What are Comets?:
Historically comets were referred to as "dirty snowballs" since they were thought to simply be large chunks of ice mixed with a small amount of ice and dust. Recently, this view has been challenged by observations made by NASA experiments such as Stardust-NExT.
While the exact nature of a comet's nucleus (see below) may not fit the mold for the dirty snowball moniker the fact remains that, according to NASA, in order for an object to officially be classified as a comet it much contain at least 85% ice (frozen water).
Comets come from great distances, such as the Kuiper belt and Oort cloud, and have highly elliptical orbits that carry them around the Sun. Occasionally they impact one of the other bodies in our solar system.
Comets are also responsible for meteor showers that light up the night sky.
The Comet Nucleus:
The primary part of a comet is known as the nucleus. A mixture of ice (its primary component), rock, dust and other frozen gases, the nucleus is actually very difficult to detect. It reflects only a small percentage of the Sun's radiation, making them almost invisible when they are in the outer reaches of the solar system.
Typical comet nuclei vary in size from about 100 meters to more than 50 kilometers (31 miles) across.
The Comet Coma and Tail:
As comets approach the Sun, radiation begins to vaporize the frozen gases and ice creating an aural glow around the object.
Known formally as the coma, this glow can extend greater than the radius of the Sun. When we observe comets from the Earth, the coma is often what we see.
The other distinctive part of a comet is the tail. Radiation pressure from the Sun pushes material form the comet forming two tails that always point away from our star.
The first tail is the dust tail, while the second is gas that has been evaporated from the nucleus. Dust from the tail is left behind like break crumbs, showing the path that the comet has traveled through the solar system. The gas tail can glow in brilliant colors and can extend a distance equal to that of the Sun to the Earth.
Short-Period Comets and the Kuiper Belt:
There are generally two types of orbital characteristics. The first are comets that have short periods.
These comets orbit the Sun every 200 years or less. (Though they are subdivided into the Jupiter family - periods less than 20 years - and Halley family - periods between 20 and 200 years.)
While some of these comets come from parts of the solar system where Jupiter and Saturn orbit, many comets of this type are thought to originate in a region of the solar system known as the Kuiper belt. Located beyond the orbit of Neptune and extending more than 55 AU (astronomical units; where the distance to the Earth from the Sun is 1 AU) the Kuiper belt may house remnants of a planet that never formed.
Long-Period Comets and the Oort Cloud:
Some comets take more than 200 years to orbit the Sun once, sometimes millions of years.
These comets come from a region outside of the Kuiper belt known as the Oort cloud. Extending more than 75,000 AU away from the Sun, the Oort cloud contains millions of comets.
Comets and Meteor Showers:
Some comets will cross the orbit that the Earth takes around the Sun. When this happens a trail of dust is left behind.
As the Earth traverses this dust trail, the tiny particles enter out atmosphere. They quickly begin to glow as they are heated-up during the fall to Earth and create a streak of light across the sky.
These events are sometimes called shooting stars or meteor showers. Since the comet tails are left behind in specific locations along Earth's path, meteor showers can be predicted with great accuracy.
Also, meteor showers are given names based on their radiant - or constellation from which they appear to originate in the sky. Some of the well known meteor showers include the Perseid meteor shower that occurs between the 9th and 13th of August and the Orionid shower that arrives in October (which can be attributed to Halley's comet).