Editing Comet (section)

=== 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>