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==Origin==
==Origin==
{{main|Accretion (astrophysics)#Accretion of comets}}
{{main|Accretion (astrophysics)#Accretion of comets}}
[[File:Comets Kick up Dust in Helix Nebula (PIA09178).jpg|thumb|The [[Helix Nebula]] has a cometary Oort cloud]]
Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation.<ref name="Comets 2015">{{cite news |url=http://phys.org/news/2015-05-comets.html |title=How comets were assembled |publisher=University of Bern ''via'' Phys.org |date=29 May 2015 |accessdate=8 January 2016}}</ref> How and when comets formed is debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals.<ref name="Cometary accretion">{{Cite journal |title=The shape and structure of cometary nuclei as a result of low-velocity accretion |journal=[[Science (journal)|Science]] |first1=M. |last1=Jutzi |first2=E. |last2=Asphaug |volume=348 |issue=6241 |pages=1355–1358 |date=June 2015 |doi=10.1126/science.aaa4747 |bibcode=2015Sci...348.1355J}}</ref><ref name="Weidenschilling1997b">{{cite journal |title=The Origin of Comets in the Solar Nebula: A Unified Model |journal=Icarus |last=Weidenschilling |first=S. J. |volume=127 |issue=2 |pages=290–306 |date=June 1997 |doi=10.1006/icar.1997.5712 |bibcode=1997Icar..127..290W}}</ref> The currently favored creation mechanism is that of the [[nebular hypothesis]], which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew.<ref name="Choi 2014">{{cite news |url=http://www.space.com/53-comets-formation-discovery-and-exploration.html/ |title=Comets: Facts About The 'Dirty Snowballs' of Space |work=Space.com |last=Choi |first=Charles Q. |date=15 November 2014 |accessdate=8 January 2016}}</ref><ref name='Hill 2000'>{{cite journal |title=Determining the ages of comets from the fraction of crystalline dust |journal=[[Nature (journal)|Nature]] |last1=Nuth |first1=Joseph A. |last2=Hill |first2=Hugh G. M. |last3=Kletetschka |first3=Gunther |volume=406 |issue=6793 |pages=275–276 |date=20 July 2000 |doi=10.1038/35018516 |bibcode=2000Natur.406..275N}}</ref><ref name='SC'>{{cite web |url=http://www.scienceclarified.com/scitech/Comets-and-Asteroids/How-Asteroids-and-Comets-Formed.html |title=How Asteroids and Comets Formed |work=Science Clarified |accessdate=16 January 2016}}</ref>

Astronomers think that comets originate in both the [[Oort cloud]] and the [[scattered disk]].<ref name="Levison2007">{{cite book |title=Encyclopedia of the Solar System |chapter=Comet Populations and Cometary Dynamics |edition=2nd |publisher=Academic Press |location=Amsterdam |last1=Levison |first1=Harold F. |first2=Luke |last2=Donnes |editor1-first=Lucy-Ann Adams |editor1-last=McFadden |editor2-first=Paul Robert |editor2-last=Weissman |editor3-first=Torrence V. |editor3-last=Johnson |pages=575–588 |date=2007 |isbn=0-12-088589-1}}</ref>


==Size==
==Size==
[[File:Tempel 1 Hartley 2 comparison.jpg|thumb|left|150px|Tempel 1 and Hartley 2 compared]]
[[File:Tempel 1 Hartley 2 comparison.jpg|thumb|left|150px|Tempel 1 and Hartley 2 compared]]
[[File:PIA02118.jpg|thumb|left|150px|Tempel 1 in X-ray light by [[Chandra X-ray Observatory|Chandra]]. (Distant imaging includes coma)]]
[[File:PIA02118.jpg|thumb|left|150px|Tempel 1 in X-ray light by [[Chandra X-ray Observatory|Chandra]]. Distant imaging includes the coma.]]
Most cometary nuclei are thought to be no more than about 10 miles (16 kilometers) across.<ref name=Yeomans/> The largest comets that have come inside the orbit of [[Saturn]] are [http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=C%2F2002%20VQ94;orb=0;cov=0;log=0;cad=0#elem C/2002 VQ94] ([http://arxiv.org/abs/1401.3137 ~100&nbsp;km]), [[Comet Hale–Bopp|Hale–Bopp]] (~60&nbsp;km), [[29P/Schwassmann–Wachmann|29P]] (~30.8&nbsp;km), [[109P/Swift–Tuttle]] (~26&nbsp;km), and [[28P/Neujmin]] (~21.4&nbsp;km).
Most cometary nuclei are thought to be no more than about 10 miles (16 kilometers) across.<ref name=Yeomans/> The largest comets that have come inside the orbit of [[Saturn]] are [http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=C%2F2002%20VQ94;orb=0;cov=0;log=0;cad=0#elem C/2002 VQ94] ([http://arxiv.org/abs/1401.3137 ~100&nbsp;km]), [[Comet Hale–Bopp|Hale–Bopp]] (~60&nbsp;km), [[29P/Schwassmann–Wachmann|29P]] (~30.8&nbsp;km), [[109P/Swift–Tuttle]] (~26&nbsp;km), and [[28P/Neujmin]] (~21.4&nbsp;km).
[[File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|200px|right|C/2006 W3 (Chistensen) - emitting carbon gas]]
[[File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|200px|right|C/2006 W3 (Chistensen) - emitting carbon gas]]
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|publisher=Astronomical Society of the Pacific (No. 6 – Fall 1986)
|publisher=Astronomical Society of the Pacific (No. 6 – Fall 1986)
|url=http://www.astrosociety.org/education/publications/tnl/06/06.html
|url=http://www.astrosociety.org/education/publications/tnl/06/06.html
|accessdate=14 December 2008}}</ref> contains equal amounts of ice and dust.
⚫
|accessdate=14 December 2008}}</ref> 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.<ref name=Yeomans/> Scientists believe that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an 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]] spacecraft observed the nucleus of Comet [[19P/Borrelly|Borrelly]] and found it to be about half the size (8×4×4&nbsp;km)<ref name=Weaver2003>{{cite journal
During a flyby in 2001, the [[Deep Space 1]] spacecraft observed the nucleus of Comet [[19P/Borrelly|Borrelly]] and found it to be about half the size (8×4×4&nbsp;km)<ref name=Weaver2003>{{cite journal
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81P/Wild: Using the [[Volume#Volume formulas|volume of an ellipsoid]] of 5.5x4.0x3.3km * a density of 0.6 g/cm<sup>3</sup> yields a mass of 2.28E+13 kg.</ref>
81P/Wild: Using the [[Volume#Volume formulas|volume of an ellipsoid]] of 5.5x4.0x3.3km * a density of 0.6 g/cm<sup>3</sup> yields a mass of 2.28E+13 kg.</ref>
|-
|-
|[[Halley's Comet]] || 15 × 8 × 8<ref name=Yeomans/><ref name=Learn/> || 0.6<ref>{{cite journal|title=Is the nucleus of Comet Halley a low density body?|author= RZ Sagdeev; PE Elyasberg; VI Moroz.|bibcode=1988Natur.331..240S|date=1988|doi=10.1038/331240a0|volume=331|issue=6153|journal=Nature|pages=240–242}}</ref> || 3{{e|14}}
|[[Halley's Comet]] || 15 × 8 × 8<ref name=Yeomans/><ref name=Learn/> || 0.6<ref>{{cite journal| title=Is the nucleus of Comet Halley a low density body?|author= RZ Sagdeev; PE Elyasberg; VI Moroz.| bibcode=1988Natur.331..240S|date=1988| doi=10.1038/331240a0| volume=331|issue=6153| journal=Nature| pages=240–242}}</ref> || 3{{e|14}}
|-
|-
|[[Tempel 1]] || 7.6×4.9<ref name=Tempel1>{{cite web |title=Comet 9P/Tempel 1 |publisher=The Planetary Society |url=http://www.planetary.org/explore/topics/asteroids_and_comets/tempel1.html |accessdate=15 December 2008}}</ref> || 0.62<ref name=Britt2006/> || 7.9{{e|13}}
|[[Tempel 1]] || 7.6×4.9<ref name=Tempel1>{{cite web |title=Comet 9P/Tempel 1 |publisher=The Planetary Society |url=http://www.planetary.org/explore/topics/asteroids_and_comets/tempel1.html |accessdate=15 December 2008}}</ref> || 0.62<ref name=Britt2006/> || 7.9{{e|13}}
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==Composition==
==Composition==
⚫
About 80% of the [[Halley's Comet]] nucleus is water ice, and frozen carbon monoxide ([[carbon monoxide|CO]]) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia.<ref name=Yeomans/> Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an 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.


The composition of [[water vapor]] from [[Churyumov–Gerasimenko]] comet, as determined by the [[Rosetta (spacecraft)|''Rosetta'' mission]], is substantially different from that found on Earth. The ratio of [[deuterium]] to [[hydrogen]] in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water on Earth came from comets such as Churyumov–Gerasimenko.<ref name="AP-20141210-SB">{{cite news |url=http://apnews.excite.com/article/20141210/us-sci-comet-water-67af853779.html |title=The mystery of where Earth's water came from deepens |work=Excite News |agency=Associated Press |first=Seth |last=Borenstein |date=10 December 2014 |accessdate=14 December 2014}}</ref><ref name="NASA-20141210-DCA">{{cite web |url=http://www.jpl.nasa.gov/news/news.php?release=2014-423 |title=Rosetta Instrument Reignites Debate on Earth's Oceans |publisher=[[NASA]] |first1=D. C. |last1=Agle |first2=Markus |last2=Bauer |date=10 December 2014 |accessdate=10 December 2014}}</ref>

==Structure==
While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed [[rubble pile]]s of smaller ice planetesimals of a previous generation,<ref name="Krishna1997">{{cite book |title=Physics of Comets |series=World Scientific Series in Astronomy and Astrophysics, Volume 2 |publisher=World Scientific |edition=2nd |last=Krishna Swamy |first=K. S. |pages=364 |date=May 1997 |isbn=981-02-2632-2}}</ref> the ''Rosetta'' mission dispelled the idea that comets are "rubble piles" of disparate material.<ref name="latimes20150731">{{cite news |url=http://www.latimes.com/science/sciencenow/la-sci-sn-rosetta-philae-comet-67p-churyumov-gerasimenko-organic-bounce-20150730-story.html |title=After a bounce, Rosetta |work=[[Los Angeles Times]] |last=Khan |first=Amina |date=31 July 2015 |accessdate=22 January 2016}}</ref><ref name="rosetta-faq">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions |title=Rosetta's frequently asked questions |publisher=European Space Agency |date=2015 |accessdate=22 January 2016}}</ref>

Measurements carried out by the ''Philae'' lander on [[67P/Churyumov–Gerasimenko]] comet, indicate that the dust layer could be as much as {{convert|20|cm|abbr=on}} thick. Beneath that is hard ice, or a mixture of ice and dust. [[Porosity]] appears to increase toward the center of the comet.<ref name="esa20141218">{{cite web |url=http://blogs.esa.int/rosetta/2014/11/18/philae-settles-in-dust-covered-ice/ |title=Philae settles in dust-covered ice |publisher=European Space Agency |first=Emily |last=Baldwin |date=18 November 2014 |accessdate=18 December 2014}}</ref>

⚫
===Splitting===
⚫
[[File:Schwassman-Wachmann3-B-HST.gif|thumb|Fragment B of Comet 73P/Schwassmann-Wachmann 3 disintegrating, as seen by the Hubble Space Telescope]]
⚫
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.<ref name=Yeomans>{{cite web|last=Yeomans|first=Donald K.|date=2005|title=Comets (World Book Online Reference Center 125580)|publisher=NASA|url=http://www.nasa.gov/worldbook/comet_worldbook.html|accessdate=20 November 2007}}{{dead link|date=May 2012}}</ref> Splitting comets include [[3D/Biela]] in 1846, [[Comet Shoemaker–Levy 9|Shoemaker–Levy 9]] in 1992,<ref name=shoemaker>{{cite web
⚫
|title=Comet Shoemaker-Levy Background
⚫
|publisher=JPL/NASA
⚫
|author=JPL Public Information Office
⚫
|url=http://www2.jpl.nasa.gov/sl9/background.html
⚫
|accessdate=25 October 2008}}</ref> and [[73P/Schwassmann–Wachmann]] from 1995 to 2006.<ref name=spitzer2006>{{cite web
⚫
|date=10 May 2006
⚫
|title=Spitzer Telescope Sees Trail of Comet Crumbs
⚫
|publisher=Spitzer Space Telescope at Caltech
⚫
|author=Whitney Clavin
⚫
|url=http://www.spitzer.caltech.edu/news/239-ssc2006-13-Spitzer-Telescope-Sees-Trail-of-Comet-Crumbs
⚫
|accessdate=25 October 2008}}</ref> Greek historian [[Ephorus]] reported that a comet split apart as far back as the winter of 372–373 BC.<ref>{{cite web|title=Great Comets in History|author=Donald K. Yeomans|work=Jet Propulsion Laboratory|url=http://ssd.jpl.nasa.gov/?great_comets|date=1998|accessdate=15 March 2007}}</ref> Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.<ref name=split>{{cite web
⚫
|title=Split Comets
⚫
|publisher=Lunar and Planetary Institute (Max-Planck-Institut für Astronomie Heidelberg)
⚫
|author=H. Boehnhardt
⚫
|url=http://www.lpi.usra.edu/books/CometsII/7011.pdf
⚫
|accessdate=25 October 2008}}</ref>

⚫
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.<ref name="DPS35">{{cite web
⚫
|date=1–6 September 2003
⚫
|title=Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?
⚫
|publisher=Bulletin of the American Astronomical Society, 35 #4
⚫
|author=J. Pittichova; K.J. Meech; G.B. Valsecch; E.M. Pittich
⚫
|url=http://aas.org/archives/BAAS/v35n4/dps2003/72.htm?q=publications/baas/v35n4/dps2003/72.htm
⚫
|accessdate=1 March 2010}}</ref>


==Albedo==
==Albedo==
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Roughly six percent of the [[near-Earth asteroid]]s are thought to be extinct nuclei of comets (see [[Extinct comets]]) which no longer experience outgassing.<ref name=dormant>{{cite journal |first=Kathryn|last=Whitman |first2=Alessandro|last2=Morbidelli |first3=Robert|last3=Jedicke |title=The Size-Frequency Distribution of Dormant Jupiter Family Comets |date=2006 |arxiv=astro-ph/0603106|bibcode = 2006Icar..183..101W |doi = 10.1016/j.icarus.2006.02.016 |volume=183 |journal=Icarus |pages=101–114}}</ref> Two near-Earth asteroids with albedos this low include [[14827 Hypnos]] and [[3552 Don Quixote]].
Roughly six percent of the [[near-Earth asteroid]]s are thought to be extinct nuclei of comets (see [[Extinct comets]]) which no longer experience outgassing.<ref name=dormant>{{cite journal |first=Kathryn|last=Whitman |first2=Alessandro|last2=Morbidelli |first3=Robert|last3=Jedicke |title=The Size-Frequency Distribution of Dormant Jupiter Family Comets |date=2006 |arxiv=astro-ph/0603106|bibcode = 2006Icar..183..101W |doi = 10.1016/j.icarus.2006.02.016 |volume=183 |journal=Icarus |pages=101–114}}</ref> Two near-Earth asteroids with albedos this low include [[14827 Hypnos]] and [[3552 Don Quixote]].


==Discovery/Visitation==
==Discovery and exploration==
The first really close mission to a comet nucleus was space probe [[Giotto (spacecraft)|Giotto]].<ref name=Giotto>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Giotto_overview|title=Giotto overview|author=esa|work=European Space Agency}}</ref> This was the first time a nucleus was imaged at such proximity, coming as near as 596&nbsp;km.<ref name=Giotto/> The data was a revelation, showing for the first time the jets, the low-albedo surface, and [[organic compounds]].<ref name=Giotto/><ref>Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see [[Organic chemistry]].</ref>
The first really close mission to a comet nucleus was space probe [[Giotto (spacecraft)|Giotto]].<ref name=Giotto>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Giotto_overview|title=Giotto overview|author=esa|work=European Space Agency}}</ref> This was the first time a nucleus was imaged at such proximity, coming as near as 596&nbsp;km.<ref name=Giotto/> The data was a revelation, showing for the first time the jets, the low-albedo surface, and [[organic compounds]].<ref name=Giotto/><ref>Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see [[Organic chemistry]].</ref>


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|}
|}


Comets already visited are:
⚫
==Splitting==
⚫
[[File:Schwassman-Wachmann3-B-HST.gif|thumb|Fragment B of Comet 73P/Schwassmann-Wachmann 3 disintegrating, as seen by the Hubble Space Telescope]]
⚫
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.<ref name=Yeomans>{{cite web|last=Yeomans|first=Donald K.|date=2005|title=Comets (World Book Online Reference Center 125580)|publisher=NASA|url=http://www.nasa.gov/worldbook/comet_worldbook.html|accessdate=20 November 2007}}{{dead link|date=May 2012}}</ref> Splitting comets include [[3D/Biela]] in 1846, [[Comet Shoemaker–Levy 9|Shoemaker–Levy 9]] in 1992,<ref name=shoemaker>{{cite web
⚫
|title=Comet Shoemaker-Levy Background
⚫
|publisher=JPL/NASA
⚫
|author=JPL Public Information Office
⚫
|url=http://www2.jpl.nasa.gov/sl9/background.html
⚫
|accessdate=25 October 2008}}</ref> and [[73P/Schwassmann–Wachmann]] from 1995 to 2006.<ref name=spitzer2006>{{cite web
⚫
|date=10 May 2006
⚫
|title=Spitzer Telescope Sees Trail of Comet Crumbs
⚫
|publisher=Spitzer Space Telescope at Caltech
⚫
|author=Whitney Clavin
⚫
|url=http://www.spitzer.caltech.edu/news/239-ssc2006-13-Spitzer-Telescope-Sees-Trail-of-Comet-Crumbs
⚫
|accessdate=25 October 2008}}</ref> Greek historian [[Ephorus]] reported that a comet split apart as far back as the winter of 372–373 BC.<ref>{{cite web|title=Great Comets in History|author=Donald K. Yeomans|work=Jet Propulsion Laboratory|url=http://ssd.jpl.nasa.gov/?great_comets|date=1998|accessdate=15 March 2007}}</ref> Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.<ref name=split>{{cite web
⚫
|title=Split Comets
⚫
|publisher=Lunar and Planetary Institute (Max-Planck-Institut für Astronomie Heidelberg)
⚫
|author=H. Boehnhardt
⚫
|url=http://www.lpi.usra.edu/books/CometsII/7011.pdf
⚫
|accessdate=25 October 2008}}</ref>

⚫
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.<ref name="DPS35">{{cite web
⚫
|date=1–6 September 2003
⚫
|title=Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?
⚫
|publisher=Bulletin of the American Astronomical Society, 35 #4
⚫
|author=J. Pittichova; K.J. Meech; G.B. Valsecch; E.M. Pittich
⚫
|url=http://aas.org/archives/BAAS/v35n4/dps2003/72.htm?q=publications/baas/v35n4/dps2003/72.htm
⚫
|accessdate=1 March 2010}}</ref>

==Visited==
*[[Halley's Comet]]
*[[Halley's Comet]]
*[[Tempel 1]] (also hit with impactor)
*[[Tempel 1]] (also hit with impactor)

Revision as of 20:33, 1 May 2016

The nucleus of Comet Tempel 1.
Surface of the nucleus of Comet 67P from 10 km away as seen by Rosetta spacecraft

The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04.[1] This is blacker than coal, and may be caused by a covering of dust.[2]

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[3][4] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[5][6] On 30 July 2015, scientists reported that the Philae spacecraft, that landed on comet 67P/Churyumov-Gerasimenko in November 2014, detected at least 16 organic compounds, of which four (including acetamide, acetone, methyl isocyanate and propionaldehyde) were detected for the first time on a comet.[7][8][9]

Origin

Main article: Accretion (astrophysics) § Accretion of comets
The Helix Nebula has a cometary Oort cloud

Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation.[10] How and when comets formed is debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals.[11][12] The currently favored creation mechanism is that of the nebular hypothesis, which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew.[13][14][15]

Astronomers think that comets originate in both the Oort cloud and the scattered disk.[16]

Size

Tempel 1 and Hartley 2 compared
Tempel 1 in X-ray light by Chandra. Distant imaging includes the coma.

Most cometary nuclei are thought to be no more than about 10 miles (16 kilometers) across.[17] The largest comets that have come inside the orbit of Saturn are C/2002 VQ94 (~100 km), Hale–Bopp (~60 km), 29P (~30.8 km), 109P/Swift–Tuttle (~26 km), and 28P/Neujmin (~21.4 km).

C/2006 W3 (Chistensen) - emitting carbon gas

The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km)[17][18] contains equal amounts of ice and dust.

During a flyby in 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size (8×4×4 km)[19] of the nucleus of Halley's Comet.[17] Borrelly's nucleus was also potato-shaped and had a dark black surface.[17] 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.[20] 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 is probably only 100–200 meters in diameter.[21]

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 (258 km), 2060 Chiron (230 km), and the currently lost 1995 SN55 (~300 km).

Known comets have been estimated to have an average density of 0.6 g/cm3.[22] Below is a list of comets that have had estimated sizes, densities, and masses.

Name Dimensions
km		 Density
g/cm3		 Mass
kg[23]
Halley's Comet 15 × 8 × 8[17][18] 0.6[24] 3×1014
Tempel 1 7.6×4.9[25] 0.62[22] 7.9×1013
19P/Borrelly 8×4×4[19] 0.3[22] 2×1013
81P/Wild 5.5×4.0×3.3[26] 0.6[22] 2.3×1013
67P/Churyumov–Gerasimenko See article on 67P 0.4[27] (1.0±0.1)×1013 [28]

Composition

About 80% of the Halley's Comet nucleus is water ice, and frozen carbon monoxide (CO) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia.[17] Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an 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.

The composition of water vapor from Churyumov–Gerasimenko comet, as determined by the Rosetta mission, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water on Earth came from comets such as Churyumov–Gerasimenko.[29][30]

Structure

While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed rubble piles of smaller ice planetesimals of a previous generation,[31] the Rosetta mission dispelled the idea that comets are "rubble piles" of disparate material.[32][33]

Measurements carried out by the Philae lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as 20 cm (7.9 in) thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet.[34]

Splitting

Fragment B of Comet 73P/Schwassmann-Wachmann 3 disintegrating, as seen by the Hubble Space Telescope

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[17] Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992,[35] and 73P/Schwassmann–Wachmann from 1995 to 2006.[36] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.[37] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[38]

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.[39]

Albedo

Comet Wild 2

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".[1] Cometary nuclei are among the darkest objects known to exist in the Solar System. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it,[40] and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it;[40] by comparison, fresh 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.

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing.[41] Two near-Earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote.

Discovery and exploration

The first really close mission to a comet nucleus was space probe Giotto.[42] This was the first time a nucleus was imaged at such proximity, coming as near as 596 km.[42] The data was a revelation, showing for the first time the jets, the low-albedo surface, and organic compounds.[42][43]

During its flyby, Giotto was hit at least 12,000 times by particles, including a 1-gram fragment that caused a temporary loss of communication with Darmstadt.[42] Halley was calculated to be ejecting three tonnes of material per second[44] from seven jets, causing it to wobble over long time periods.[2] Comet Grigg–Skjellerup's nucleus was visited after Halley, with Giotto approaching 100–200 km.[42]

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[3][4] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[5][6]

Tempel 1
Deep Impact 		Tempel 1
Stardust 		Borrelly
Deep Space 1 		Wild 2
Stardust 		Hartley 2
Deep Impact 		C-G
Rosetta

Comets already visited are:

See also

References

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  2. ^ a b "ESA Science & Technology: Halley". ESA. 10 March 2006. Retrieved 22 February 2009.
  3. ^ a b Bauer, Markus (14 April 2015). "Rosetta and Philae Find Comet Not Magnetised". European Space Agency. Retrieved 14 April 2015.
  4. ^ a b Schiermeier, Quirin (14 April 2015). "Rosetta's comet has no magnetic field". Nature. doi:10.1038/nature.2015.17327.
  5. ^ a b Agle, DC; Brown, Dwayne; Fohn, Joe; Bauer, Markus (2 June 2015). "NASA Instrument on Rosetta Makes Comet Atmosphere Discovery". NASA. Retrieved 2 June 2015.
  6. ^ a b Feldman, Paul D.; A'Hearn, Michael F.; Bertaux, Jean-Loup; Feaga, Lori M.; Parker, Joel Wm.; et al. (2 June 2015). "Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta" (PDF). Astronomy and Astrophysics. 583: A8. arXiv:1506.01203. Bibcode:2015A&A...583A...8F. doi:10.1051/0004-6361/201525925.
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  9. ^ Bibring, J.-P.; Taylor, M.G.G.T.; Alexander, C.; Auster, U.; Biele, J.; Finzi, A. Ercoli; Goesmann, F.; Klingehoefer, G.; Kofman, W.; Mottola, S.; Seidenstiker, K.J.; Spohn, T.; Wright, I. (31 July 2015). "Philae's First Days on the Comet - Introduction to Special Issue". Science. 349 (6247): 493. Bibcode:2015Sci...349..493B. doi:10.1126/science.aac5116. Retrieved 30 July 2015.
  10. ^ "How comets were assembled". University of Bern via Phys.org. 29 May 2015. Retrieved 8 January 2016. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)
  11. ^ Jutzi, M.; Asphaug, E. (June 2015). "The shape and structure of cometary nuclei as a result of low-velocity accretion". Science. 348 (6241): 1355–1358. Bibcode:2015Sci...348.1355J. doi:10.1126/science.aaa4747.
  12. ^ Weidenschilling, S. J. (June 1997). "The Origin of Comets in the Solar Nebula: A Unified Model". Icarus. 127 (2): 290–306. Bibcode:1997Icar..127..290W. doi:10.1006/icar.1997.5712.
  13. ^ Choi, Charles Q. (15 November 2014). "Comets: Facts About The 'Dirty Snowballs' of Space". Space.com. Retrieved 8 January 2016.
  14. ^ Nuth, Joseph A.; Hill, Hugh G. M.; Kletetschka, Gunther (20 July 2000). "Determining the ages of comets from the fraction of crystalline dust". Nature. 406 (6793): 275–276. Bibcode:2000Natur.406..275N. doi:10.1038/35018516.
  15. ^ "How Asteroids and Comets Formed". Science Clarified. Retrieved 16 January 2016.
  16. ^ Levison, Harold F.; Donnes, Luke (2007). "Comet Populations and Cometary Dynamics". In McFadden, Lucy-Ann Adams; Weissman, Paul Robert; Johnson, Torrence V. (eds.). Encyclopedia of the Solar System (2nd ed.). Amsterdam: Academic Press. pp. 575–588. ISBN 0-12-088589-1.
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  18. ^ a b "What Have We Learned About Halley's Comet?". Astronomical Society of the Pacific (No. 6 – Fall 1986). 1986. Retrieved 14 December 2008.
  19. ^ a b Weaver, H. A.; Stern, S.A.; Parker, J. Wm. (2003). "Hubble Space Telescope STIS Observations of Comet 19P/BORRELLY during the Deep Space 1 Encounter". The American Astronomical Society. 126 (1): 444–451. Bibcode:2003AJ....126..444W. doi:10.1086/375752. Retrieved 14 December 2008.
  20. ^ Fernández, Yanga R. (2002). "The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity". Earth, Moon, and Planets. 89 (1): 3–25. Bibcode:2000EM&P...89....3F. doi:10.1023/A:1021545031431.
  21. ^ "SOHO's new catch: its first officially periodic comet". European Space Agency. 25 September 2007. Retrieved 20 November 2007.
  22. ^ a b c d D. T. Britt; G. J. Consol-magno SJ; W. J. Merline (2006). "Small Body Density and Porosity: New Data, New Insights" (PDF). Lunar and Planetary Science XXXVII. Retrieved 14 December 2008.{{cite web}}: CS1 maint: multiple names: authors list (link)
  23. ^ Halley: Using the volume of an ellipsoid of 15x8x8km * a rubble pile density of 0.6 g/cm3 yields a mass (m=d*v) of 3.02E+14 kg.
    Tempel 1: Using a spherical diameter of 6.25 km; volume of a sphere * a density of 0.62 g/cm3 yields a mass of 7.9E+13 kg.
    19P/Borrelly: Using the volume of an ellipsoid of 8x4x4km * a density of 0.3 g/cm3 yields a mass of 2.0E+13 kg.
    81P/Wild: Using the volume of an ellipsoid of 5.5x4.0x3.3km * a density of 0.6 g/cm3 yields a mass of 2.28E+13 kg.
  24. ^ RZ Sagdeev; PE Elyasberg; VI Moroz. (1988). "Is the nucleus of Comet Halley a low density body?". Nature. 331 (6153): 240–242. Bibcode:1988Natur.331..240S. doi:10.1038/331240a0.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  26. ^ "Comet 81P/Wild 2". The Planetary Society. Retrieved 20 November 2007.
  27. ^ Baldwin, Emily (6 October 2014). "Measuring Comet 67P/C-G". European Space Agency. Retrieved 16 November 2014.
  28. ^ Baldwin, Emily (21 August 2014). "Determining the mass of comet 67P/C-G". European Space Agency. Retrieved 21 August 2014.
  29. ^ Borenstein, Seth (10 December 2014). "The mystery of where Earth's water came from deepens". Excite News. Associated Press. Retrieved 14 December 2014.
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  32. ^ Khan, Amina (31 July 2015). "After a bounce, Rosetta". Los Angeles Times. Retrieved 22 January 2016.
  33. ^ "Rosetta's frequently asked questions". European Space Agency. 2015. Retrieved 22 January 2016.
  34. ^ Baldwin, Emily (18 November 2014). "Philae settles in dust-covered ice". European Space Agency. Retrieved 18 December 2014.
  35. ^ JPL Public Information Office. "Comet Shoemaker-Levy Background". JPL/NASA. Retrieved 25 October 2008.
  36. ^ Whitney Clavin (10 May 2006). "Spitzer Telescope Sees Trail of Comet Crumbs". Spitzer Space Telescope at Caltech. Retrieved 25 October 2008.
  37. ^ Donald K. Yeomans (1998). "Great Comets in History". Jet Propulsion Laboratory. Retrieved 15 March 2007.
  38. ^ H. Boehnhardt. "Split Comets" (PDF). Lunar and Planetary Institute (Max-Planck-Institut für Astronomie Heidelberg). Retrieved 25 October 2008.
  39. ^ J. Pittichova; K.J. Meech; G.B. Valsecch; E.M. Pittich (1–6 September 2003). "Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?". Bulletin of the American Astronomical Society, 35 #4. Retrieved 1 March 2010.{{cite web}}: CS1 maint: multiple names: authors list (link)
  40. ^ a b "Comet May Be the Darkest Object Yet Seen". The New York Times. 14 December 2001. Retrieved 9 May 2011.
  41. ^ Whitman, Kathryn; Morbidelli, Alessandro; Jedicke, Robert (2006). "The Size-Frequency Distribution of Dormant Jupiter Family Comets". Icarus. 183: 101–114. arXiv:astro-ph/0603106. Bibcode:2006Icar..183..101W. doi:10.1016/j.icarus.2006.02.016.
  42. ^ a b c d e esa. "Giotto overview". European Space Agency.
  43. ^ Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see Organic chemistry.
  44. ^ J. A. M. McDonnell; et al. (15 May 1986). "Dust density and mass distribution near comet Halley from Giotto observations". Nature. 321: 338–341. Bibcode:1986Natur.321..338M. doi:10.1038/321338a0.
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