comet.
Scientists classify comets by predicting their orbit period. These two classifications are Short Term Comets and Long Term Comets. Short term cometstake less that 200 years to orbit the Sun while Long Term Comets take more than 200 years. These two types of comets come from different places in the Universe. Short term comets come from th Kuiper belt, which lies beyond the orbit of Pluto. Long term comets come from come from the Oort cloud. This cloud lies 1 000 times farher away from Pluto's Orbit.
Eventually, comets lose their ice and the comets breaks up into objects similar to asteroids.
A meteor is a small piece of rock or metallic material from space. Meteors are also know as shooting stars or falling stars. Most meteors disintergrate before reaching the Earth's surface. Meteors that reach the Earth's surface are called meteorites.
Meteors become visible at about 65 to 120 kilometres above the Earth's surface, and disintergrate at about 50 to 95 kilometres above the surface. Meteorites hit the Earth at about 70 kilometres per hour.
There are 3 main types of meteors, stony, iron and stony iron. There are 2 groups of stony meteors. Chondrites (the same material that makes up the planets) and achondrites(which were part of a parent body such as an asteroid). Stony meteors are made up of silicon and oxygen, with small amounts of iron and magnesium. Iron meteors are made up of iron and nickel.
Nearly all meteors are small. A meteor has been found with a weight of 60 metric tons. A very large meteorite exploded above the Tunguska River in Siberia in 1908, leaving hundreds of trees burnt and felled.
Impact sites of
meteorites are called Impact Craters or Impact Basins. The
impact areas are around 25 kilometres in diameter, and have slightly
uplifted centres and are usually shallow.