Examine individual changes

'{{multiple image |align=right |direction=horizontal |total_width=325 |image1=Comet Parts.svg |caption1=Diagram of a comet showing the dust tail, the dust trail (or [[antitail]]) and the ion gas tail, which is formed by the solar wind flow. |image2=Iss030e015472 Edit.jpg |caption2=[[C/2011 W3 (Lovejoy)|Comet Lovejoy]] photographed in 2011 by astronaut Dan Burbank from the International Space Station }} [[File:17pHolmes 071104 eder vga.jpg|thumb|[[Comet Holmes]] (17P/Holmes) in 2007 showing blue ion tail on right]] [[File:Comet and tail animation.gif|thumb|Animation of a comet's tail]] A '''comet tail'''—and [[Coma (cometary)|coma]]—are features visible in [[Comet|comets]] when they are illuminated by the [[Sun]] and may become visible from [[Earth]] when a [[comet]] passes through the inner [[Solar System]]. 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. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from [[ion]]isation. Most comets are too faint to be seen by a minion from the rise of gru.visible without the aid of a [[telescope]], but a few each decade become bright enough to be visible to the [[naked eye]]. == Tail formation == [[File:Cometorbit01.svg|thumb|left|250px|A comet's orbit showing the different directions of the gas and dust tails as the comet passes the Sun]] In the outer [[Solar System]], comets remain frozen 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 the [[Hubble Space Telescope]] observations,<ref name="Cochran1995">{{cite journal |last1=Cochran |first1=A. L. |last2=Levison |first2=H. F. |last3=Stern |first3=S. A. |last4=Duncan |first4=J. |title=The Discovery of Halley-sized Kuiper Belt Objects Using the Hubble Space Telescope |bibcode=1995ApJ...455..342C |journal=[[The Astrophysical Journal]] |volume=455 |page=342 |year=1995 |doi=10.1086/176581 |arxiv=astro-ph/9509100|s2cid=118159645 }}</ref><ref name="Cochran1998">{{cite journal |last1=Cochran |first1=A. L. |last2=Levison |first2=H. F. |last3=Tamblyn |first3=P. |last4=Stern |first4=S. A. |last5=Duncan |first5=J. |title=The Calibration of the Hubble Space Telescope Kuiper Belt Object Search: Setting the Record Straight |journal=[[Astrophysical Journal Letters]] |volume=503 |issue=1 |page=L89 |year=1998 |doi=10.1086/311515 |bibcode=1998ApJ...503L..89C |arxiv=astro-ph/9806210|s2cid=18215327 }}</ref> but these detections have been questioned,<ref name="Brown1997">{{cite journal |last1=Brown |first1=Michael E. |last2=Kulkarni |first2=S. R. |last3=Liggett |first3=T. J. |title=An Analysis of the Statistics of the Hubble Space Telescope Kuiper Belt Object Search |journal=[[Astrophysical Journal Letters]] |volume=490 |issue=1 |page=L119 |year=1997 |doi=10.1086/311009 |bibcode=1997ApJ...490L.119B|doi-access=free }}</ref><ref name="Jewitt1996">{{cite journal |last1=Jewitt |first1=David C. |author-link=David C. Jewitt |last2=Luu |first2=Jane |last3=Chen |first3=J. |title=The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey |bibcode=1996AJ....112.1225J |journal=[[The Astronomical Journal]] |volume=112 |issue=3 |page=1225 |year=1996 |doi=10.1086/118093}}</ref> and have not yet been independently confirmed. 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 [[Cosmic dust|dust]] and gas thus released form a huge, extremely tenuous atmosphere around the comet called the ''[[coma (cometary)|coma]]'', and 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. 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 [[antitail]], only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. [[Parallax]] viewing from the Earth may sometimes mean the tails appear to point in opposite directions.<ref>{{cite web |last=McKenna |first=M. |title=Chasing an Anti-Tail |url=http://www.asod.info/?p=1019 |publisher=Astronomy Sketch of the Day |date=May 20, 2008 |access-date=February 25, 2009}}</ref> == Size == While the solid nucleus of comets is generally less than 30&nbsp;km across, the coma may be larger than the Sun, and ion tails have been observed to extend {{convert|3.8|AU|abbr=out|lk=on|Gm e6mi}}.<ref name="Yeoman">{{cite web |last=Yeomans |first=Donald K. |title=Comet |url=http://www.nasa.gov/worldbook/comet_worldbook.html |archive-url=https://web.archive.org/web/20050429084558/http://www.nasa.gov/worldbook/comet_worldbook.html |url-status=dead |archive-date=April 29, 2005 |work=World Book Online Reference Center |publisher=[[World Book]] |year=2005 |access-date=December 27, 2008 }}</ref> The [[Ulysses (spacecraft)|''Ulysses'' spacecraft]] made an unexpected pass through the tail of the comet [[C/2006 P1]] (Comet McNaught), on February 3, 2007.<ref name="chance encounter">{{cite news |publisher=Astronomy |title=A chance encounter with a comet |date=October 2, 2007 |url=http://www.astronomy.com/asy/default.aspx?c=a&id=6065}}</ref> Evidence of the encounter was published in the October 1, 2007, issue of ''[[The Astrophysical Journal]]''.<ref>{{cite journal |url=http://www.iop.org/EJ/article/0004-637X/667/2/1262/72016.html |author=Neugebauer |title=Encounter of the Ulysses Spacecraft with the Ion Tail of Comet MCNaught |journal=[[The Astrophysical Journal]] |volume=667 |issue=2 |year=2007 |pages=1262–1266 |doi=10.1086/521019 |bibcode=2007ApJ...667.1262N |display-authors=etal|doi-access=free }}</ref> == Magnetosphere == The observation of antitails contributed significantly to the discovery of [[solar wind]].<ref>{{cite journal |last=Biermann |first=L. |title=The plasma tails of comets and the interplanetary plasma |journal=[[Space Science Reviews]] |volume=1 |issue=3 |page=553 |year=1963 |doi=10.1007/BF00225271 |bibcode=1963SSRv....1..553B|s2cid=120731934 }}</ref> The ion tail is the result of [[ultraviolet]] radiation ejecting electrons off particles in the coma. Once the particles have been ionised, they form a plasma which in turn induces a [[magnetosphere]] around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the solar wind, so a [[bow shock]] is formed upstream of the comet (i.e. facing the Sun), 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 (physics)|plasma]]. The field lines "drape" around the comet forming the ion tail.<ref name="pp 864">{{cite book |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |title=An Introduction to Modern Astrophysics |pages=864–874 |publisher=[[Addison-Wesley]] |year=1996 |isbn=978-0-201-54730-6}}</ref> (This is similar to the formation of planetary magnetospheres.) == Tail loss == [[File:Encke tail rip off.ogg|thumb|right|Comet Encke loses its tail]] If the ion tail loading is sufficient, then 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, notable among which was on the 20th of April 2007 when the ion tail of [[comet Encke]] was completely severed as the comet passed through a [[coronal mass ejection]].<ref name="CME2007">{{cite web |date=October 1, 2007 |title=The Sun Rips Off a Comet's Tail |publisher=Science@NASA |url=https://science.nasa.gov/headlines/y2007/01oct_encke.htm |access-date=October 20, 2009 |archive-url=https://web.archive.org/web/20091104041757/http://science.nasa.gov/headlines/y2007/01oct_encke.htm |archive-date=November 4, 2009 |url-status=dead }}</ref> This event was observed by the [[STEREO|STEREO spacecraft]].<ref>{{cite journal |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. |title=The Heliospheric Imagers Onboard the STEREO Mission |journal=[[Solar Physics (journal)|Solar Physics]] |volume=254 |issue=2 |pages=387–445 |year=2009 |doi=10.1007/s11207-008-9299-0 |last10=Rochus |first10=P. |bibcode=2009SoPh..254..387E|s2cid=54977854 |url=http://orbi.ulg.ac.be/handle/2268/15675 }}</ref> A disconnection event was also seen with [[C/2009 R1]] (McNaught) on May 26, 2010.<ref name="C2009R1-DE">{{cite web |date=May 30, 2010 |title=Comet C/2009 R1 (McNaught) - Animation & Images |publisher=Remanzacco Observatory |url=http://remanzacco.blogspot.com/2010/05/comet-c2009-r1-mcnaught-animation.html |access-date=June 7, 2011}}</ref> {{clear left}} == Analogues == Mercury and Venus possess similar tails caused by interaction of the solar wind with their atmospheres. On January 29, 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 |author=Staff |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=January 29, 2013 |publisher=[[ESA]] |access-date=January 31, 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=January 30, 2013 |publisher=[[Space.com]] |access-date=January 31, 2013}}</ref> The [[MESSENGER]] mission observed Magnesium and Sodium being primary components of the Mercury tail.<ref name=McClintock2009-610>[[#McClintock2009|McClintock]] 2009, p. 610–611</ref> {{-}} == References == {{Reflist}} == External links == {{Commons category|Comet tails}} * {{dmoz|Science/Astronomy/Solar_System/Small_Bodies/Comets/|Comets}} * [https://web.archive.org/web/20190105230832/https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/overview/ Comets page] at [https://solarsystem.nasa.gov/ NASA's Solar System Exploration] * [http://www.icq.eps.harvard.edu/icq.html International Comet Quarterly] at Harvard.edu {{Comets|nonobject=yes}} {{Small Solar System bodies}} [[Category:Comets]] [[Category:Articles containing video clips]]'