Cloud of gas or a trail around a comet or asteroid

Coma (comet)

Coma (comet)

The coma is the nebulous envelope around the <a href="/en/Comet_nucleus" title="Comet nucleus" class="wl">nucleus</a> of a <a href="/en/Comet" title="Comet" class="wl">comet</a>, formed when the comet passes close to the <a href="/en/Sun" title="Sun" class="wl">Sun</a> on its highly <a href="/en/Ellipse" title="Ellipse" class="wl">elliptical</a> <a href="/en/Orbit" title="Orbit" class="wl">orbit</a>; as the comet warms, parts of it <a href="/en/Sublimation_(chemistry)" title="Sublimation (chemistry)" class="mw-redirect wl">sublimate</a>.<a href="#cite_note-Combi2004-1" style="counter-reset: mw-Ref 1;">[1]</a> This gives a comet a "fuzzy" appearance when viewed in <a href="/en/Telescope" title="Telescope" class="wl">telescopes</a> and distinguishes it from <a href="/en/Star" title="Star" class="wl">stars</a>. The word coma comes from the Greek "kome" (κόμη), which means "hair" and is the origin of the word comet itself.<a href="#cite_note-chap14-2" style="counter-reset: mw-Ref 2;">[2]</a><a href="#cite_note-3" style="counter-reset: mw-Ref 3;">[3]</a>

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Cloud of gas or a trail around a comet or asteroid</div>
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<figure class="wiki-thumb"><a href="/en/File:Infrared_Structure_of_Comet_Holmes.jpg" class="mw-file-description image media-thumb" data-hash="File:Infrared_Structure_of_Comet_Holmes.jpg"><img loading="lazy" src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Infrared_Structure_of_Comet_Holmes.jpg/400px-Infrared_Structure_of_Comet_Holmes.jpg" decoding="async" data-file-width="3000" data-file-height="2400" data-file-type="bitmap" height="320" width="400" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Infrared_Structure_of_Comet_Holmes.jpg/600px-Infrared_Structure_of_Comet_Holmes.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Infrared_Structure_of_Comet_Holmes.jpg/800px-Infrared_Structure_of_Comet_Holmes.jpg 2x" alt="Infrared_Structure_of_Comet_Holmes.jpg"></a><figcaption>Structure of Comet Holmes in infrared, as seen by an infrared space telescope</figcaption></figure>
The coma is generally made of <a href="/en/Volatiles" title="Volatiles" class="wl">ice</a> and <a href="/en/Comet_dust" title="Comet dust" class="wl">comet dust</a>.<a href="#cite_note-Combi2004-1" style="counter-reset: mw-Ref 1;">[1]</a> Water composes up to 90% of the <a href="/en/Volatiles" title="Volatiles" class="wl">volatiles</a> that outflow from the nucleus when the comet is within 3-4&nbsp;<a href="/en/Astronomical_unit" title="Astronomical unit" class="wl">AU</a> of the Sun.<a href="#cite_note-Combi2004-1" style="counter-reset: mw-Ref 1;">[1]</a> The <a href="/en/Water" title="Water" class="wl">H2O</a> parent molecule is destroyed primarily through <a href="/en/Photodissociation" title="Photodissociation" class="wl">photodissociation</a> and to a much smaller extent <a href="/en/Photoionization" title="Photoionization" class="wl">photoionization</a>.<a href="#cite_note-Combi2004-1" style="counter-reset: mw-Ref 1;">[1]</a> The <a href="/en/Solar_wind" title="Solar wind" class="wl">solar wind</a> plays a minor role in the destruction of water compared to <a href="/en/Photochemistry" title="Photochemistry" class="wl">photochemistry</a>.<a href="#cite_note-Combi2004-1" style="counter-reset: mw-Ref 1;">[1]</a> Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's <a href="/en/Comet_tail" title="Comet tail" class="wl">tail</a> by <a href="/en/Radiation_pressure" title="Radiation pressure" class="wl">light pressure</a>.


On 11 August 2014, astronomers released studies, using the <a href="/en/Atacama_Large_Millimeter_Array" title="Atacama Large Millimeter Array" class="wl">Atacama Large Millimeter/Submillimeter Array (ALMA)</a> for the first time, that detailed the distribution of <a href="/en/Hydrogen_cyanide" title="Hydrogen cyanide" class="wl">HCN</a>, <a href="/en/Hydrogen_isocyanide" title="Hydrogen isocyanide" class="wl">HNC</a>, <a href="/en/Formaldehyde" title="Formaldehyde" class="wl">H2CO</a>, and <a href="/en/Dust" title="Dust" class="wl">dust</a> inside the comae of <a href="/en/Comet" title="Comet" class="wl">comets</a> <a href="/en/C/2012_F6_(Lemmon)" title="C/2012 F6 (Lemmon)" class="wl">C/2012 F6 (Lemmon)</a> and <a href="/en/Comet_ISON" title="Comet ISON" class="wl">C/2012 S1 (ISON)</a>.<a href="#cite_note-NASA-20140811-4" style="counter-reset: mw-Ref 4;">[4]</a><a href="#cite_note-AJL-20140811-5" style="counter-reset: mw-Ref 5;">[5]</a> On 2 June 2015, NASA reported that the <a href="/en/Rosetta_(spacecraft)#Instruments" title="Rosetta (spacecraft)" class="wl">ALICE spectrograph</a> on the <a href="/en/Rosetta_(spacecraft)" title="Rosetta (spacecraft)" class="wl">Rosetta space probe</a> studying comet <a href="/en/67P/Churyumov–Gerasimenko" title="67P/Churyumov–Gerasimenko" class="wl">67P/Churyumov–Gerasimenko</a> determined that <a href="/en/Electron" title="Electron" class="wl">electrons</a> (within 1&nbsp;km (0.62&nbsp;mi) above the <a href="/en/Comet_nucleus" title="Comet nucleus" class="wl">comet nucleus</a>) produced from <a href="/en/Photoionization" title="Photoionization" class="wl">photoionization</a> of <a href="/en/Water" title="Water" class="wl">water</a> <a href="/en/Molecule" title="Molecule" class="wl">molecules</a> by <a href="/en/Sunlight" title="Sunlight" class="wl">solar radiation</a>, and not <a href="/en/Photon" title="Photon" class="wl">photons</a> from the Sun as thought earlier, are responsible for the liberation of water and <a href="/en/Carbon_dioxide" title="Carbon dioxide" class="wl">carbon dioxide</a> molecules released from the comet nucleus into its coma.<a href="#cite_note-NASA-20150602-6" style="counter-reset: mw-Ref 6;">[6]</a><a href="#cite_note-AA-20150602-7" style="counter-reset: mw-Ref 7;">[7]</a>

The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate. The coma is generally made of ice and comet dust. Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure. On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 and C/2012 S1 . 