A comet is a very small solar system body made mostly of ices mixed with smaller amounts of dust and rock.
“Comet” comes from the Latin “cometes”, which in turn comes from the Greek “kometes”, meaning “long-haired”.
A comet has four components: a nucleus, a coma, a dust tail and an ion tail.
The nucleus of a comet is composed of ice and rocky material and it ranges from about few hundred meters to 100 km ( 62 miles) in diameter.
The cloud of gases that forms around the nucleus as the coma is heated is know as the coma. These gases are usually a mixture of water vapor, ammonia, carbon dioxide.
The dust tail of a comet is composed of gases and tiny dust particles blown away from the nucelus as the comet is heated. The dust tail is the most visible part of a comet.
The ion tail is a stream of ionized gases that are blown directly away from the Sun as a result of the
comet’s contact with the solar wind.
When a comet is heated by the Sun, its ices begin to sublimate. The mixture of ice crystals and dust blows away from the comet nucleus in the solar wind, creating a pair of tails. The dust tail is what we normally see when we view comets from Earth.
Comet tails can be over 1 million km (600,000 miles) long.
Comets come in several categories. The most common are periodic and non-periodic.
Periodic comets or short-period comets are generally defined as having orbital periods of less than 200 years.
Well-known comets include the non-periodic comets Hale-Bopp (C/1995 O1), Hyakutake (C/1996 B2), McNaught (C2006 P1), and Lovejoy (C/2011 W3). These flared brightly in our skies and then faded into obscurity.
In addition, Comet Shoemaker-Levy 9 (D/1993 F2) was spotted after it had broken up after a close call with Jupiter.
The periodic Comet Halley (1P/Halley) is the most famous in history. It returns to the inner solar system once every 76 years. Other well-known periodic comets include 2P/Encke, which appears ever 3.3 years and 9P/Tempel (Tempel 2), which was visited by the Deep Impact and Stardust probes, and makes perihelion around the Sun every 5.5 years.
In 1705, while studying the orbits of several known comets, Edmond Halley found that the comet observed in 1531, 1607 and 1682 was one in the same. As a result of Halley’s discovery, the comet was named after him. Halley’s Comet is visible every 75 to 76 years.
While the coma over Halley’s Comet can stretch up to 100,000 km across, the nucleus is actually small, only around 15 km (9.3 miles) long, 8 km (5 miles) wide and 8 km (5 miles) thick.
Comet Hale–Bopp was perhaps the most widely observed comet of the 20th century and one of the brightest seen for many decades. It was discovered on July 23, 1995, at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.
Comets orbit the Sun in elliptical paths – just like the planets. The path of a comet though is far more elliptical than that of any planet.
The closest point in a comet’s orbit to the Sun is called “perihelion”. The most distant point is called “aphelion”.
Comets have two tails: a dust tail (which you can see with the naked eye) and a plasma tail, which is easily photographed but difficult to see with your eyes.
Sometimes comets are referred to as “dirty snowballs” or “cosmic snowballs”. This is because they are composed mostly of ice, rock, gas and dust.
There are over 3,000 currently known comets.