Comets

Their name comes from latin “Stella cometa”, star with hair.

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; (planet size: 40,000 km in diameter)
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 periods
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).

Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.

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.

Comet Halley (1P/Halley)

Comet Halley, officially designated 1P/Halley, more generally known as Halley's Comet after Edmond Halley, is a comet that can be seen every 75-76 years. It is the most famous of all periodic comets, even though in every century many comets appear brighter and more spectacular. Halley's comet last appeared in the inner Solar System in 1986, and will next appear in the summer of 2061.

Halley's Comet was the first to be recognized as periodic. Having perceived that the observed characteristics of the comet of 1682 were nearly the same as those of two comets which had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler in Prague), Halley concluded that all three comets were in fact the same object returning every 76 years (a period that has since been amended to every 75–76 years). After a rough estimate of the perturbations the comet would sustain from the attraction of the planets, he predicted its return for 1757. Halley's prediction of the comet's return proved to be correct, although it was not seen until 25 December 1758 by Johann Georg Palitzsch, a German farmer and amateur astronomer, and did not pass through its perihelion until March 1759; the attraction of Jupiter and Saturn having caused a retardation of 618 days, as was computed by a team of three French mathematicians, Alexis Clairault, Joseph Lalande, and Nicole-Reine Lepaute, previous to its return. Halley did not live to see the comet's return, having died in 1742.

The most recent appearance of Halley was in 1986 and although it was not as spectacular as the two previous occasions (1834 and 1910), it was very important for the astronomy because a spaceship could be brought near to study it for the first time.

On March 13, 1986, the spacecraft Giotto approached at a 596 kilometer distance from Halley's nucleus.
Images showed Halley's nucleus to be a dark peanut-shaped body, 15 km long, 7 to 10 km wide. Only 10% of the surface was active, with at least three outgassing jets seen on the sunlit side.
Of the volume of material ejected by Halley: 80% was water, 10% carbon monoxide, and 2.5% a mix of methane and ammonia. Other hydrocarbons, iron, and sodium were detected in trace amounts.
Halley's nucleus was blacker than soot, which suggests there is proportionally more dust than ice.
The nucleus's surface was rough and of a porous quality, with the density of whole nucleus only 0.3 kg/m3.
The quantity of material ejected was found to be 3 tons per second for seven jets, and these caused the comet to wobble over long time periods.
The dust ejected was mostly only the size of cigarette smoke particles, the largest being 40 milligram. Although the one that sent Giotto spinning was not measured, from its effects its mass has been estimated to lie between 0.1 and 1 gram.
Analysis showed the comet formed 4.5 billion years ago from volatiles (mainly ice) that had condensed onto interstellar dust particles. It had remained practically unaltered since its formation.

The next Halley approach will be in the summer of 2061.

Diagram of the spacecraft Giotto encounter with Halley on March 13 of 1986.
Blue: Giotto orbit. Green: Earth orbit. Yellow: Halley orbit. (a): Launching point on July 2, 1985. (b): Rendezvous point on 03/13/1986. (c): Earth’s position at rendezvous date. - ESA diagram.

Comet Hyakutake images from Hubble Space Telescope, taken on March 27, 1996, during their pass by to Earth at 14.96 millions of kilometers.

Comet Linear
Comet that was observed to break apart as it approached the Sun. C/1999 S4 was discovered by the LINEAR program in September 1999. On Jul. 21-22, 2000, a couple of days before perihelion on Jul. 24 (0.75 AU). The disintegration of C/1999 S4 after perihelion had more to do with solar heating than with the Sun’s gravitational force. The comet receives their name after the short for “Lincoln Near Earth Asteroid Research”.

Information came from Wikipedia, Bill Arnett’s Nin...Eight Planets, ESA-Giotto's WEB page and cometography.com.