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(Left) Image of comet Holmes from the 3.6-meter Canada-France-Hawaii telescope on Mauna Kea showing the large expanding dust coma. On the left, a 'raw' image is shown, in which the brightness reflects the distribution of dust in the coma of the comet (the nucleus is in the bright, point-like region to the upper left of center). On the right is shown the same image after application of the Laplacian spatial filter, to emphasize fine structures. The white/black circular objects are background stars enhanced by the Laplacian filter.
The first of the famous comets is Halley’s Comet. Formally known as 1P/Halley, this comet is the most famous of the periodic comets and can currently be seen every 75–76 years. There are many comets that are more spectacular, but Halley’s is the only one that can be seen by the unaided eye and the only the only unaided eye comet certain to return within a human lifetime. During its returns to the inner solar system, it has been observed by astronomers since at least 240 BC, but it was not recognized as a periodic comet until the eighteenth century when its orbit was computed by English astronomer Edmond Halley. This comet last appeared in the inner solar system in 1986, and will next appear in mid-2061.
Another famous comet is Hale Bopp. Designated as C/1995 O, Comet Hale Bopp was the most observed comet of the twentieth century. It was visible to the unaided eye for a record 18 months. That is twice as long as the Great Comet of 1811. Hale Bopp comet was discovered on July 23, 1995 at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed perihelion on April 1, 1997. Hale-Bopp’s orbital position was calculated as 7.2 AU from the Sun, by far the greatest distance from Earth at which a comet had been discovered. Most comets at this distance are extremely faint, and show no discernible activity, but Hale-Bopp already had an observable coma. An image taken at the Anglo-Australian Telescope in 1993 was found to show the then-unnoticed comet some 13 AU from the sun, a distance at which most comets are essentially unobservable.
The third of the famous comets Comet Holmes, 17P/Holmes, is a periodic comet discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its 2007 return when it temporarily brightened by a factor of about half a million, in what was the largest known outburst by a comet, and became visible to the unaided eye. It also briefly became the largest object in the solar system, as its coma expanded to a diameter greater than that of the Sun. Its mass did not increase.
There are articles of great depth on each of the three famous comets. You can find Comet Halley’s here, Hale-Bopp here, and Comet Holmes here. Astronomy Cast has a great episode on how amateurs can contribute to astronomy.
Source: NASA
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