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Rainer Kracht, a German Amateur astronomer, has found the 500th comet discovered in images of the Sun taken by the NASA/ESA Solar & Heliospheric Observatory (SOHO). Comet C/2002 P3 (SOHO) is the 63rd discovered by Kracht and is part of a group of comets that bear his name. The SOHO spacecraft was designed to study the Sun, however, images from the Large Angle and Spectrometric Coronagraph (LASCO) have been used by amateur and professional astronomers to search for new comets. Close to the Sun most comets are lost in the glare of sunlight, however, LASCO blocks out the Sun to create an artificial eclipse which allows the solar corona to be viewed. Comets present within the images can also be seen.
Real-time LASCO images, available on the mission website, have been used by amateurs to scan for comets. Once they find one they seek confirmation from others on-line. More than 75 percent of the comet discoveries have come from amateurs all over the world. "Analyzing SOHO data is a great challenge and you have to combine many skills," said Rainer Kracht, the discoverer of C/2002 P3 (SOHO). "But every amateur astronomer dreams of finding a comet." To encourage amateurs to continue looking for comets the SOHO team had challenged amateurs to guess the perihelion time (the time at which the comet passes close to the Sun) of the 500th discovery. Comet C/2002 P3 (SOHO) had a perihelion time of August 12 at 12:04:48 p.m. EST and the sweep stake was won by Diane McElhiney.
Amongst the 500 comets discovered by SOHO are groups with similar orbits. Two of these groups were previously unknown and have been named the Kracht and Meyer groups after their discovers. There is also a third group, the Kreutz group, in which the individual members are often only 10 metres in diameter. All the groups of sun-grazing comet are thought to have formed due to the break-up of a larger parent comet possibly during a close encounter with the Sun. The Kreutz group, for example, may have formed by the break-up of a 100 km wide comet around 2000 years ago. Because comets are weak mixtures of ice and dust, often described as dirty snowballs, they can be split apart readily by the differences in gravitational force exerted as they pass planets or the Sun.
More info: SOHO Website
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