Comets sweep around the Sun as ghosts in the night. They appear, seemingly from nowhere, and disappear just as mysteriously. Every once in awhile, a comet will grace the sky with a spectacular display. This section is devoted to the observation of these exotic celestial creatures.
The mid-1990's was a magical time for astronomy enthusiasts. Jupiter was assaulted by comet Shoemaker-Levy 9 in July of 1994. That series of impacts gave professional and amateur astronomers an opportunity to observe for the first time the level of destruction a small comet can induce. One year later in July 1995, two amateur astronomers observing independently in Arizona and New Mexico found a new comet in the constellation Sagittarius. Two years later, Comet Hale-Bopp would become the most widely observed comet in history. In January 1996, a Japanese amateur astronomer discovered his second comet in as many months. Comet Hyakutake rushed on stage, producing a display in March and April which had not been seen for 20 years. Three great comets over a span of four years.
The illustration below identifies the primary features of a comet.
Comets tend to be small as celestial bodies go. Whereas Pluto, with a diameter approaching 2,300 kilometers (km), is considered a runt among the major planets, a 20 km diameter comet is considered large. Comets are composed of a mixture of ices and rocky material. Astronomers sometimes refer to them as dirty snowballs. When a comet enters the inner Solar System, the Sun's energy begins to transform the comet's appearance. Carbon dioxide ice near the surface sublimates. The carbon dioxide changes from a solid to a gaseous state. Dust and ice particles are released during sublimation. This material forms a shell around the comet called a coma. The densest portion of the coma is in the immediate vicinity of the comet. This can appear as a pinpoint of brightness called a nucleus. Despite its appearance, the nucleus is at least several thousand kilometers across. The coma often has an oval or teardrop shape.
The comet's gravity is not strong enough to retain the gas, ice and dust spouting from its surface. These particles lag behind, tracing the path of the comet's orbit. This cosmic flotsam reflects sunlight and, if dense enough, we are able to see it from Earth as a dust tail. The carbon dioxide gas molecules are so light that the Solar wind carries them away from the comet. Ultraviolet radiation interacts with the carbon dioxide gas and it becomes ionized. The ionized gas fluoresces, emitting its own light. This is visible as a plasma tail. Whereas the dust tail has the yellow hue of sunlight, plasma tails are blue. This is because the light emitted by ionized carbon dioxide is blue.
Use the below links to access my comet observation pages. These pages include sketches and detailed notes.
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Revised: April 15, 2006 [WDF]