Editing Comet (section)

=== 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>