Some of the strangest objects in the Solar System are comets. These are balls of rock, ice and dust which orbit the Sun in elliptical orbits (oval-shaped orbits, not round). They can go very close to the Sun, and then they glide across the solar system, travelling many millions of miles beyond the most distant planets before making their return to the Sun.
Comets are recognisable because of their tails. This tail is only visible when the comet is close enough to the Sun, usually when it is at about the same distance from Earth as Mars is. This is because heat from the Sun warms up the comet, causing its ice and dust to melt and burn away and leave a tail. The tail is visible because sunlight reflects off the ice and dust particles. When seen from Earth, a comet can have the appearance of a blurred star. The tail of a comet can be up to over 1 million kilometres long!
The most recent comet seen recently from Earth was the Hale-Bopp comet in 1997 (pictured in a photograph at the top of this page and to the right). This comet was visible from all over the planet and looked like a star with a blurry tail. When this page was written in February 2000, Hale Bopp was at a distance of approximately half way between the orbits of Jupiter and Saturn. Now, in December 2011, it is beyond the orbit of Neptune. This comet won't be seen again from Earth for over two thousands years. However, one comet which makes regular appearances is Halley's Comet. This comet goes as far away from the Sun as almost Neptune, but comes almost as close as Mercury to the Sun. This comet is visible from Earth every 76 years, meaning that the average person can see it once during his/her lifetime. Its next appearance will be in 2061, after last appearing in 1986. The picture below is of Halley's Comet from 1986.
Comets orbit the Sun by travelling through the Solar System. This means that they cross the paths of the planets. Occasionally, a planet may also be in the way as the comet crosses its path. This happened in 1994 when the comet, Shoemaker-Levy 9, collided with Jupiter. This is the first time a comet has ever been seen to collide with a planet, and several telescopes were pointed at Jupiter to witness the event.
Comets are usually discovered about 2 to 3 months before they are clearly visible from Earth. This is because they are so small and hardly glow at all when they are at a great distance from Earth. The Hale-Bopp comet, discovered by Alan Hale in July 1995, is an exception. It would not be seen at its maximum brightness from Earth for nearly two more years. The comet was discovered so early because it was so big and would, Hale believed, leave a tail four times wider than a full moon. In April 1997, the comet was a prominent object in the night sky and was easy to spot.
There are several missions to explore comets. In the past these have involved space craft observing comets as they fly past each other, but have also included attempts to collect some of the dust that comes off a comet as it makes its trip through the solar system.