The nucleus is the solid, central part of a comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

, popularly termed a dirty snowball. A cometary nucleus is composed of rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

, and frozen gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es. When heated by the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, the gases sublimate

Sublimation (physics)

Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase...

 and produce an atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 surrounding the nucleus known as the coma

Coma (cometary)

frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...

. The force exerted on the coma by the Sun's radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 and solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04.

Splitting

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. Splitting comets include 3D/Biela

3D/Biela

Biela's Comet or Comet Biela was a periodic comet first recorded in 1772 by Montaigne and Messier and finally identified as periodic in 1826 by Wilhelm von Biela. It was subsequently observed to split in two and has not been seen since 1852...

 in 1846, Shoemaker–Levy 9 in 1992, and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus

Ephorus

Ephorus or Ephoros , of Cyme in Aeolia, in Asia Minor, was an ancient Greek historian. Information on his biography is limited; he was the father of Demophilus, who followed in his footsteps as a historian, and to Plutarch's claim that Ephorus declined Alexander the Great's offer to join him on his...

 reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.

Comets 42P/Neujmin

42P/Neujmin

42P/Neujmin, also known as Neujmin 3, is a periodic comet in our solar system.This comet and 53P/Van Biesbroeck are fragments of a parent comet that split in March 1845....

 and 53P/Van Biesbroeck

53P/Van Biesbroeck

53P/Van Biesbroeck is a periodic comet in our solar system.This comet and 42P/Neujmin are fragments of a parent comet that split in March 1845.-External links:*...

 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.

Size

Most cometary nuclei are thought to be no more than about 10 miles (16 kilometers) across. The largest comets that have come inside the orbit of Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 are Hale–Bopp (~60 km), 29P (~30.8 km), 109P/Swift–Tuttle (~26 km), and 28P/Neujmin

28P/Neujmin

28P/Neujmin, also known as Neujmin 1, is a large periodic comet in our Solar System. With a perihelion distance of 1.5AU, this comet does not make close approaches to the Earth....

 (~21.4 km).

The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km) contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also extremely dark black. Scientists believe that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the sun, exposing the interior ice to the warming sunlight.

During a flyby in 2001, the Deep Space 1

Deep Space 1

Deep Space 1 is a spacecraft of the NASA New Millennium Program dedicated to testing a payload of advanced, high risk technologies....

 spacecraft observed the nucleus of Comet Borrelly

19P/Borrelly

Comet Borrelly or Borrelly's Comet is a periodic comet, which was visited by the spacecraft Deep Space 1 in 2001.- Discovery :...

 and found it to be about half the size (8×4×4 km) of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.

The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas.

The nucleus of P/2007 R5

P/2007 R5

Comet P/2007 R5 , also designated P/1999 R1 and P/2003 R5, is the first periodic comet to be discovered using the automated telescopes of the SOHO spacecraft....

 is probably only 100–200 meters in diameter.

The largest centaurs (unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo

10199 Chariklo

10199 Chariklo is the largest known centaur. It orbits the Sun between Saturn and Uranus, grazing Uranus.Chariklo was discovered by James V. Scotti of the Spacewatch program on February 15, 1997. Chariklo is named after the nymph Chariclo , the wife of Chiron and the daughter of Apollo.A...

 (258 km), 2060 Chiron

2060 Chiron

2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

 (230 km), and the currently lost 1995 SN55

1995 SN55

', also written as 1995 SN55, is a lost centaur with a perihelion greater than Jupiter and a semi-major axis that may be less than Neptune's. This object could be the largest known centaur.-Size:...

 (~300 km).

Known comets have been estimated to have an average density of 0.6 g

Gram

The gram is a metric system unit of mass....

/cm³. Below is a list of comets that have had estimated sizes, densities, and masses.

Name	Dimensions km	Density g Gram The gram is a metric system unit of mass.... /cm3	Mass kg Kilogram The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
Halley's Comet	15 × 8 × 8	0.6	3
Tempel 1	7.6×4.9	0.62	7.9
19P/Borrelly 19P/Borrelly Comet Borrelly or Borrelly's Comet is a periodic comet, which was visited by the spacecraft Deep Space 1 in 2001.- Discovery :...	8×4×4	0.3	2
81P/Wild 81P/Wild Comet 81P/Wild, also known as Wild 2 , is a comet named after Swiss astronomer Paul Wild, who discovered it in 1978 using a 40-cm Schmidt telescope at Zimmerwald....	5.5×4.0×3.3	0.6	2.3

Albedo

Comets are often described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust. It has been suggested that comets should be referred to as "Icy dirtballs". Cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto

Giotto mission

Giotto was a European robotic spacecraft mission from the European Space Agency, intended to fly by and study Halley's Comet. On 13 March 1986, the mission succeeded in approaching Halley's nucleus at a distance of 596 kilometers....

 probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1

Deep Space 1

Deep Space 1 is a spacecraft of the NASA New Millennium Program dedicated to testing a payload of advanced, high risk technologies....

 discovered that Comet Borrelly's

19P/Borrelly

Comet Borrelly or Borrelly's Comet is a periodic comet, which was visited by the spacecraft Deep Space 1 in 2001.- Discovery :...

 surface reflects only 2.5–3.0% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing

Outgassing

Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...

.

Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing. Two near-earth asteroids with albedos this low include 14827 Hypnos

14827 Hypnos

14827 Hypnos is an Apollo near-Earth asteroid discovered by Carolyn S. Shoemaker and Eugene Merle Shoemaker at Palomar Observatory on May 5, 1986...

 and 3552 Don Quixote

3552 Don Quixote

3552 Don Quixote is a small main-belt, Amor, Mars crossing, potentially hazardous asteroid. It has a highly inclined comet-like orbit, and measures about 19 km in diameter. Its rotation period is 7.7 hours...

.

External links

• Nucleus of Halley's Comet (15×8×8 km)
• Nucleus of Comet Wild 2 (5.5×4.0×3.3 km)
• International Comet Quarterly: Split Comets