Comet: Difference between revisions

Revision as of 21:29, 11 December 2013

This article is about the astronomical objects. For other uses, see Comet (disambiguation).

Comet Holmes (17P/Holmes) in 2007, showing blue ion tail on right

A comet is an icy

solar radiation and the solar wind

upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger, and if sufficiently bright may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many different cultures.

Comets have a wide range of

interstellar space along hyperbolic trajectories

.

Comets are distinguished from

centaurs has blurred the distinction between asteroids and comets

.

As of July 2013[update] there were 4,894 known comets,

Great Comets

".

Etymology

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant "long-haired star, comet" in Greek. Κομήτης was derived from κομᾶν ("to wear the hair long"), which was itself derived from κόμη ("the hair of the head") and was used to mean "the tail of a comet".^[7][8]

The astronomical symbol for comets is (☄), consisting of a small disc with three hairlike extensions.^[9]

Physical characteristics

Nucleus

Main article: Comet nucleus

Nucleus of Comet 103P/Hartley with jets streaming out; photography by a visiting space probe. The nucleus is about 2 km in length.

C/2011 W3 (Lovejoy) heads into the Sun

Comet Wild 2

exhibits jets on light side and dark side, stark relief, and is dry.

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of

Fred Whipple's model.^[11] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".^[12]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of

asteroids and comets.^[16][17]

The outer surfaces of cometary nuclei have a very low

organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes.^[19]

Properties of some comets

Name	Dimensions km	Density g/cm3	Mass kg[20]
Halley's Comet	15 × 8 × 8[21]	0.6[22]	3×1014
Tempel 1	7.6 × 4.9[23]	0.62[24]	7.9×1013
19P/Borrelly	8 × 4×4	0.3[24]	2×1013
81P/Wild	5.5 × 4.0 × 3.3[25]	0.6[24]	2.3×1013

Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed,

P/2007 R5 is probably only 100–200 metres in diameter.^[28] A lack of smaller comets being detected despite the increased sensitivity of instruments has lead some to suggest that there is a real lack of comets smaller than 100 metres (330 ft) across.^[29] Known comets have been estimated to have an average density of 0.6 g/cm³.^[24] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.^[30]

Roughly six percent of the

extinct nuclei of comets that no longer experience outgassing,^[31] including 14827 Hypnos and 3552 Don Quixote

.

Coma

Perihelion.^[32]

The streams of dust and gas thus released form a huge and extremely thin 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.^[33]

The coma is generally made of H₂O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun.^[34] The H₂O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.^[34] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.^[35]

Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.

17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.^[37] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.^[38] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun.^[38] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.^[38] Ion tails have been observed to extend one astronomical unit (150 million km) or more.^[37]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from

Comet Holmes.^[37]

Tails

Main article: Comet tail

Coma (cometary)

In the outer

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.

The observation of antitails contributed significantly to the discovery of solar wind.[47] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.^[48]

If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail,

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Orbital characteristics

Most comets are

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.^[65] Short-period comets display a tendency for their aphelia to coincide with a gas giant's orbital radius, with the Jupiter family of comets being the largest.^[60] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.^[66][67]

Based on their orbital characteristics, short-period comets are thought to originate from the

Long-period comets have highly

Single-apparition or non-periodic comets are similar to long-period comets because they also have

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[78] whereas others use it to mean exclusively short-period comets.^[72] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded

A number of periodic comets discovered in earlier decades or previous centuries are now

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Other researchers have cast doubt on this theory.[86] The detection of organic molecules in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.^[87]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's

In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.^[90]

Fate of comets

Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System; such is the case for hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.^[91]

Volatiles exhausted

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.^[31] Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.^[92] Some asteroids in elliptical orbits are now identified as extinct comets.^[93] Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.^[31]

Breakup

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.^[100]

Some comets have been observed to break up during their perihelion passage, including great comets

Some comets meet a more spectacular end – either falling into the Sun^[102] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.^[103]

Nomenclature

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "

After

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named its discoverers, or an instrument or program that helped to find it.[107]

History of study

Early observations and thought

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.^[108] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.^[109][110]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.^[111] Pliny the Elder believed that comets were connected with political unrest and death.^[112]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.^[113][114]

Orbital studies

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[120]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.^[121] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.^[122]

In 1950,

Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly's.^[125]

In July 2005, the

on November 4, 2010.

Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire," at extremely high temperatures of over 1,000 °C (1,830 °F).^[127][128] Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,^[129] so comets also contain crystalline grains that formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials".^[130] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.^[131]

The Rosetta probe is presently en route to Comet Churyumov–Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.^[132]

Examples

• 
  Comet

  C/2006 P1

  (McNaught) taken from Victoria, Australia 2007
• 
  The

  Kreutz group
• 
  Great Comet 1861

Great comets

Main article:

Great Comet

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as

Great Comets.^[98]

Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.^[133] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.^[134] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.^[135]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—

C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.^[136]

Sungrazing comets

Main article: Sungrazing comet

Kreutz Sungrazer

with a prominent tail, plunging towards the Sun

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.

tidal forces they experience often lead to their fragmentation.^[138]

About 90% of the sungrazers observed with

Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.^[139] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.^[140]

Unusual comets

See also Fate of comets.

Of the thousands of known comets, some exhibit unusual properties. Encke's Comet orbits from outside the asteroid belt to just inside the orbit of the planet

Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL₃.^[143]

Observation

A comet may be discovered photographically using a wide-field

amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.^[28] SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010.^[144]

Comets & Culture

See also: Comets in fiction and Category:Impact events in fiction

Comet Hale-Bopp, as seen in Pazin, Croatia

1997.

The depiction of comets in

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,^[146] whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.^[147]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (Deep Impact, 1998), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or of waves of zombies (Night of the Comet, 1984).^[145] In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.^[148]

See also

Template:Wikipedia books

• Comet vintages
• List of comets
• The Big Splash (book)