Editing Coma (comet) (section)

{{Short description|Cloud of gas or a trail around a comet or asteroid}}
{{Other uses|Coma (disambiguation)}}
[[File:Infrared Structure of Comet Holmes.jpg|thumb|400px|Structure of Comet Holmes in infrared, as seen by an infrared space telescope]]

The '''coma''' is the nebulous envelope around the [[comet nucleus|nucleus]] of a [[comet]], formed when the comet passes near the [[Sun]] in its [[highly elliptical orbit]]. As the comet warms, parts of it [[sublimation (chemistry)|sublimate]];<ref name=Combi2004>{{cite journal |last1=Combi|first1=Michael R. |last2=Harris|first2=W. M. |last3=Smyth|first3=W. H. |title=Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations |journal=Comets II |publisher=[[Lunar and Planetary Institute]] |volume=745 |pages=523–552 |date=2004 |bibcode=2004come.book..523C |url=https://lpi.usra.edu/books/CometsII/7023.pdf
}}</ref> this gives a comet a diffuse appearance when viewed through [[telescope]]s and distinguishes it from [[star]]s. The word ''coma'' comes from the Greek {{Wikt-lang|grc|κόμη}} ({{grc-transl|κόμη}}), which means "hair" and is the origin of the word ''comet'' itself.<ref name=chap14>{{cite web |url=http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT314/HTML/AT31402.HTM |publisher= |website=lifeng.lamost.org |title=Chapter 14, Section 2 {{pipe}} Comet appearance and structure |access-date=2017-01-08}}</ref><ref>{{cite Dictionary.com|comet |access-date=2016-01-02}}</ref>

The coma is generally made of [[volatile (astrogeology)|ice]] and [[comet dust]].<ref name=Combi2004/> Water composes up to 90% of the [[volatile (astrogeology)|volatiles]] that outflow from the nucleus when the comet is within {{convert|3–4|au|e6mi e6km|lk=on|abbr=unit}} from the Sun.<ref name=Combi2004/> The [[water|H<sub>2</sub>O]] parent molecule is destroyed primarily through [[photodissociation]] and to a much smaller extent [[photoionization]].<ref name=Combi2004/> The [[solar wind]] plays 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 while smaller particles are pushed away from the Sun into the comet's [[Comet tail|tail]] by [[radiation pressure|light pressure]].
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[[Stardust (spacecraft)|Stardust]] was a [[NASA]] mission to recover samples of a comet's coma.
In some cases, such as the [[Great Comet of 1882]], a comet develops a visible [[antitail]] or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail.-->

On 11 August 2014, astronomers released studies, using the [[Atacama Large Millimeter Array|Atacama Large Millimeter/Submillimeter Array (ALMA)]] for the first time, that detailed the distribution of [[Hydrogen cyanide|HCN]], [[Hydrogen isocyanide|HNC]], [[Formaldehyde|H<sub>2</sub>CO]], and [[dust]] inside the comae of [[comet]]s [[C/2012 F6 (Lemmon)]] and [[Comet ISON|C/2012 S1 (ISON)]].<ref name="NASA-20140811">{{cite web |last1=Zubritsky |first1=Elizabeth |last2=Neal-Jones |first2=Nancy |title=RELEASE 14-038 - NASA's 3-D Study of Comets Reveals Chemical Factory at Work |url=http://www.nasa.gov/press/2014/august/goddard/nasa-s-3-d-study-of-comets-reveals-chemical-factory-at-work |date=11 August 2014 |work=[[NASA]] |access-date=2014-08-12 }}</ref><ref name="AJL-20140811">{{cite journal |author=Cordiner, M.A. |title=Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array |date=11 August 2014 |journal=[[The Astrophysical Journal]] |volume=792 |pages=L2 |number=1  |doi=10.1088/2041-8205/792/1/L2 |display-authors=etal|arxiv = 1408.2458 |bibcode = 2014ApJ...792L...2C |s2cid=26277035 }}</ref> On 2 June 2015, NASA reported that the [[Rosetta (spacecraft)#Instruments|ALICE spectrograph]] on the [[Rosetta (spacecraft)|''Rosetta'' space probe]] studying comet [[67P/Churyumov–Gerasimenko]] determined that [[electron]]s (within {{convert|1|km|mi|abbr=on}} above the [[comet nucleus]]) produced from [[photoionization]] of [[water]] [[molecule]]s by [[Sunlight|solar radiation]], and not [[photon]]s from the Sun as thought earlier, are responsible for the liberation of water and [[carbon dioxide]] molecules released from the comet nucleus into its coma.<ref name="NASA-20150602">{{cite web |last1=Agle |first1=DC |last2=Brown |first2=Dwayne |last3=Fohn |first3=Joe |last4=Bauer |first4=Markus |title=NASA Instrument on Rosetta Makes Comet Atmosphere Discovery |url=http://www.jpl.nasa.gov/news/news.php?feature=4609 |date=2 June 2015 |work=[[NASA]] |access-date=2015-06-02 }}</ref><ref name="AA-20150602">{{cite journal |url=http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |title=Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta |journal=[[Astronomy and Astrophysics]] |volume=583 |pages=A8 |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=5 |date=2 June 2015 |doi=10.1051/0004-6361/201525925|arxiv = 1506.01203 |bibcode = 2015A&A...583A...8F |s2cid=119104807 }}</ref>