Comets have been observed and regarded with awe throughout history. Astronomers believe that comets were created about 4.6 billion years ago -- at the time when our solar system formed, along with the planets, moons and chunks of rock (meteors). The most popular astronomical theory to explain comets was advanced in 1950 by Fred L. Whipple of the Harvard-Smithsonian Center for Astrophysics. This so-called "dirty snowball" theory asserts that the comet nucleus (it's usually tens of kilometers across -- or no bigger than Manhattan), is composed mostly of various kinds of ices -- frozen water, carbon dioxide, ammonia and methane -- with a little dust thrown in for good measure.Why should we learn about Comets?
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They usually follow highly elongated paths around the Sun. Most become visible, even in telescopes, only when they get near enough to the Sun for the Sun's radiation to start subliming the volatile gases, which in turn blow away small bits of the solid material. These materials expand into an enormous escaping atmosphere called the coma, which becomes far bigger than a planet, and they are forced back into long tails of dust and gas by radiation and charged particles flowing from the Sun -- the Solar Wind. Comets are cold bodies, and we only see them because the gases in their comae and tails fluoresce in sunlight (somewhat akin to a fluorescent light) and because of sunlight reflected from the solids. Comets are regular members of the solar system family, gravitationally bound to the Sun. They are generally believed to be made of material originally in the outer part of the solar system that didn't get incorporated into the planets. Basically, they are leftover debris. It is because they are thought to be composed of such unchanged primitive material that makes them extremely interesting to scientists learning about conditions during the earliest period of the solar system.
Comets are very small in size relative to planets at large distances away from the sun. Diameters: .750 km (2,460 feet) or less - 20 km (12 miles). Recently, evidence has been found for much larger distant comets, perhaps having diameters of 300 km (186 miles) or more. As comets approach the Sun (between 1.5 to 2.0 AU), the coma becomes far bigger than planets, ranging in diameters of 105 to 106 km in diameter. Evidence suggests that comets are very fragile. Their tensile strength (the stress they can take without being pulled apart) appears to be only about 1,000 dynes/cm^2 (about 2 lb./ft.^2). Basically, you could take a big piece of cometary material and simply pull it apart with your bare hands, just like a loosely compacted snowball.