thumb|400px|Structure of Comet Holmes in infrared, as seen by an infrared space telescope
The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes near the Sun in its highly elliptical orbit. As the comet warms, parts of it sublimate; this gives a comet a diffuse appearance when viewed through telescopes and distinguishes it from stars. The word coma comes from the Greek (), which means "hair" and is the origin of the word comet itself.
The coma is generally made of ice and comet dust. On 2 June 2015, NASA reported that the ALICE spectrograph on the Rosetta space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the liberation of water and carbon dioxide molecules released from the comet nucleus into its coma.
Size
left|Comet [[17P/Holmes, 2007/11/02|thumb]]
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. 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. This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. Thomas E. Cravens was the first to propose an explanation in early 1997. 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 off" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.
Observation
With a 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 to pass through the field of view. Alice is an ultraviolet spectrograph, and it found that electrons created by UV light were colliding and breaking up molecules of water and carbon monoxide. Space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did a close flyby of the comet in 1986. A hydrogen gas halo was detected to be 15 times the diameter of Sun (12.5 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. The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft. One of the issues is where the hydrogen is coming from and how (e.g. Water splitting):
A hydrogen gas halo three times the size of the Sun was detected by Skylab around Comet Kohoutek in the 1970s. SOHO detected a hydrogen gas halo bigger than 1 AU in radius around Comet Hale–Bopp. Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light. The halos have been measured to be ten billion meters across, many times bigger than the Sun.
The four top gases in 67P's halo were water, carbon dioxide, carbon monoxide, and oxygen. The ratio of oxygen to water coming off the comet remained constant for several months.