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Cometary String Of Pearls To Swing Past Earth In May

Comet 73P breaking up in 1995. Image credit: Jim V. Scotti.
by Staff Writers
Pasadena CA (SPX) Mar 27, 2006
Starting on May 12, Earth-based sky watchers will be treated to the unusual sight of a string of cometary fragments sweeping across the night. The fragments belong to Comet 73P/Schwassmann-Wachmann 3, which fell apart in 1995.

For still unknown reasons, the comet's nucleus split into at least three mini-comets flying single file through space - very much like the 21 known fragments of Comet Shoemaker-Levy, which smashed into Jupiter sequentially in July 1994.

Astronomers observed the breakup of 73P, as they call the comet, but their view was quite fuzzy, even through large telescopes, because the event happened when the object was about 150-million miles away, but in May, the fragments will make their closest approach in more than 20 years.

"This is a rare opportunity to watch a comet in its death throes — from very close range," said Don Yeomans, head of NASA's Near Earth Object Program at Jet Propulsion Laboratory.

Close, but not close enough to threaten Earth. "Goodness, no," Yeomans said. "The closest fragment will be about 6-million miles away - or 25 times farther than the Moon."

Yeomans said astronomers plan to focus the Hubble Space Telescope on 73P for this encounter. "Also, the giant Arecibo radar in Puerto Rico will 'ping' the fragments to determine their shape and spin," he said, but added even backyard astronomers can photograph the mini-comets as they file through the constellations Cygnus and Pegasus on the evenings of May 12 through 14.

One problem will be that the fragments will not be very bright. The largest are expected to reflect the Sun's light only at the third or fourth magnitude, which makes them only dimly visible to the unaided eye, and probably invisible from urban and suburban areas.

"Remember," Yeomans noted, "these are mini-comets," not like the Hayutake of 1996 or Hale-Bopp of 1997, which could be seen with the naked eye even from cities. The fragments of 73P are best viewed from the countryside, he said, with binoculars.

The number of fragments is changing constantly. When the breakup began in 1995 there were only three, named A, B and C. Astronomers now count at least eight, including big fragments B and C, plus smaller fragments named G, H, J, L, M and N.

"It looks as though some of the fragments are themselves forming their own sub-fragments," Yeomans said. That means the number could increase as 73P approaches Earth. Because of the uncertainty, it is possible 73P could produce a meteor shower as it passes by.

"We believe the (meteor) cloud is expanding too slowly to reach Earth only 11 years after the break-up," said astronomer Paul Wiegert of the University of Western Ontario in London, "but it all depends on what caused the comet to fly apart—and that we don't know."

Regarding the breakup, Wiegert said the most likely explanation is thermal stress. The comet's icy nucleus may have cracked like an ice cube dropped into hot soup, "breaking apart as it approached the Sun after a long sojourn in the frigid outer solar system. If this is truly what happened, then the debris cloud should be expanding slowly, and there will be no strong meteor shower."

Another possibility is a collision, he said. What if "the comet was shattered by a hit from a small interplanetary boulder?" A violent collision would produce faster-moving debris that could reach Earth and produce meteors.

Wiegert said he expects to see nothing, but it would not be the first time a dying comet produced a meteor shower.

"One outstanding example is comet Biela, which was seen to split in 1846, and had completely broken apart by 1872," he said. "At least three very intense meteor showers (3,000 to 15,000 meteors per hour) were produced by this dying comet in 1872, 1885 and 1892."

Assuming a thermal breakup for 73P, Wiegert and colleagues have calculated the most likely trajectory of its dust cloud and concluded that it will not reach Earth until 2022, "producing a minor meteor shower - nothing spectacular." He added, however, that "the ongoing splitting of the comet means new meteoroids are being sent in new directions, so a future strong meteor shower from 73P remains a real possibility."

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Cometary String-Of-Pearls To Pass By Earth In May 2006
Huntsville AL (SPX) Mar 27, 2006
In 1995, Comet 73P/Schwassmann-Wachmann 3 did something unexpected: it fell apart. For no apparent reason, the comet's nucleus split into at least three "mini-comets" flying single file through space. Astronomers watched with interest, but the view was blurry even through large telescopes. "73P" was a hundred and fifty million miles away.

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