A comet is a small, icy, dusty celestial body that, as it approaches the Sun, warms up and releases gas and dust. This gas and dust forms a tail that points away from the Sun. Comets orbit the Sun just like planets do, but their orbits are usually much more elliptical. A comet’s Tail always points away from the Sun because of the solar wind – a stream of charged particles that flow outward from the Sun.
Comets are fascinating to scientists because they are relatively unchanged since the formation of our solar system 4.6 billion years ago. They provide clues about what conditions were like when our solar system first formed. Comets also help us understand how life may have come to Earth; some scientists think that comets delivered water and organic molecules – building blocks of life – to our planet long ago.
Comets are named after the astronomer who discovers them or after their appearance. For example, Comet Hale-Bopp was named after its discoverers, Alan Hale and Thomas Bopp; it was nicknamed the “Great Comet of 1997” because it was so bright.
There are three main types of comets: short-period comets, long-period comets, and sungrazing comets. Short-period comets have orbital periods shorter than 200 years; they originate in either the Kuiper Belt or the Oort Cloud (a cloud of icy bodies thought to surround our solar system at great distances). Long-period comets have orbital periods longer than 200 years; they also originate in either the Kuiper Belt or Oort Cloud, but their orbits take them much farther out into space before they swing back toward the Sun (and us). Sungrazing comets approach so close to the Sun that they’re sometimes torn apart by its intense gravitational field and heat; most sungrazers come from a family of related objects called Kreutz sun-grazing comet fragments.
The best known short-period comet is Halley’s Comet; it has an orbit that brings it close to Earth every 75-76 years (its last visit was in 1986). The best known long-period comet is probably Shoemaker Levy 9 which hit Jupiter in 1994; pieces of this comet had been observed orbiting Jupiter for several years before impact!
Discovered in 1995 by Scottish astronomer Robert McNaught while working at Siding Spring Observatory in New South Wales, Australia, McNaught–Hartley comet became one of only 20 identified periodic cometary nuclei as it passed 0.12 AU (18 million km) from Earth on 20 November 2010—the closest pass by any known periodic comet since Lexell’s Comet approached to 0