Comets
A comet is a small icy object found in the Solar System that is orbits the sun. When it gets close to the sun it shows a coma (temporary atmosphere) and a tail. Comets contain three parts: a nucleus, a coma, and a tail. They can be from a few years old to hundreds of thousands of years old. Comets have a wide range of orbital periods. Long-period comets originate in the Oort Cloud and short period comets come from the Kuiper Belt. The Oort Cloud is a spherical region that surrounds the solar system and extends almost halfway to the nearest star. The Kuiper Belt is a region of the solar system that begins just beyond the orbit of Neptune. From January 2011 there are about 4,185 reported comets. 484 of them are short-period. However the numbers in comets are increasing. Those numbers represent only a tiny fraction of the comet population. Usually there is one comet per year that humans are able to see through their naked eye. Bright and spectacular comets are called Great Comets, although many of these are faint and not that spectacular.
Fate of Comets
Comets can be ejected from the solar system. If they are ejected it is through another solar system. Comets may also break up into fragments. The break-up might be caused by tidal gravitational waves from the sun or another planet by an explosion of volatile material. Jupiter-Family comets can have a lifetime up to 10,000 years or 1,000 revolutions. 10% of long period comets survive more than 50 passages and 1% survives more than 2,000 passages. Many comets that have been discovered in earlier decades or centuries are now lost comets. Sometimes when a new comet is discovered and they see its orbit they notice that it is an old lost comet. Some of the comets collide with other planets, moons, comets, or asteroids. The craters on the moon might have been caused by comets. Organic molecules on comets has had some impact on people to think that life on earth may have been caused by meteorites or comets.
Characteristics
Short period comets take about 200 years or less to complete one orbit. Comets leave a trail of debris that can lead to meteor showers on earth. Nuclei of comets can measure up about ten miles or less. Some comets that crash into the sun or burn up when they get close to the sun are called sun grazers. Scientists found out that comets in the main asteroid belt may be a key source of water to the terrestrial planets. The Nucleus of a comet might have a small rocky core. Astronomers think that comets are leftovers from ice, dust, gas, and rocks that formed about 4.6 billion years ago.
Resources
http://www.space.com/53-comets-formation-discovery-and-exploration.html
http://en.wikipedia.org/wiki/Comet
http://lordsong.deviantart.com/art/Chasing-Comets-299553083
http://www.flickr.com/photos/nasamarshall/3766373964/