“Coma” comes from the Greek word for “hair”, and it’s also the origins of the word comet. When comets appear in the sky, they don’t look like a normal star. Instead, they are surrounded by a glowing halo, and sometimes that halo appears to trail behind them, like long hair in the wind.
However, most comets spend most of their lives without a coma. It only appears when a comet gets too close to the Sun. When that happens, the heat of the Sun turns ices on the surface of the comet into vapors, which forms the coma. As the comet gets closer to the sun, the coma grows larger. In some cases, the coma can get very big, as large as the diameter of Jupiter. The coma of the Great Comet of 1811 was as large as the diameter of the Sun.
The solar wind pushes the coma in a direction pointing away from the Sun, forming a tail. So when a comet is leaving the inner solar system, the tail leads in front of the body of the comet itself. When the comet gets too far away, the surface can freeze once again and the coma disappears.
The typical coma consists of 90% water. The remainder is carbon dioxide, carbon monoxide, ammonia, methane, and oxygen. Dust grains form a tiny percentage.
Comets have a very special place in astronomical history. For centuries European astronomers thought that they were atmospheric phenomena. But in the late 1500’s astronomer Tycho Brahe used precise parallax measurements to estimate the distance to a new comet. He found that the comet had to be further away than the Moon, proving that they had celestial origins.
