Editing Coma (comet) (section)

==Hydrogen gas halo==
[[File:Kohoutek-uv.jpg|thumb|Artificially colored far-ultraviolet image (with film) of Comet Kohoutek (Skylab, 1973)]]
[[OAO-2]] ('Stargazer') discovered large halos of hydrogen gas around comets.<ref name=o2>{{cite web|url=http://www.sal.wisc.edu/~meade/OAO/|publisher=sal.wisc.edu|title=Orbiting Astronomical Observatory OAO-2|access-date=2017-01-08}}</ref> Space probe Giotto detected hydrogen ions at distance of 7.8&nbsp;million km away from Halley when it did a close flyby of the comet in 1986.<ref name=Giotto>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Giotto_overview|publisher=esa.int|title=Giotto overview / Space Science / Our Activities / ESA |access-date=2017-01-08}}</ref> A hydrogen gas halo was detected to be 15 times the diameter of Sun (12.5&nbsp;million miles). This triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet was emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earth's surface because those wavelengths are blocked by the atmosphere.<ref name="usra">{{cite web|url=http://www.lpi.usra.edu/education/explore/comets/background/|publisher=lpi.usra.edu|title=About Comets|access-date=2017-01-08}}</ref> The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft.<ref name="blogs.esa.int">{{cite web|url=http://blogs.esa.int/rosetta/2015/06/02/ultraviolet-study-reveals-surprises-in-comet-coma/|publisher=blogs.esa.int|title=Ultraviolet study reveals surprises in comet coma {{pipe}} Rosetta – ESA's comet chaser|access-date=2017-01-08}}</ref> One of the issues is where the hydrogen is coming from and how (e.g. [[Water splitting]]):

{{blockquote|First, an ultraviolet photon from the Sun hits a water molecule in the comet's coma and ionises it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into two hydrogen atoms and one oxygen, and energising them in the process. These atoms then emit ultraviolet light that is detected at characteristic wavelengths by Alice.<ref name="blogs.esa.int">{{cite web|url=http://blogs.esa.int/rosetta/2015/06/02/ultraviolet-study-reveals-surprises-in-comet-coma/|publisher=blogs.esa.int|title=Ultraviolet study reveals surprises in comet coma {{pipe}} Rosetta – ESA's comet chaser|access-date=2017-01-08}}</ref>}}

A hydrogen gas halo three times the size of the Sun was detected by [[Skylab]] around [[Comet Kohoutek]] in the 1970s.<ref name="nasa">{{cite web|url=https://history.nasa.gov/SP-404/ch4.htm|publisher=history.nasa.gov|title= SP-404 Skylab's Astronomy and Space Sciences Chapter 4 Observations of Comet Kohoutek |date=January 1979 |access-date=2017-01-08 |last1=Lundquist |first1=C. A. }}</ref> SOHO detected a hydrogen gas halo bigger than 1 AU in radius around [[Comet Hale–Bopp]].<ref name="google">{{cite book|title=Great Comets|author=Burnham, R.|date=2000|publisher=Cambridge University Press|isbn=9780521646000|url=https://books.google.com/books?id=HkFKMksEwPcC&pg=PA127|page=127|access-date=2017-01-08}}</ref> Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light.<ref name="ase.tufts.edu">{{cite web|url=https://ase.tufts.edu/cosmos/view_chapter.asp?id=12&page=5|publisher=ase.tufts.edu|title=NASA's Cosmos|access-date=2017-01-08}}</ref> The halos have been measured to be ten billion meters across, many times bigger than the Sun.<ref name="ase.tufts.edu"/> The hydrogen atom are very light so they can travel a long distance before they are themselves ionized by the Sun.<ref name="ase.tufts.edu"/> When the hydrogen atoms are ionized they are especially swept away by the solar wind.<ref name="ase.tufts.edu"/>