Coma (comet)

On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

[6][7] 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.

[9] 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.

[12] 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.

[13] 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.

[14] 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.

[16] The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft.

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.