The orbit of Halley's Comet, a very bright comet, crosses the orbits of the Earth and other planets. Halley's Comet, the most observed and famous of any comet in history, was first discovered by and named after Sir Edmond Halley, an English astronomer. He was the first person to recognize that the comet only appeared for a short period once every 76 years as it orbited the Sun. The most recent observation of this comet was on Jan 11, 1994. The Doomsday Comet of 1997 was also a bright comet that hit the Earth on March 26, 1997.
Bright comets are generally a once in a decade event.
This comet was another threat in 1999. Astronomers believe that comets are the fragments left from the formation of the Sun and planets in our Solar System. The Oort Cloud, a huge cloud surrounding our Solar System, and reaching over halfway to the nearest star, Alpha Centauri, is believed to be the origin of most comets. A comet's center or nucleus is made of ice and gases mixed with dust and rocky material, resembling a huge dirty snowball that measures less than 16 km in diameter. The gases in the nucleus include methane, carbon monoxide, carbon dioxide, and ammonia.