Coma (cometary)

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Structure of Comet Holmes in infrared, as seen by an infrared space telescope

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.[1] 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".[2]

The coma is generally made of ice and dust.[1] Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun.[1] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.[1] The solar wind plays a minor role in the destruction of water compared to photochemistry.[1] 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.


Comet 17P/Holmes, 2007/11/02

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.[2] About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[3] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[4] 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.[4] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.[4]


Tempel 1 in X-ray light by Chandra

Comets were found to emit X-rays in 1996.[5] 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.[6]


With basic Earth-surface based telescope and some technique, the size of the Coma can be calculated.[7] Called the drift method, one locks the telescope in position and measures the time for the visible disc pass through the field of view.[7] That time multiplied by the cosine of comet's declination, times .25 should equal the coma's diameter in arcminutes.[7] If the distance to the comet is known, then the apparent size of the coma can be determined.[7]

Hydrogen gas halo[edit]

OAO-2 ('Stargazer') discovered large halos of hydrogen gas around comets.[8] 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.[9]

Coma spectrum[edit]

Three coma spectra compared

External links[edit]

See also[edit]


  1. ^ a b c d e 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. 
  2. ^ a b Comet appearance and structure
  3. ^ Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. Retrieved 2007-11-17. 
  4. ^ a b c Gary W. Kronk. "The Comet Primer". Retrieved 2011-04-05. 
  5. ^ "First X-Rays from a Comet Discovered". Goddard Spaceflight Center. Retrieved 2006-03-05. 
  6. ^ "Interaction model – Probing space weather with comets". KVI atomics physics. Archived from the original on 2006-02-13. Retrieved 2009-04-26. 
  7. ^ a b c d David. H. Levy - David Levy's Guide to Observing and Discovering Comets - Page 127
  8. ^ Orbiting Astronomical Observatory OAO-2
  9. ^ ESA - Giotto