A comet is classified as an icy body that discharges gas or dust. Most comets visible from Earth appear to travel in long ovals around the sun. Comets consist of a nucleus bounded by a fuzzy or cloudy atmosphere and may have one or two tails. The gases in the comet's coma and tail are reflective and bright, which allows us to see comets much better when they are closer to the sun. Astronomers also believe that comets release the energy they store from the sun causing them to shine. Astronomers believe comets are remnants of ice, rock and gas from what shaped the planets billions of years ago. They also believe that comets are strongly linked to life on earth, bringing some water and carbon-based molecules that fuel life on the ground. When astronomers reflect on the comet's nucleus, they reflect on a dirty snowball. It's basically a ball of ice and rock dust particles. The coma forms as the sun approaches the sun and some of the ice vaporizes to form cloud dust around the comet. Solar radiation also pushes dust particles away from the comet and this is how the tail is shaped. Each time a comet orbits and arrives in the inner circle of the solar system, it melts a little more, as eventually some comets will not be able to find the complete merger. Astronomers classify comets into two groups. Short-period and long-period comets due to the time they take to orbit the sun. Short-period comets take less than two hundred years while long-period comets take two hundred or more years. Astronomers doubt that short-period comets come from the Kuiper Belt, which is basically a band of objects close to Pluto's orbit. They believe that the gravitational pull of planets can cause an object to be… in the center of the paper… a close-range reflection where space probes fly at close range to asteroids and collect detailed data. It is also a continuing theory that perhaps an asteroid crashed into Earth causing climate and environmental changes that caused the extinction of the dinosaurs. The asteroid is believed to have created a huge depression in Mexico's Yucatan Peninsula. Asteroids are classified into different types based on their spectra. Type C, it includes more than 75% of known asteroids: tremendously dark; similar to carbonaceous chondrite meteorites; approximately the same chemical arrangement as the Sun minus hydrogen, helium, and other volatiles; Type S, seventeen percent: relatively bright; metallic iron-nickel assorted with iron and magnesium silicates; Type M, most of the rest: bright; pure iron-nickel. There are also a dozen or so additional rare types.