Coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars. The word coma comes from the Greek "kome", in Greek written as κόμη, and can be translated as "hair".
The coma is generally made of ice and dust. Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization. The solar wind plays a minor role in the destruction of water compared to photochemistry. 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.
Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, even though the density is very low. About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.
Comets were found to emit X-rays in 1996. This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping of" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.
With basic Earth-surface based telescope and some technique, the size of the Coma can be calculated. Called the drift method, one locks the telescope in position and measures the time for the visible disc pass through the field of view. That time multiplied by the cosine of comet's declination, times .25 should equal the coma's diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined.
Hydrogen gas halo
OAO-2 ('Stargazer') discovered large halos of hydrogen gas around comets. Space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did close flyby of the comet in 1986.
- Combi, Michael R.; Harris; Smyth; Harris, W. M.; Smyth, W. H. (2004). "Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations". Lunar and Planetary Institute (Comets II) 745: 523–552. Bibcode:2004come.book..523C.
- Comet appearance and structure
- Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. Retrieved 2007-11-17.
- Gary W. Kronk. "The Comet Primer". Cometography.com. Retrieved 2011-04-05.
- "First X-Rays from a Comet Discovered". Goddard Spaceflight Center. Retrieved 2006-03-05.
- "Interaction model – Probing space weather with comets". KVI atomics physics. Archived from the original on 2006-02-13. Retrieved 2009-04-26.
- David. H. Levy - David Levy's Guide to Observing and Discovering Comets - Page 127
- Orbiting Astronomical Observatory OAO-2
- ESA - Giotto
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