Comets & Cometary Activity: Structures & Vocabulary

Comets are ice-rich small bodies that become active when approaching the Sun. Solar heating causes volatile ices to sublimate, releasing gas and dust that form a temporary atmosphere called a coma and tails that can extend millions of kilometers.

As the nucleus warms, volatile molecules sublimate and drag dust from the surface. Radiation pressure pushes dust away gradually, while ionized gas couples to the solar wind and is carried anti-sunward. Repeated perihelion passages erode and alter the nucleus.

Spacecraft such as Giotto, Deep Impact, Stardust, and Rosetta have imaged nuclei directly and sampled comet material. Spectroscopy identifies water, carbon-bearing molecules, and other volatiles in comae, while tail geometry confirms different forces acting on gas ions and dust.

An active comet contains a nucleus, expanding coma, dust tail, and ion tail. Jets can emerge from localized regions, while dust grains span wide size ranges. Short-period comets are linked mainly to trans-Neptunian reservoirs, whereas many long-period comets originate from the distant Oort cloud.