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Sun-grazers: a hot road to the end.

Sun-grazers: A hot road to the end

They seem to evoke the death of mythic Icarus, killed after the wax holding together his homemade wing melted when he felw too close to the sun. On rare occasions, astronomers have observed comets passing so near the sun that the heat has caused them to fragment or even disappear completely. Only eight such sun-grazers, plus three or four "possibles," have been discovered over the centuries with ground-based telescopes, and another six appeared in photos taken by a U.S. Air Force satellite equipped with a coronagraph for solar studies.

Now NASA's Solar Maximum Mission satellite (SMM), or Solar Max, has turned up two more and a possible third.

The air Force satellite, known as P78-1, was launched in 1979, but its career ended six years later when it was deliberately destroyed in a U.S. antsatellite test (SN: 11/28/85, p. 197). Solar Max went into orbit in 1980, and although it did detect one of the Air Force craft's six comets, it failed until recently to find any sun-grazers of its own.

The new pair, dubbed SMM-1 and SMM-2, was announced July 1 by the National Center for Atmospheric Research in Boulder, Colo. Photographed last Oct. 5 and 17 by Solar Max's coronagraph -- which blocks out the sun's central disk to highlight its rim -- they were subsequently spotted in the images by Sharon Beck of the center's High Altitude Observatory. In both cases, the coronagraphic observations continued for many hours beyond the time of the initial discovery images, but later photos did not show either comet emergin from behind the instrument's sun-masking "occulting disk."

The coronagraph photographed a still more recent candidate on June 27. It has not yet been dubbed SMM-3, since scientists have yet to complete their analysis of its orbit. Such analysis is necessary to determine not only that it is a previously unknown comet in need fo a name but also that it is indeed a sun-grazer.

A reasonable assumption, says Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., is that all but one of the known sun-grazers were once part of a single comet, which long ago passed so close to the sun that it fragmented. They are called the Kreutz group, after German astronomer Heinrich Kreutz, who around the turn of the century conducted an extensive study of the ones known at that time. He determined that all of them circled the sun in a clockwise or retrograde direction, in orbits titled about 40[deg.] to the plane of the ecliptic and with the orbits' major axes aligned in the same direction.

The Kreutz group now also seems to include all of the sun-grazers detected by the two satellites. All appear to have the required orbital tilt, though Marsden notes it is impossible to determine from the coronagraphic images whether they are also circling in the necessary retrograde direction. If they are, he says, the orbits' major axes are oriented in the proper direction for the comets to be defined as members of the Kreutz group.

"The fact that this one assumption [that he orbits are retrograde] allows each of the sun-grazers found from space to fit with the Kreutz group suggests that the assumption is valid," Marsden says. If so, he adds, their present orbits suggest that some of the initial fragments split again in subsequent trips past the sun.
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Title Annotation:comets that pass near to the sun
Author:Eberhart, Jonathan
Publication:Science News
Date:Jul 16, 1988
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