From Academic Kids

, showing a white dust tail and blue gas tail (February 1997)
Comet Hale-Bopp, showing a white dust tail and blue gas tail (February 1997)

A comet is a small astronomical object similar to an asteroid but composed largely of ice. Comets typically move in highly elliptical orbits, the aphelia of which may be many times more distant than Pluto's orbit. Often described as "dirty snowballs", comets are composed largely of frozen carbon dioxide, methane and water with dust and various mineral aggregates mixed in.

Comets are believed to originate in a cloud (the Oort cloud) at large distances from the sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.


Physical characteristics

Comets are believed to originate in a distant cloud known as the Oort cloud, after the astronomer Jan Hendrik Oort who hypothesised its existence. They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that bring them very close to the Sun. When a comet approaches the inner solar system, radiation from the Sun causes its outer layers of ice to evaporate. The streams of dust and gas this releases form a huge but extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form pointing away from the sun. The dust and gas each form their own distinct tail, pointed in slightly different directions — dust being left behind in the comet's orbit (so that it often forms a curved tail) and the ion tail (gas) always pointing directly away from the Sun, since the gas is more strongly affected by the solar wind than dust is, and follows magnetic field lines rather than an orbital trajectory. While the solid body of the comet (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the tails can extend over 150 million km (1 Astronomical unit) or more.


Comets have highly elliptical orbits. Note the two distinct tails.
Comets have highly elliptical orbits. Note the two distinct tails.


In 1996, comets were found to emit X-rays [1] (http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html). These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons [2] (http://www.kvi.nl/~bodewits).

Orbital characteristics

Orbits of  and , illustrating the high  of the orbit and more rapid motion when closer to the

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. Single-apparition comets have parabolic and hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun. On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.


Also because of gravitational interactions, a number of periodic comets discovered in earlier decades or previous centuries are now lost, since their orbits were never known well enough to know where to look for their future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, which was discovered in 1869 but became unobservable after 1908 due to perturbations by Jupiter, and was not found again until accidentally rediscovered by LINEAR in 2001.Template:Hnote

Comet nomenclature

The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680," the "Great September Comet of 1882," and the "Daylight Comet of 1910." After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke Template:Hnote and Comet Biela Template:Hnote, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred by the year of their apparition.

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered on some instruments has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion. (As of May 2005, SOHO has discovered over 950 comets and is conducting a contest to guess the date of its 1000th comet discovery.Template:Hnote)



History of comet study

Early observations and thought




In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.Template:Hnote

Orbital studies

Missing image


The first suggestion that Kepler's laws of planetary notion should also apply to the comets was made by William Lower in 1610.Template:Hnote In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, and Jean-Dominique Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.Template:Hnote

The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In his Principia Mathematica of 1687, Isaac Newton proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.Template:Hnote

In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9. Template:Hnote (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, Template:Hnote while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. Template:Hnote Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. Template:Hnote When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.


Studies of physical characteristics

Hast thou ne'er seen the Comet's flaming flight?



"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,


However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 18641866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.Template:Hnote A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.


Forthcoming space missions will add greater detail to our understanding of what comets are made of. The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. In 2005, the Deep Impact probe will blast a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta space probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.

Great comets




Peculiar comets

Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.Template:Hnote 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.Template:Hnote Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.Template:Hnote Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.




Comets in fiction

Comets are popular subjects for science fiction authors and filmmakers although they are often misrepresented as fiery objects, rather than icy.

See also


  1. Aristotle (ca. 350 B.C.) Meteorologia. An English translation by E.W. Webster is available online (http://classics.mit.edu/Aristotle/meteorology.1.i.html).
  2. Bill Arnett. (2000). "Astronomical Names." Available online (http://www.nineplanets.org/names.html).
  3. Committee on Small Body Nomenclature (1994). "Cometary Designation System." Available online (http://cfa-www.harvard.edu/cfa/ps/lists/CometResolution.html).
  4. European Southern Observatory. (2003). "A Brief History of Comets." Available online: Part I (http://www.eso.org/outreach/info-events/hale-bopp/comet-history-1.html), Part II (http://www.eso.org/outreach/info-events/hale-bopp/comet-history-2.html).
  5. Template:Journal reference
  6. Gary W. Kronk. (2001–2005). Cometography. Available online (http://cometography.com).
  7. I.S. Newton (1687). Philosophiæ Naturalis Principia Mathematica. Londoni: Josephi Streater.
  8. Samuel Pepys (1893). The Diary of Samuel Pepys, M.A., F.R.S.. London: George Bell & Sons.
  9. Vigyan Prasar (2001). "Development of Cometary Thought." Available online: Part I (http://www.vigyanprasar.com/dream/mar2001/comets.htm), Part II (http://www.vigyanprasar.com/dream/apr2001/comets.htm).
  10. Reading Museum Service (2000-2004). Britain's Bayeux Tapestry. Available online (http://www.bayeuxtapestry.org.uk/). Accessed 22 April 2005.
  11. Template:Book reference
  12. Solar and Heliospheric Observatory. (2005). "The SOHO 1000th Comet Contest." Available online (http://soho.nascom.nasa.gov/comet1000/).
  13. Template:Journal reference

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