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Beginner's Guide

1.   What is a comet?

A comet is a dirty snowball. A comet is an icy mud ball. A comet is a demon in the sky! At least that's what some ancient and even recent observers imagined.

Comets, from the Greek (aster) kometes meaning long-haired (star), have been observed for thousands of years. The Europeans documented the 1066 AD apparition of Comet Halley in the Bayeax Tapestry, while the Chinese have observations of Comet Halley going back to 240 BC! However, it wasn't until 1705 when Edmond Halley predicted the return in 1758 of a particular comet that astronomers realized that comets were objects far out in the solar system rather than vaporous demons in the atmosphere.

There are three basic parts to a comet:

  1. the nucleus
  2. the coma
  3. the tails

photo of Hale-Bopp
Fig 1: Comet Hale-Bopp
photo by Elizabeth Warner

comet drawing with parts labeled
Fig 2: Drawing by W.D. Ferris

The nucleus is the innermost region of the comet. It is also the hardest part to see. While the exact composition and structure of a cometary nucleus is unknown (the Deep Impact mission contributed to our knowledge of this topic), much can be inferred from observations of the more visible coma and tails. In general though, the nucleus is believed to be a dirty snowball. Images of the nuclei of comets Halley, Borrelly, and Wild 2 showed a much darker nucleus than had been expected, forcing astronomers to rethink their ideas on the composition, structure, and evolution of cometary nuclei. Observations of Tempel 1 showed that it is also very dark.

Comet Orbits

animated gif of a comet orbit
Fig 3: Animated graphic depicting difference between a planetary and cometary orbit.

While some comets do cross Earth's orbit, many do not. The orbits of Tempel 1 and Hartley 2 do not cross Earth's orbit, so we'll never see a meteor shower caused by them.

More On...



The coma is a huge cloud of dust and gas surrounding the nucleus. Most of the gas comes from ices which have sublimated (having gone from the solid phase to the gas phase skipping the liquid phase) from the nucleus. While the nucleus of a comet may only be a few to a couple tens of kilometers in diameter, the coma can extend for hundreds of thousands of kilometers.

A comet, while close to the sun, usually has two tails. One is made primarily of gas and always points away from the sun. This can look pretty weird since sometimes this tail can be in front of the comet as the comet heads away from the sun along its orbit. The other tail is made of dust that is left behind like a trail of crumbs. If a comet has an orbit that occasionally crosses the Earth's orbit, we might pass through that trail and have a meteor shower.

Comets, like the planets, orbit around the sun. But the orbits of comets tend to be much more elliptical (oval or egg-shaped).




What are meteors?
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Updated: 10-Dec-2018