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Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley.
As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure.
Comets are sometimes called dirty snowballs or "icy mudballs". They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.
When they are near the Sun and active, comets have several distinct parts:
nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;
coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;
hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;
dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;
ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.
Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits.
After 500 or so passes near the Sun off most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. (Perhaps half of the near-Earth asteroids may be "dead" comets.) A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter (esp. with Jupiter).
By far the most famous comet is Comet Halley but SL 9 was a "big hit" for a week in the summer of 1994.
Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.
Many comets are first discovered by amateur astronomers. Since comets are brightest when near the Sun, they are usually visible only at sunrise or sunset.
Comet Halley
In 1705 Edmond Halley predicted, using Newton's newly formulated laws of motion, that the comet seen in 1531, 1607, and 1682 would return in 1758 (which was, alas, after his death). The comet did indeed return as predicted and was later named in his honor.
The average period of Halley's orbit is 76 years but you cannot calculate the dates of its reappearances by simply subtracting multiples of 76 years from 1986. The gravitational pull of the major planets alters the orbital period from revolution to revolution. Nongravitational effects (such as the reaction from gasses boiled off during its passage near the Sun) also play an important, but smaller, role in altering the orbit. Between the years 239 BC and 1986 AD the orbital period has varied from 76.0 years (in 1986) to 79.3 years (in 451 and 1066). The closest perihelion passage to the time of Jesus are 11 BC and 66 AD; neither event took place in Jesus' lifetime. Its most famous appearance was in 1066 when it was seen at the Battle of Hastings, an event commemorated in the Bayeux Tapestry.
Comet Halley was visible in 1910 and again in 1986. Its next perihelion passage will be in early 2062.
Halley's orbit is retrograde and inclined 18 degrees to the ecliptic. And, like all comets, highly eccentric.
Only four comets have been visited by spacecraft. NASA's ICE passed through the tail of Comet Giacobini-Zinner in 1985; Comet Grigg Skjellerup was visited by Giotto in 1989. In 1986, five spacecraft from the USSR, Japan, and the European Community visited Comet Halley; ESA's Giotto obtained close-up photos of Halley's nucleus (above and right). NASA's technology demonstration spacecraft DS1 imaged the nucleus of Comet Borrelly in 2001.
The nucleus of Comet Halley is approximately 16x8x8 kilometers.
Contrary to prior expectations, Halley's nucleus is very dark: its albedo is only about 0.03 making it darker than coal and one of the darkest objects in the solar system.
The density of Halley's nucleus is very low: about 0.1 gm/cm3 indicating that it is probably porous, perhaps because it is largely dust remaining after the ices have sublimed away.
Halley is almost unique among comets in that it is both large and active and has a well defined, regular orbit. This made it a relatively easy target for Giotto et al. but may not be representative of comets in general.
Comet Halley will return to the inner solar system in the year 2061
Comet Shoemaker-Levy 9
Comet Shoemaker-Levy 9 was discovered by Eugene and Carolyn Shoemaker and David Levy in 1993. Shortly after its discovery it was determined to be in a highly elliptical path near Jupiter and on a collision course. It was difficult to calculate its orbit prior to its 1992 pass near the giant planet.
In 1992, SL 9 passed by Jupiter within the Roche limit. It was broken into at least 21 separate fragments which were dispersed several million kilometers along its orbit.
The size and mass of the original body and the individual fragments is as of this writing still highly uncertain. The estimates range from 2 to 10 km in diameter for the original body and from 1 to 3 km for the largest fragments.
Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter. This was the first time that scientists had an opportunity to witness the collision of two extraterrestrial bodies.
The impacts were observed by virtually every large ground based observatory, thousands of small and amateur telescopes, and several spacecraft including HST and Galileo.
The pictures were posted to the Net within hours of the impacts and caused severe overloading on some ftp and WWW sites.
The after-effects of the impacts were visible on Jupiter for nearly a year after the event.
There are linear chains of craters on Ganymede and Callisto that are believed to have been formed by the impacts of bodies similar to SL 9.
SL 9 is no more, but its scientific legacy will be studied for years.
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