Comets are icy bodies that generally spend most of their time in the outer solar system past the orbit of the last known planet, Pluto. Occasionally they enter the realm of the inner solar system and become visible to the naked eye. Most visiting comets return to the inner solar system only every few thousand years or even longer. A few become trapped in orbits that cause them to visit the inner solar system every few years or decades. The most famous of these is Comet Halley (Halley's Comet) which returns about every 76 years on average. Sightings of Comet Halley have been recorded since at least 240 B.C. and possibly earlier.
Comets are composed of three structures.
The nucleus is an irregularly shaped low density mass consisting of frozen gases, water ice, and dark dust. This is why comets are sometimes called "dirty snowballs."
The coma of gas and dust surrounds the nucleus and forms the "atmosphere" of the comet. The coma forms when the comet approaches relatively close to the Sun (e.g., inside the orbit of Jupiter). Fountains of sublimating dust and gas in the nucleus sustain the coma. The size of the coma may exceed the size of the planet Jupiter, yet consists mostly of empty space.
The gas and dust tails consist of trains of gas and dust whose length can exceed the distance of the Earth from the Sun (1 AU). The cometary gases become ionized by the action of the Sun's ultraviolet radiation and react to pressure from the solar wind, forming the comet's ion gas tail. The ion gas tail points away from the Sun. Dust particles are generally neutral so they are less affected by the solar wind. The dust tail typically just trails behind the comet. Occasionally a comet exhibits a third tail composed of sodium. Comet Hale-Bopp displayed a sodium tail in addition to the usual ion and dust tails.
Most comets appear to be less then twenty kilometers in diameter -- five kilometers is a typical size -- but some are much larger. For example, the giant comet 2060 Chiron, also known as 95P/Chiron, follows an orbit which lies mostly between Saturn and Uranus. Chiron appears to be somewhere between 148 and 208 kilometers in diameter. The photo at right shows the famous periodic Comet Halley as it appeared on May 13, 1910 over Flagstaff, Arizona. The bright dot below the comet is the planet Venus.
Comets are divided into two groups on the basis of the length of their orbits and their assumed place of origin.
Short period comets orbit the Sun in less than a few hundred years. Most probably originate from the Edgeworth/Kuiper belt, a flattened disk of comets outside the orbit of Neptune. The Edgeworth/Kuiper belt is thought to contain from 108 to 1010 comets. Many Edgeworth/Kuiper belt objects have now been directly observed. The shortest known period for a periodic comet is 3.3 years for Comet Encke. The period of the famous Comet Halley averages 76 years.
Long period comets orbit the Sun in thousands to millions of years. These probably originate from the Oort cloud, a vast spherical halo of comets thought to surround the solar system. The Oort cloud may extend as far as one light year in distance from the Sun and contain as many as 1013 comets. Two spectacular naked-eye comets of the 1990s were both long-period comets: Comet C/1996 B2 (Hyakutake) and Comet C/1995 O1 Hale-Bopp.
Most comets follow highly elliptical eccentric orbits which are not confined to the plane of the planets. As a comet approaches the Sun it heats up and some of its volatile ices vaporize to form a huge coma surrounding the cometary nucleus as well as tails of gas and dust.
Like near-Earth asteroids, long period comets can also impact the Earth on their way in from the outer solar system. Some researchers believe that disintegrating giant comets injected into short period orbits by the giant outer planets may contribute significantly to the near-Earth object population and increase the risk of damaging impacts over short time scales of a few decades to a few centuries. This idea remains controversial.
Some comets masquerade as asteroids when a very dark lag deposit covers their surface, choking off all further solar heating of the interior. Such comets do not outgas. Comet Wilson-Harrington (1949) provides an example. In 1979 it was rediscovered and designated as asteroid 1979 VA. The comet had turned into an "asteroid" in the intervening thirty years. Another candidate may be the Apollo asteroid 3200 Phaeton whose elongated orbit takes it from beyond the orbit of Mars to within the orbit of Mercury. While 3200 Phaeton looks like an ordinary asteroid, its orbit is identical with that of the Geminid meteors. Since it is believed that meteors are cometary debris, this may indicate than Phaeton is an extinct comet. 2101 Adonis and 2201 Oljato are two more asteroids considered to be possible extinct comets.
The reverse can also occur: a dormant "asteroidal" comet can start outgassing again after a long period of inactivity. Periodic Comet Encke was apparently dormant, perhaps for centuries, before suddenly springing back to life again in 1786. Comet Encke is the only known active cometary member of the Taurid complex. However, all ten of the numbered asteroids in the Taurid complex appear to be associated with meteor showers. These ten "asteroids" may all be extinct comets.
Probably about half of the near-Earth asteroid population consists of "real" asteroids while the other half consists of comets masquerading as asteroids.
Several comets have now been visited by spacecraft. NASA's ICE passed through the tail of Comet Giacobini-Zinner in 1985. In 1986 five spacecraft from the USSR, Japan, and the European community probed Comet Halley. ESA's Giotto shot close-up photos of Halley's nucleus showing it be both larger and darker than previously thought. Giotto also visited Comet Grigg Skjellerup in 1989.
On February 2, 1999 NASA launched the Stardust probe which flew by a comet (Comet Wild 2) and returned with a sample of cometary material for detailed analysis in 2006.
In 2002 NASA's CONTOUR mission flew by three comets: Encke, Schwassmann-Wachmann-3 and d'Arrest. CONTOUR is intended to improve our knowledge of the structure and characteristics of comet nuclei.
In 2004 NASA's Deep Impact mission flew to comet Tempel 1 and shot it with a large projectile to create a crater. This allowed detailed examination of the ejecta some of which will be material from the interior of the comet.
Throughout history most cultures believed the appearance of a comet portended doom. The comet's sudden appearance and irregular behavior challenged the apparent regularity of the heavens. Its ominous sword-like shape reinforced the belief that it signalled death and disaster. In fact, the word "disaster" means "evil star" and refers in part to the supposedly baleful effects of comets. Only a few cultures believed that comets brought good. The Kung of what is now Namibia believe that comets guarantee good times ahead.
The word "comet" comes from the Greek "kometes" meaning "hair." To the Greeks comets were "hairy stars." Other cultures also referred to comets similarly. For example, the Tshi people of Zaire called comets "hairy stars" too. The Chinese called comets "broom stars," some Polynsians referred to them as "dust stars," and the Mesoamericans called them "smoking stars."
Noah Goldman discusses Comets in Ancient Cultures .
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Last modified by pib on May 12, 2009.