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New research published in the current edition of the journal Science suggests that many comets fall apart and are destroyed after only a few orbits of the Sun. The researchers, led by Harold Levison from the Southwest Research Institute, performed calculations to see just how many new comets get captured into smaller orbits such that they orbit the Sun in less than 200 years. The calculations showed that around a 100 times more long period comets, which fall in from the Oort cloud that surrounds the Solar System, should be captured than we actually observe. Comets close to the Sun are known to become more asteroid-like with time due to the build-up of dust on their surface which protects their ice from heating and eventually can prevent the comet from developing a tail. Levison and colleages, however, suggest there are too few asteroids to account for the new comets. Ninety-nine percent of new comets are, therefore, missing.
The researchers propose that the break-up of new comets into small pieces explains their disappearance. Comets are mixtures of ice and dust, similar to dirty snowballs, and are, therefore, very weak objects. Comet Shoemaker-Levy 9, for example, broke up into a string of fragments as it passed close to Jupiter prior to its collision in 1994. The comet's icy nucleus was broken up by forces exerted by differences in Jupiter's gravity which were less than that required to lift a feather from the surface of the Earth.
The latest research suggests that long period comets, which formed in the same region as Jupiter and Saturn before being thrown out into the Oort cloud, consist of weaker materials than comets that come from the Kuiper Belt in the plane of the Solar System beyond Neptune. Calculations show that far fewer Kuiper Belt Objects are completely destroyed by disintegration before they become Jupiter-family comets. The results suggest that there may be real physical differences between comets which formed in the early Solar System in the giant planet region and those which formed further away from the Sun in the Kuiper Belt. Understanding these differences in detail, however, can only be achieved by space missions to different comets such as the upcoming Rosetta (ESA) and CONTOUR (NASA) missions.
More info: The Journal Science
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