Comets consist of rock, ice and organic compounds and they can be several miles in diameter (NASA, 2006). They are thought to originate from a region which is beyond the orbits of the outermost planets, and scientists believe that gravitational perturbations jolt them out of this area and set them on orbital courses which bring them closer to the Sun. Short-period comets travel in short orbits near the Sun while long-period comets travel in elliptical orbits around the Sun which take them far out beyond the planets and back.
When comets reach the more intense sunlight of the Solar System, the ices in the nucleus of the comet begin to vaporize and fall away from the comet (NASA, 2006). The gases which evolve as the ice is vaporized form a delicate atmosphere around the comet's nucleus called a coma, and the dust which was in the nucleus previously forms the tail of the comet that can be thousands of miles long and is sometimes visible from Earth. Comets are thought to have struck the Earth billions of years ago and caused major changes in Earth's early oceans, atmosphere and climate, and they may have brought to Earth the first carbon-based molecules, triggering the origins of life.
In 1994, the Hubble telescope recorded a large comet named P/Shoemaker-Levy 9 impacting on Jupiter (Armageddon, 2006). As the comet entered Jupiter's atmosphere, the temperature reached over 20,000 degrees. Fireballs from the impact were sent thousands of miles into space. Had it impacted on Earth, scientist predict it would cause massive earthquakes around the globe which would jet out of the ground at thousands of miles an hour and kill all life and destroy any buildings in its path (Armageddon, 2006: The Threat, 2006). It would also trigger volcanic activity, and set off volcanic eruptions and explosions all around the world, they predict. The impact would send an enormous dust cloud into the atmosphere, and fireballs would be sent miles...