Editing Comet (section)

== Physical characteristics ==
[[File:Structure of a comet.jpg|thumb|Structure of a comet]]

=== Nucleus ===
[[File:Comet Hartley 2.jpg|thumb|Nucleus of [[103P/Hartley]] as imaged during a [[Deep Impact (spacecraft)|spacecraft flyby]]. The nucleus is about 2&nbsp;km in length.]]
{{Main|Comet nucleus}}
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of [[rock (geology)|rock]], [[Comet dust|dust]], [[ice|water ice]], and frozen [[carbon dioxide]], [[carbon monoxide]], [[methane]], and [[ammonia]].<ref>{{cite journal |bibcode=1998A&A...330..375G |title=Making a comet nucleus |last1=Greenberg |first1=J. Mayo |volume=330 |date=1998 |pages=375 |journal=[[Astronomy & Astrophysics]]}}</ref> As such, they are popularly described as "dirty snowballs" after [[Fred Whipple]]'s model.<ref>{{cite web |url=http://starryskies.com/solar_system/Comet/dirty_snowballs.html |title=Dirty Snowballs in Space |publisher=Starryskies |access-date=15 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130129035627/http://starryskies.com/solar_system/Comet/dirty_snowballs.html |archive-date=29 January 2013}}</ref> Comets with a higher dust content have been called "icy dirtballs".<ref>{{cite news |url=http://www.timeshighereducation.co.uk/news/evidence-from-esas-rosetta-spacecraft-suggests-that-comets-are-more-icy-dirtball-than-dirty-snowball/199168.article |title=Evidence from ESA's Rosetta Spacecraft Suggests that Comets are more "Icy Dirtball" than "Dirty Snowball" |date=21 October 2005 |work=Times Higher Education}}</ref> The term "icy dirtballs" arose after observation of [[Tempel 1|Comet 9P/Tempel 1]] collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "[[Fried ice cream|deep fried ice cream]]", in that their surfaces are formed of dense crystalline ice mixed with [[organic compound]]s, while the interior ice is colder and less dense.<ref name="NASA-20150210" />

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. Nuclei contain a variety of organic compounds, which may include [[methanol]], [[hydrogen cyanide]], [[formaldehyde]], [[ethanol]], [[ethane]], and perhaps more complex molecules such as long-chain [[hydrocarbon]]s and [[amino acid]]s.<ref>{{cite web |last=Meech |first=M. |title=1997 Apparition of Comet Hale–Bopp: What We Can Learn from Bright Comets |url=http://www.psrd.hawaii.edu/Feb97/Bright.html |publisher=Planetary Science Research Discoveries |date=24 March 1997 |access-date=30 April 2013}}</ref><ref>{{cite web |title=Stardust Findings Suggest Comets More Complex Than Thought |url=http://stardust.jpl.nasa.gov/news/news110.html |publisher=NASA |date=14 December 2006 |access-date=31 July 2013}}</ref> In 2009, it was confirmed that the amino acid [[glycine]] had been found in the comet dust recovered by NASA's [[Stardust (spacecraft)|Stardust mission]].<ref>{{cite journal |doi=10.1111/j.1945-5100.2009.tb01224.x |title=Cometary glycine detected in samples returned by Stardust |date=2009 |last1=Elsila |first1=Jamie E. |last2=Glavin |first2=Daniel P. |last3=Dworkin |first3=Jason P. |display-authors=1 |journal=Meteoritics & Planetary Science |volume=44 |issue=9 |pages=1323 |bibcode=2009M&PS...44.1323E|doi-access=free }}</ref> In August 2011, a report, based on [[NASA]] studies of [[meteorite]]s found on Earth, was published suggesting [[DNA]] and [[RNA]] components ([[adenine]], [[guanine]], and related organic molecules) may have been formed on [[asteroid]]s and comets.<ref name="Callahan">{{cite journal |doi=10.1073/pnas.1106493108 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |date=2011 |last1=Callahan |first1=M. P. |last2=Smith |first2=K. E. |last3=Cleaves |first3=H. J. |last4=Ruzicka |first4=J. |last5=Stern |first5=J. C. |last6=Glavin |first6=D. P. |last7=House |first7=C. H. |last8=Dworkin |first8=J. P. |display-authors=1 |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=34 |pages=13995–8 |bibcode=2011PNAS..10813995C |pmid=21836052 |pmc=3161613|doi-access=free }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=NASA |date=8 August 2011 |access-date=31 July 2013 |archive-date=26 April 2020 |archive-url=https://web.archive.org/web/20200426055700/https://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |url-status=dead }}</ref>

The outer surfaces of cometary nuclei have a very low [[albedo]], making them among the least reflective objects found in the Solar System. The [[Giotto (spacecraft)|Giotto]] [[space probe]] found that the nucleus of [[Halley's Comet]] (1P/Halley) reflects about four percent of the light that falls on it,<ref name="dark">{{cite journal |title=The Activity and Size of the Nucleus of Comet Hale-Bopp (C/1995 O1) |journal=Science |last1=Weaver |first1=H. A. |last2=Feldman |first2=P. D. |last3=a'Hearn |first3=M. F. |last4=Arpigny |first4=C. |last5=Brandt |first5=J. C. |last6=Festou |first6=M. C. |last7=Haken |first7=M. |last8=McPhate |first8=J. B. |last9=Stern |first9=S. A. |last10=Tozzi |first10=G. P. |display-authors=1 |volume=275 |issue=5308 |pages=1900–1904 |date=1997 |pmid=9072959 |doi=10.1126/science.275.5308.1900 |bibcode=1997Sci...275.1900W|s2cid=25489175 }}</ref> and [[Deep Space 1]] discovered that [[19P/Borrelly|Comet Borrelly]]'s surface reflects less than 3.0%;<ref name="dark" /> by comparison, [[Bitumen|asphalt]] reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter [[Volatility (chemistry)|volatile]] [[Chemical compound|compounds]], leaving behind larger organic compounds that tend to be very dark, like [[tar]] or [[petroleum|crude oil]]. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their [[outgassing]] processes.<ref>{{cite book |url=https://books.google.com/books?id=PRqVqQKao9QC&pg=PA91 |page=91 |title=Habitability and Cosmic Catastrophes |isbn=978-3-540-76945-3 |last1=Hanslmeier |first1=Arnold |date=2008|publisher=Springer }}</ref>

Comet nuclei with radii of up to {{convert|30|km|mi|sp=us}} have been observed,<ref>{{cite journal |doi=10.1023/A:1021545031431 |title=The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity |date=2000 |last1=Fernández |first1=Yanga R. |journal=Earth, Moon, and Planets |volume=89 |issue=1 |pages=3–25 |bibcode=2002EM&P...89....3F|s2cid=189899565 }}</ref> but ascertaining their exact size is difficult.<ref>{{cite web |url=http://www2.ess.ucla.edu/~jewitt/nucleus.html | first=David | last=Jewitt |title=The Cometary Nucleus |publisher=Department of Earth and Space Sciences, UCLA |date=April 2003 |access-date=31 July 2013}}</ref> The nucleus of [[322P/SOHO]] is probably only {{convert|100|-|200|m|ft|sp=us}} in diameter.<ref name="soho1">{{cite web |title=SOHO's new catch: its first officially periodic comet |publisher=European Space Agency |url=http://www.esa.int/Our_Activities/Space_Science/SOHO_s_new_catch_its_first_officially_periodic_comet |access-date=16 August 2013}}</ref> A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than {{convert|100|m|ft|sp=us}} across.<ref>{{harvnb|Sagan|Druyan|1997|p=137}}</ref> Known comets have been estimated to have an average density of {{convert|0.6|g/cm3|oz/cuin|abbr=on}}.<ref name="Britt2006">{{cite journal |bibcode=2006LPI....37.2214B |title=Small Body Density and Porosity: New Data, New Insights |last1=Britt |first1=D. T. |last2=Consolmagno |first2=G. J. |last3=Merline |first3=W. J. |display-authors=1 |volume=37 |date=2006 |pages=2214 |journal=37th Annual Lunar and Planetary Science Conference |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |access-date=25 August 2013 |archive-url=https://web.archive.org/web/20081217064607/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |archive-date=17 December 2008 |url-status=dead}}</ref> Because of their low mass, comet nuclei do not [[gravitational collapse|become spherical]] under their own [[gravity]] and therefore have irregular shapes.<ref>{{cite web |url=https://history.nasa.gov/SP-467/ch7.htm |title=The Geology of Small Bodies |date=January 1984 |publisher=NASA |access-date=15 August 2013 |last1=Veverka |first1=J. }}</ref>

[[File:Comet wild 2.jpg|thumb|220px|Comet [[81P/Wild]] exhibits jets on light side and dark side, stark relief, and is dry.]]
Roughly six percent of the [[near-Earth asteroid]]s are thought to be the [[Extinct comet|extinct nuclei of comets]] that no longer experience outgassing,<ref name="dormant">{{cite journal |doi=10.1016/j.icarus.2006.02.016 |arxiv=astro-ph/0603106v2 |date=2006 |title=The size–frequency distribution of dormant Jupiter family comets |last1=Whitman |first1=K. |last2=Morbidelli |first2=A. |last3=Jedicke |first3=R. |display-authors=1 |journal=Icarus |volume=183 |issue=1 |pages=101–114 |bibcode=2006Icar..183..101W|s2cid=14026673 }}</ref> including [[14827 Hypnos]] and [[3552 Don Quixote]].

Results from the [[Rosetta (spacecraft)|''Rosetta'']] and [[Philae (spacecraft)|''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 [[planetesimal]]s.<ref name="esa20150414">{{cite news |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_find_comet_not_magnetised |title=Rosetta and Philae Find Comet Not Magnetised |publisher=European Space Agency |first=Markus |last=Bauer |date=14 April 2015 |access-date=14 April 2015}}</ref><ref name="nature20150414">{{cite journal |title=Rosetta's comet has no magnetic field |journal=[[Nature (journal)|Nature]] |first=Quirin |last=Schiermeier |date=14 April 2015 |doi=10.1038/nature.2015.17327|s2cid=123964604 }}</ref> Further, the [[Rosetta (spacecraft)#Instruments|ALICE spectrograph]] on ''Rosetta'' determined that [[electron]]s (within {{convert|1|km|mi|abbr=on}} above the [[comet nucleus]]) produced from [[photoionization]] of water molecules by [[Sunlight|solar radiation]], and not [[photon]]s from the Sun as thought earlier, are responsible for the degradation of water and [[carbon dioxide]] [[molecule]]s released from the comet nucleus into its coma.<ref name="NASA-20150602">{{cite web |last1=Agle |first1=D. C. |last2=Brown |first2=Dwayne |last3=Fohn |first3=Joe |last4=Bauer |first4=Markus |display-authors=1 |title=NASA Instrument on Rosetta Makes Comet Atmosphere Discovery |url=http://www.jpl.nasa.gov/news/news.php?feature=4609 |date=2 June 2015 |publisher=[[NASA]] |access-date=2 June 2015}}</ref><ref name="AA-20150602">{{cite journal |last1=Feldman |first1=Paul D. |last2=A'Hearn |first2=Michael F. |last3=Bertaux |first3=Jean-Loup |last4=Feaga |first4=Lori M. |last5=Parker |first5=Joel Wm. |last6=Schindhelm |first6=Eric |last7=Steiffl |first7=Andrew J. |last8=Stern |first8=S. Alan |last9=Weaver |first9=Harold A. |last10=Sierks |first10=Holger |last11=Vincent |first11=Jean-Baptiste |display-authors=1 |title=Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta |url=http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-url=https://web.archive.org/web/20150608071334/http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-date=2015-06-08 |url-status=live |date=2 June 2015 |journal=[[Astronomy & Astrophysics]] |doi=10.1051/0004-6361/201525925 |access-date=3 June 2015 |arxiv=1506.01203 |bibcode=2015A&A...583A...8F |volume=583 |pages=A8|s2cid=119104807 }}</ref> Instruments on the ''Philae'' lander found at least sixteen organic compounds at the comet's surface, four of which ([[acetamide]], [[acetone]], [[methyl isocyanate]] and [[propionaldehyde]]) have been detected for the first time on a comet.<ref name="wapo20150730">{{cite news |url=https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |archive-url=https://web.archive.org/web/20181223235109/https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |url-status=dead |archive-date=23 December 2018 |title=Philae probe finds evidence that comets can be cosmic labs |newspaper=The Washington Post |agency=Associated Press |first=Frank |last=Jordans |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="esa20150730">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Science_on_the_surface_of_a_comet |title=Science on the Surface of a Comet |publisher=European Space Agency |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="SCI-20150731">{{cite journal |last1=Bibring |first1=J.-P. |last2=Taylor |first2=M.G.G.T. |last3=Alexander |first3=C. |last4=Auster |first4=U. |last5=Biele |first5=J. |last6=Finzi |first6=A. Ercoli |last7=Goesmann |first7=F. |last8=Klingehoefer |first8=G. |last9=Kofman |first9=W. |last10=Mottola |first10=S. |last11=Seidenstiker |first11=K.J. |last12=Spohn |first12=T. |last13=Wright |first13=I. |display-authors=1 |title=Philae's First Days on the Comet – Introduction to Special Issue |date=31 July 2015 |journal=[[Science (journal)|Science]] |volume=349 |number=6247 |page=493 |doi=10.1126/science.aac5116 |bibcode=2015Sci...349..493B |pmid=26228139|doi-access=free }}</ref>

{| class="wikitable center" style="text-align: center; width: 530px; margin: 0.1em auto;"
|+Properties of some comets
|-
! width="120" | Name
! width="120" | Dimensions<br />(km)
! width="120" | Density<br />([[Gram|g]]/cm<sup>3</sup>)
! width="120" | Mass<br />([[Kilogram|kg]])<ref name="mass">
Halley: Using the [[Volume#Formulas|volume of an ellipsoid]] of 15×8×8&nbsp;km * a [[rubble pile]] density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass (m=d*v) of 3.02E+14&nbsp;kg.<br />
Tempel 1: Using a spherical diameter of 6.25&nbsp;km; [[Volume#Formulas|volume of a sphere]] * a density of 0.62&nbsp;g/cm<sup>3</sup> yields a mass of 7.9E+13&nbsp;kg.<br />
19P/Borrelly: Using the [[Volume#Formulas|volume of an ellipsoid]] of 8x4x4km * a density of 0.3&nbsp;g/cm<sup>3</sup> yields a mass of 2.0E+13&nbsp;kg.<br />
81P/Wild: Using the [[Volume#Formulas|volume of an ellipsoid]] of 5.5x4.0x3.3&nbsp;km * a density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass of 2.28E+13&nbsp;kg.</ref>
!Refs
|-
| align="left" | [[Halley's Comet]]
| 15 × 8 × 8
| 0.6
| 3{{e|14}}
|<ref>{{cite web |url=https://astrosociety.org/file_download/inline/6f0b9235-b5eb-40e6-88a3-6dd45e3b6f92 |title=What Have We Learned About Halley's Comet? |date=1986 |publisher=Astronomical Society of the Pacific |access-date=4 October 2013}}</ref><ref>{{cite journal |title=Is the nucleus of Comet Halley a low density body? |journal=Nature |last1=Sagdeev |first1=R. Z. |last2=Elyasberg |first2=P. E. |last3=Moroz |first3=V. I. |display-authors=1 |volume=331 |issue=6153 |pages=240 |date=1988 |issn=0028-0836 |doi=10.1038/331240a0 |bibcode=1988Natur.331..240S|s2cid=4335780 }}</ref>
|-
| align="left" | [[Tempel 1]]
| 7.6 × 4.9
| 0.62
| 7.9{{e|13}}
|<ref name="Britt2006" /><ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9P |title=9P/Tempel 1 |publisher=JPL |access-date=16 August 2013}}</ref>
|-
| align="left" | [[19P/Borrelly]]
| 8 × 4 × 4
| 0.3
| 2.0{{e|13}}
|<ref name="Britt2006" />
|-
| align="left" | [[81P/Wild]]
| 5.5 × 4.0 × 3.3
| 0.6
| 2.3{{e|13}}
|<ref name="Britt2006" /><ref name="wild2">{{cite web |title=Comet 81P/Wild 2 |publisher=The Planetary Society |url=http://www.planetary.org/explore/topics/asteroids_and_comets/wild2.html |access-date=20 November 2007 |url-status=dead |archive-url=https://web.archive.org/web/20090106004009/http://planetary.org./explore/topics/asteroids_and_comets/wild2.html |archive-date=6 January 2009 }}</ref>
|-
| align="left" | [[67P/Churyumov–Gerasimenko]]
| 4.1 × 3.3 × 1.8
| 0.47
| 1.0{{e|13}}
|<ref>{{cite web |url=http://www.esa.int/spaceinimages/Images/2015/01/Comet_vital_statistics |title=Comet vital statistics |publisher=European Space Agency |date=22 January 2015 |access-date=24 January 2015}}</ref><ref>{{cite web |url=http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/ |title=Determining the mass of comet 67P/C-G |publisher=European Space Agency |first=Emily |last=Baldwin |date=21 August 2014 |access-date=21 August 2014}}</ref>
|}

=== Coma ===
{{Main|Coma (cometary)}}

[[File:Hubble's Last Look at Comet ISON Before Perihelion.jpg|thumb|[[Hubble Space Telescope|Hubble]] image of [[Comet ISON]] shortly before [[perihelion]].<ref>{{cite web |url=http://www.spacetelescope.org/images/opo1347a/ |title=Hubble's Last Look at Comet ISON Before Perihelion |publisher=European Space Agency |date=19 November 2013 |access-date=20 November 2013}}</ref>]]

[[File:Comet borrelly.jpg|thumb|left|[[19P/Borrelly|Comet Borrelly]] exhibits jets, but has no surface ice.]]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". 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.<ref>{{cite book |url=https://books.google.com/books?id=4zjv84hHNPcC&pg=PA66 |title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations |last1=Clay Sherrod |first1=P.  |last2=Koed |first2=Thomas L. |name-list-style=amp |page=66 |date=2003 |publisher=Courier Corporation |isbn=978-0-486-15216-5}}</ref>

The coma is generally made of water and dust, with water making up to 90% of the [[Volatile (astrogeology)|volatiles]] that outflow from the nucleus when the comet is within 3 to 4 [[astronomical unit]]s (450,000,000 to 600,000,000&nbsp;km; 280,000,000 to 370,000,000&nbsp;mi) of the Sun.<ref name=Combi2004>{{cite book |url=http://www.lpi.usra.edu/books/CometsII/7023.pdf |archive-url=https://web.archive.org/web/20070315183630/http://www.lpi.usra.edu/books/CometsII/7023.pdf |archive-date=2007-03-15 |url-status=live |title=Gas dynamics and kinetics in the cometary coma: Theory and observations |journal=Comets II |last1=Combi |first1=Michael R. |last2=Harris |first2=Walter M. |last3=Smyth |first3=William H. |display-authors=1 |pages=523 |date=2004 |doi=10.2307/j.ctv1v7zdq5.34 |bibcode=2004come.book..523C}}</ref> 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]].<ref name=Combi2004/> 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 [[radiation pressure|light pressure]].<ref>{{cite web |url=http://migall.fastmail.fm/astronomy/solar_system/small_bodies/hale_bop/jpl/define.htm |title=Comet Definitions |publisher=Michael Gallagher |last=Morris |first=Charles S. |access-date=31 August 2013}}</ref>

Although the solid nucleus of comets is generally less than {{convert|60|km|mi|sp=us}} across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun.<ref>{{cite journal |doi=10.1023/A:1021512317744 |bibcode=2002EM&P...90...67L |date=2002 |first1=Rosine |last1=Lallement |last2=Bertaux |first2=Jean-Loup |last3=Szegö |first3=Karöly |last4=Nemeth |first4=Szilvia |display-authors=1 |journal=Earth, Moon, and Planets |volume=90 |pages=67–76 |title=The Shadow of Comet Hale–Bopp in Lyman-Alpha|issue=1 |s2cid=118200399 }}</ref> For example, about a month after an outburst in October 2007, comet [[17P/Holmes]] briefly had a tenuous dust atmosphere larger than the Sun.<ref name=atmosphere2>{{cite web |author-link=David C. Jewitt |last=Jewitt |first=David |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=The Splintering of Comet 17P/Holmes During a Mega-Outburst |publisher=University of Hawaii |access-date=30 August 2013}}</ref> The [[Great Comet of 1811]] had a coma roughly the diameter of the Sun.<ref name="primer">{{cite web |title=The Comet Primer |work=Gary W. Kronk's Cometography |last=Kronk |first=Gary W. |url=http://cometography.com/educate/comintro.html |access-date=30 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20110517043903/http://cometography.com/educate/comintro.html |archive-date=17 May 2011 }}</ref> Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of [[Mars]] around {{convert|1.5|AU}} from the Sun.<ref name="primer"/> At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.<ref name="primer"/> Ion tails have been observed to extend one astronomical unit (150&nbsp;million km) or more.<ref name=atmosphere2/>

[[File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|C/2006 W3 (Christensen) emitting carbon gas (IR image)]]
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 reflects sunlight directly while the gases glow from [[ion]]isation.<ref name="le">{{cite web |url=http://www.le.ac.uk/ph/faulkes/web/planets/r_pl_comets.html |title=Comets |publisher=University of Leicester |last1=Brinkworth |first1=Carolyn |last2=Thomas |first2=Claire |name-list-style=amp |access-date=31 July 2013}}</ref> Most comets are too faint to be visible without the aid of a [[telescope]], but a few each decade become bright enough to be visible to the naked eye.<ref>{{cite book |url=https://books.google.com/books?id=caYpAQAAMAAJ |page=75 |title=A field guide to the stars and planets |isbn=978-0-395-93432-6 |last=Pasachoff |first=Jay M |date=2000|publisher=Houghton Mifflin }}</ref> Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to [[17P/Holmes|Comet Holmes]].<ref name=atmosphere1>{{cite web |last=Jewitt |first=David |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=Comet Holmes Bigger Than The Sun |publisher=Institute for Astronomy at the University of Hawaii |access-date=31 July 2013}}</ref>

In 1996, comets were found to emit [[X-ray]]s.<ref>{{cite journal |doi=10.1126/science.274.5285.205 |title=Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2 |date=1996 |last1=Lisse |first1=C. M. |last2=Dennerl |first2=K. |last3=Englhauser |first3=J. |last4=Harden |first4=M. |last5=Marshall |first5=F. E. |last6=Mumma |first6=M. J. |last7=Petre |first7=R. |last8=Pye |first8=J. P. |last9=Ricketts |first9=M. J. |display-authors=1 |journal=Science |volume=274 |issue=5285 |pages=205 |last10=Schmitt |first10=J. |last11=Trumper |first11=J. |last12=West |first12=R. G. |bibcode=1996Sci...274..205L |s2cid=122700701 |url=https://zenodo.org/record/1231082}}</ref> This greatly surprised astronomers because X-ray emission is usually associated with very [[black-body radiation|high-temperature bodies]]. [[Thomas E. Cravens]] was the first to propose an explanation in early 1997.<ref>{{cite journal
| last = Cravens
| first = T. E.
| date = 1997
| title = Comet Hyakutake x-ray source: Charge transfer of solar wind heavy ions
| journal = Geophysical Research Letters
| volume = 24
| issue = 1
}}</ref>  The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and [[far ultraviolet]] photons.<ref>{{cite journal |title=Charge Exchange-Induced X-Ray Emission from Comet C/1999 S4 (LINEAR) |journal=Science |last1=Lisse |first1=C. M. |last2=Christian |first2=D. J. |last3=Dennerl |first3=K. |last4=Meech |first4=K. J. |last5=Petre |first5=R. |last6=Weaver |first6=H. A. |last7=Wolk |first7=S. J. |display-authors=1 |volume=292 |issue=5520 |pages=1343–8 |date=2001 |doi=10.1126/science.292.5520.1343 |bibcode=2001Sci...292.1343L |pmid=11359004}}</ref>

===Bow shock===
[[Bow shock]]s form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 1990s as several spacecraft flew by comets [[21P/Giacobini–Zinner]],<ref>{{cite journal |title=The Bow wave of Comet Giacobini-Zinner – ICE magnetic field observations |journal=Geophysical Research Letters |last1=Jones |first1=D. E. |last2=Smith |first2=E. J. |last3=Slavin |first3=J. A. |last4=Tsurutani |first4=B. T. |last5=Siscoe |first5=G. L. |last6=Mendis |first6=D. A. |display-authors=1 |volume=13 |issue=3 |pages=243–246 |date=March 1986 |bibcode=1986GeoRL..13..243J |doi=10.1029/GL013i003p00243}}</ref> 1P/Halley,<ref>{{cite journal |title=First in situ plasma and neutral gas measurements at comet Halley |journal=Nature |first1=K. I.|last1=Gringauz|first2= T. I. |last2=Gombosi |first3=A. P. |last3=Remizov |first4=I. |last4=Apáthy |first5=I. |last5=Szemerey |first6=M. I. |last6=Verigin |first7=L. I. |last7=Denchikova |first8=A. V. |last8=Dyachkov |first9=E. |last9=Keppler |first10=I. N. |last10=Klimenko |first11=A. K. |last11=Richter |first12=A. J. |last12=Somogyi |first13=K. |last13=Szegő |first14=S. |last14=Szendrő |first15=M. |last15=Tátrallyay |first16=A. |last16=Varga |first17= G. A. |last17=Vladimirova |display-authors=1 |volume=321 |pages=282–285 |date=15 May 1986 |bibcode=1986Natur.321..282G |doi=10.1038/321282a0|s2cid=117920356 }}</ref> and [[26P/Grigg–Skjellerup]].<ref>{{cite journal |title=First results from the Giotto magnetometer experiment during the P/Grigg-Skjellerup encounter |journal=[[Astronomy & Astrophysics]] |first1=F. M. |last1=Neubauer |first2=H. |last2=Marschall |first3=M. |last3=Pohl |first4=K.-H. |last4=Glassmeier |first5=G. |last5=Musmann |first6=F. |last6=Mariani |first7=M. H. |last7=Acuna |first8=L. F. |last8=Burlaga |first9=N. F. |last9=Ness |first10=M. K. |last10=Wallis |first11=H. U. |last11=Schmidt |first12=E. |last12=Ungstrup |display-authors=1 |volume=268 |issue=2 |pages=L5–L8 |date=February 1993 |bibcode=1993A&A...268L...5N}}</ref> It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near [[perihelion]] when the bow shocks already were fully developed.

The [[Rosetta (spacecraft)|''Rosetta'']] spacecraft observed the bow shock at comet [[67P/Churyumov–Gerasimenko]] at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.<ref>{{cite journal |title=The infant bow shock: a new frontier at a weak activity comet |journal=[[Astronomy & Astrophysics]] |last1=Gunell |first1=H. |last2=Goetz |first2=C. |last3=Simon Wedlund |first3=C. |last4=Lindkvist |first4=J. |last5=Hamrin |first5=M. |last6=Nilsson |first6=H. |last7=LLera |first7=K. |last8=Eriksson |first8=A. |last9=Holmström |first9=M. |display-authors=1 |volume=619 |at=L2 |date=November 2018 |doi=10.1051/0004-6361/201834225 |bibcode=2018A&A...619L...2G |url=https://www.duo.uio.no/bitstream/10852/67125/1/aa34225-18.pdf |archive-url=https://web.archive.org/web/20190430061032/https://www.duo.uio.no/bitstream/10852/67125/1/aa34225-18.pdf |archive-date=2019-04-30 |url-status=live|doi-access=free }}</ref>

=== Tails ===
{{Main|Comet tail}}
[[File:Cometorbit01.svg|thumb|left|280px|Typical direction of tails during a comet's orbit near the Sun]]
In the outer [[Solar System]], comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the [[Kuiper belt]] have been reported from observations by the [[Hubble Space Telescope]]<ref name="Cochran1995">{{cite journal |bibcode=1995ApJ...455..342C |title=The Discovery of Halley-sized Kuiper Belt Objects Using the Hubble Space Telescope |last1=Cochran |first1=Anita L. |last2=Levison |first2=Harold F. |last3=Stern |first3=S. Alan |last4=Duncan |first4=Martin J. |display-authors=1 |volume=455 |date=1995 |pages=342 |journal=The Astrophysical Journal |doi=10.1086/176581 |arxiv=astro-ph/9509100|s2cid=118159645 }}</ref><ref name="Cochran1998">{{cite journal |doi=10.1086/311515 |title=The Calibration of the Hubble Space Telescope Kuiper Belt Object Search:Setting the Record Straight |date=1998 |last1=Cochran |first1=Anita L. |last2=Levison |first2=Harold F. |last3=Tamblyn |first3=Peter |last4=Stern |first4=S. Alan |last5=Duncan |first5=Martin J. |display-authors=1 |journal=The Astrophysical Journal |volume=503 |issue=1 |pages=L89 |arxiv=astro-ph/9806210 |bibcode=1998ApJ...503L..89C|s2cid=18215327 }}</ref> but these detections have been questioned.<ref name="Brown1997">{{cite journal |doi=10.1086/311009 |title=An Analysis of the Statistics of the \ITAL Hubble Space Telescope\/ITAL] Kuiper Belt Object Search |date=1997 |last1=Brown |first1=Michael E. |last2=Kulkarni |first2=Shrinivas R. |last3=Liggett |first3=Timothy J. |display-authors=1 |journal=The Astrophysical Journal |volume=490 |issue=1 |pages=L119–L122 |bibcode=1997ApJ...490L.119B|doi-access=free }}</ref><ref name="Jewitt1996">{{cite journal |bibcode=1996AJ....112.1225J |title=The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey |last1=Jewitt |first1=David |last2=Luu |first2=Jane |last3=Chen |first3=Jun |display-authors=1 |volume=112 |date=1996 |pages=1225 |journal=The Astronomical Journal |doi=10.1086/118093}}</ref> As a comet approaches the inner Solar System, [[solar radiation]] causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

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.<ref name="le"/> At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.<ref>{{cite book |url=https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA422 |page=422 |title=The Cambridge Guide to the Solar System |isbn=978-1-139-49417-5 |last=Lang |first=Kenneth R. |date=2011|publisher=Cambridge University Press }}</ref> On occasions—such as when Earth passes through a comet's orbital plane, the [[antitail]], pointing in the opposite direction to the ion and dust tails, may be seen.<ref>{{Cite APOD |title=PanSTARRS: The Anti Tail Comet |date=29 June 2013 |access-date=31 July 2013}}</ref>

[[File:Comet Parts.svg|thumb|upright|Diagram of a comet showing the [[Antitail|dust trail]], the dust tail, and the ion gas tail formed by [[solar wind]].]]
The observation of antitails contributed significantly to the discovery of solar wind.<ref>{{cite journal |doi=10.1007/BF00225271 |title=The plasma tails of comets and the interplanetary plasma |date=1963 |last1=Biermann |first1=L. |journal=Space Science Reviews |volume=1 |issue=3 |page=553 |bibcode=1963SSRv....1..553B|s2cid=120731934 }}</ref> The ion tail is formed as a result of the ionization 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.<ref name="pp 864">{{cite book |title=An Introduction to Modern Astrophysics |publisher=Addison-Wesley |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |name-list-style=amp |pages=864–874 |date=1996 |isbn=0-201-54730-9}}</ref>

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, [[magnetic reconnection]] occurs. This leads to a "tail disconnection event".<ref name="pp 864" /> This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of [[Encke's Comet]] was completely severed while the comet passed through a [[coronal mass ejection]]. This event was observed by the [[STEREO|STEREO space probe]].<ref>{{cite journal |title=The Heliospheric Imagers Onboard the STEREO Mission |journal=Solar Physics |last1=Eyles |first1=C. J. |last2=Harrison |first2=R. A. |last3=Davis |first3=C. J. |last4=Waltham |first4=N. R. |last5=Shaughnessy |first5=B. M. |last6=Mapson-Menard |first6=H. C. A. |last7=Bewsher |first7=D. |last8=Crothers |first8=S. R. |last9=Davies |first9=J. A. |last10=Simnett |first10=G. M. |last11=Howard |first11=R. A. |last12=Moses |first12=J. D. |last13=Newmark |first13=J. S. |last14=Socker |first14=D. G. |last15=Halain |first15=J.-P. |last16=Defise |first16=J.-M. |last17=Mazy |first17=E. |last18=Rochus |first18=P. |display-authors=1 |volume=254 |issue=2 |pages=387 |date=2008 |doi=10.1007/s11207-008-9299-0 |bibcode=2009SoPh..254..387E |hdl=2268/15675 |s2cid=54977854 |url=https://orbi.uliege.be/bitstream/2268/15675/1/The%20Heliospheric%20Imagers%20Onboard%20the%20STEREO.pdf |archive-url=https://web.archive.org/web/20180722165455/https://orbi.uliege.be/bitstream/2268/15675/1/The%20Heliospheric%20Imagers%20Onboard%20the%20STEREO.pdf |archive-date=2018-07-22 |url-status=live}}</ref>

In 2013, [[ESA]] scientists reported that the [[ionosphere]] of the planet [[Venus]] streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."<ref name="ESA-20130129">{{cite web |title=When A Planet Behaves Like A Comet |url=http://www.esa.int/Our_Activities/Space_Science/When_a_planet_behaves_like_a_comet |date=29 January 2013 |publisher=European Space Agency |access-date=30 August 2013}}</ref><ref name="Space-20130130">{{cite web |last=Kramer |first=Miriam |title=Venus Can Have 'Comet-Like' Atmosphere |url=http://www.space.com/19537-venus-comet-atmosphere.html |date=30 January 2013 |publisher=Space.com |access-date=30 August 2013}}</ref>

=== Jets ===
[[File:Hartley2jets2 epoxi big.jpg|thumb|170px|Gas and snow jets of [[103P/Hartley]]]]
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.<ref name=jets>{{cite web |url=http://hubblesite.org/hubble_discoveries/comet_ison/blogs/comets-and-jets |title=Comets and Jets |work=Hubblesite.org |date=12 November 2013}}</ref> These streams of gas and dust can cause the nucleus to spin, and even split apart.<ref name=jets/> In 2010 it was revealed that [[Sublimation (phase transition)|sublimation]] of [[dry ice]] (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.<ref>{{cite news |url=http://www.astronomynow.com/news/n1011/11hartley/ |title=Dry ice fuels comet jets |work=Astronomy Now |first=Emily |last=Baldwin |date=11 November 2010 |archive-url=https://web.archive.org/web/20131217034053/http://www.astronomynow.com/news/n1011/11hartley/ |archive-date=17 December 2013}}</ref> Infrared imaging of Hartley&nbsp;2 shows such jets exiting and carrying with it dust grains into the coma.<ref>{{cite news |url=https://www.nytimes.com/2010/11/19/science/space/19comet.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2010/11/19/science/space/19comet.html |archive-date=2022-01-01 |url-access=limited |title=Comet Hartley 2 Is Spewing Ice, NASA Photos Show |work=The New York Times |last1=Chang |first1=Kenneth |date=18 November 2010}}{{cbignore}}</ref>