DEFINITIONS OF TERMS ON THE EDUCATION AND OUTREACH WEB

HOMEPAGE

A comet is a small fragile body made of ice and dust that is irregular in shape. Comets that we see have orbits around the Sun that are usually highly elliptical in shape and, therefore, they spend most of their time at the edges of our Solar System or beyond where temperatures are typically colder than -200C. A cold comet remains dormant. However, when a comet gets to about 1.5 A.U. from the Sun its ices begin to warm, and the comet becomes active. This means the ices begin to sublimate more rapidly into the vacuum of space. This is the beginning of the process that can make a comet form a tail ! Here is how it works. Typically, the comet body of dust and ice, called the comet nucleus, is a few to 10's of kilometers across. The sublimating ices, however, form a ball of gas molecules and dust grains around the nucleus, called the coma which can be 100's of thousands of kilometers across. Some gases in the coma absorb ultraviolet radiation from the Sun and form ions. These ions are pushed by the Solar wind into an ion tail and are visible because they fluoresce which means they emit visible light after first absorbing ultraviolet light. Also dust in the coma is pushed by the Sun's radiation pressure forming a dust tail. The dust is visible because it reflects the sunlight. These tails always point away from the Sun and can be millions of kilometers long. A comet's bright tail is the feature that we can often see and recognize. Comets are the largest objects visible in the Solar system! Here is a cartoon drawing showing the parts of a comet.

Compare this diagram with this photograph of comet Hale-Bopp made by Fred Espenak, a scientist at NASA's Goddard Space Flight Center. It is easy to identify all of the parts of the comet except the nucleus which is an insignificant speck obscured in the coma.

About half the material in a comet is ice and the other half is dust. Some of the dust is composed of silicon and oxygen (silicates) and the ice is mostly water. However, there are small amounts of other molecules mixed with the water. There are many molecules and ions that have been found in comets.

Scientists believe that perhaps a trillion or so comets were made during the formation of the Solar system, 4.5 billion years ago. These comets are stored in the "deep freeze" of space, far from the Sun in regions called the Oort cloud and the Kuiper Belt. On rare occasions, comets in these regions can be disturbed (for example) by the gravity of a nearby star and some can be moved in the direction of our Sun along highly elliptical orbits where they become visible to us from time to time. Less than a thousand individual comets have been recorded by scientists, and only a few hundred have been seen in more than one passage by the Sun and Earth. Comets are usually not detectable until they are near or within the orbit of Mars (1.5 A. U.) where increased sublimation results in increase comet brightness. Comets vary greatly in brightness, of course, and often they can only be seen through a large telescope. The appearance of two bright naked-eye comets, Hyakutake (fall 1996) and Hale-Bopp (spring 1997) was unusual. Comets are usually named after their discoverers, such as Comet West, Comet Hyakutake and Comet Hale-Bopp (two discoverers). Sometimes comets are named for the person who first recognized that the comets seen two or more times were really the return of the same comet, Halley's Comet is one example.

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Code 691

TERMS LAST UPDATED:  May 31, 2000            Page last updated:  June 26, 2001