Editing Coma (comet) (section)

==Observation==
With a basic Earth-surface based telescope and some technique, the size of the coma can be calculated.<ref name=levy>{{cite book|title=David Levy's Guide to Observing and Discovering Comets|author=Levy, D.H.|date=2003|publisher=[[Cambridge University Press]]|isbn=9780521520515|url=https://books.google.com/books?id=2AzBYCYV9ucC&pg=PA127|page=127|access-date=2017-01-08}}</ref> 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.<ref name=levy/> That time multiplied by the cosine of the comet's declination, times .25, should equal the coma's diameter in arcminutes.<ref name=levy/> If the distance to the comet is known, then the apparent size of the coma can be determined.<ref name=levy/>

In 2015, it was noted that the ALICE instrument on the ESA Rosetta spacecraft to comet 67/P, detected hydrogen, oxygen, carbon and nitrogen in the coma, which they also called the comet's atmosphere.<ref name="esa.int">{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Ultraviolet_study_reveals_surprises_in_comet_coma|publisher=esa.int|title=Ultraviolet study reveals surprises in comet coma / Rosetta / Space Science / Our Activities / ESA |access-date=2017-01-08}}</ref> 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.<ref name="esa.int"/>