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Three Basic Parts of a Comet

Comets derive their name from the Greek word "kometes," which means a long-haired star. Once considered harbingers of bad tidings or even seen as demons in the sky, comets are remnants from the formation of our solar system. Orbiting the sun in a wide elliptical arc, some comets, like Halley's, swing by close enough to be observed on Earth in a predictable timetable. Much of what you can see in a comet is composed of vapors.

Nucleus
- The nucleus is dark and mostly shielded from view. The nucleus is the innermost region of a comet and is typically a dirty, dusty, icy object about 1 to 10 miles across. Think of the nucleus as a dirty snowball and you have a pretty good idea of the concept. The exact composition of nucleus materials is unknown and may vary between comets.
Coma
- The nucleus is surrounded by a large cloud of dust and gases called the coma. Sunlight reflecting on the coma makes it visible on Earth. About 90 percent of the gas is water vapor from ice that sublimated from the nucleus. Sublimation is the process of ice moving from a solid state to a gaseous state without existing as a liquid in between. A typical comet can outgas about 5,000 gallons of water per second. The remaining 10 percent of the coma is thought to be made up of carbon monoxide, cyanide, ammonia and soot.
Tail
- The third basic part of a comet is the tail. Like the coma, the tail becomes visible as the sun's light filters through the dust, particles and debris contained in the tail. Although it is an entirely different part of a comet's anatomy, the tail consists of the same gases, vapor and debris as the coma. As the comet nears the sun, two distinct tails are usually the result.
Dust Tail
- The dust tail is the part of the comet made up mostly of dust particles from the disintegration of the nucleus. Because the Earth and comets orbit the sun in different planes, Earth sometimes passes through the dust tail of a comet. When this happens, the result is usually a meteor shower that earthbound stargazers can enjoy.
Ion Tail
- The second tail that develops as the comet approaches the sun is the result of ionized gases that become attracted to solar wind, a cloud of charged gases emitted by the sun. Ultraviolet sunlight breaks apart molecules in gases within the tail, which strips out electrons. The ion tail always points away from the sun, a phenomenon that gives comets a peculiar appearance because the tail is in front of the comet once it travels past the sun.

Astronomy