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# Comets

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Comets are thought to be small pieces of debris left over from the formation of the solar system. A comet consists of a nucleus, which is a small body, similar to an asteroid, but mostly made up of water ice, CO-ice and dust. When the nucleus get close to the sun, the ices start to evaporate, dragging some of the dust with them. The gas and the dust creates a coma, that surrounds the nucleus, and one or more tail, consisting of material forced away from the comet by the solar [radiation pressure]?. The coma and the tails shine because of reflected solar radiation and heated gas emitting its own radiation.

Most comets are too faint to be visible without the aid of a telescope. A few each year become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that there appearance have been noticed by humans for millennia.

It was not settled whether comets are atmospheric phenomena or interplanetary objects until the 16th century, when Tycho Brahe measured that they must be outside of the Earth's atmosphere. In the 17th century, [Edmond Halley]? used the gravitation theory, recently developed by Isaac Newton to try to calculate the orbit?s of comets. He then found that one of them periodically came back to the vicinity of the sun every 76-77th year. Soon this comet became known as [Halley's comet]?, and from ancient sources it is known to have been observed by humans at least since 66 BC. The second comet to be discovered to have a periodic orbit, was Encke's comet, in 1821. Like Halley's comet, it is named after its calculator, the German mathematician and physicist [Johann Franz Encke]?, that found it to be a periodic comet. Usually, comets get their names after their discoverer(s). Encke's comet has the shortest period of any comet, only 3.3 years and, because of this, more recorded appearances than any other comet. It was also the first comet whose orbit was noticed to be influenced by non-gravitional forces (see below). Although it is now usually to faint to be visible with the naked eye, it may have been a bright comet a few thousand years ago. So far, it is not known to have been observed before 1786. Maybe an improved analysis of its orbit before that will find that it actually is noted in ancient sources.

The actual nature of comets were speculated over for centuries. In the early 19th century, another German mathematician, Friedrich Wilhelm Bessel, was on the right track. He made a theory about the brightness of the comets coming from the evaporation from a solid object and that the non-gravitational forces of comet Encke were caused by the jetforce?s created as the material evaporated from the surface of the object. His idea was forgotten for more than 100 years, before [Fred Lawrence Whipple]? independently proposed the same idea in 1950. It soon became the accepted comet model and was confirmed when an armada of spacecrafts flew through the coma of Halley's comet in 1986 and photographed the nucleus and the jets of evaporating material.

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Last edited August 5, 2001 3:19 pm (diff)
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