[MIham] Mini-Comets Approaching Earth 03.24.2006

[FSK]

     A cometary "string-of-pearls" will fly past Earth in May 2006 giving
astronomers a fantastic view of a dying comet.


      March 24, 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.


      We're about to get a much closer look. In May 2006 the fragments are
going to fly past Earth closer than any comet has come in more than twenty
years.

      Right: Comet 73P breaking up in 1995. Photo credit: Jim V. Scotti.
[More]

      "This is a rare opportunity to watch a comet in its death throes-from
very close range," says Don Yeomans, head of NASA's Near Earth Object
Program at JPL.

      There's no danger of a collision. "Goodness, no," says Yeomans. "The
closest fragment will be about six million miles away--or twenty-five times
farther than the Moon." That's close without actually being scary.


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      The flyby is a big deal. "The Hubble Space Telescope will be
watching," says Yeomans. "Also, the giant Arecibo radar in Puerto Rico will
'ping' the fragments to determine their shape and spin." Even backyard
astronomers will be able to take pictures as the mini-comets file through
the constellations Cygnus and Pegasus on May 12, 13 and 14.

      Ironically these comets, so nearby, will not be very bright. The
largest fragments are expected to glow like 3rd or 4th magnitude stars, only
dimly visible to the unaided eye.

      "Remember," says Yeomans, "these are mini-comets." They're not like
the Great Comets Hayutake and Hale-Bopp of 1996 and 1997. Those could be
seen with the naked eye from light-polluted cities. The fragments of 73P, on
the other hand, are best viewed from the countryside--and don't forget your
binoculars.

      The number of fragments is constantly changing. When the breakup began
in 1995 there were only three: A, B and C. Astronomers now count at least
eight: big fragments B and C plus smaller fragments G, H, J, L, M and N. "It
looks as though some of the fragments are themselves forming their own
sub-fragments," says Yeomans, which means the number could multiply further
as 73P approaches. No knows how long the "string of pearls" will be when it
finally arrives.



      Above: Fragments B and C approaching Earth on Feb. 26, 2006. Credit:
Giovanni Sostero and Ernesto Guido of the Remanzacco Observatory in Italy
using a remote-controlled 14-inch telescope in New Mexico.

      Bonus: There could be a meteor shower, too.

      This is very uncertain, indeed, forecasters consider it unlikely. But
an expanding cloud of dust from the 1995 break-up of the comet could brush
past Earth in May 2006 producing a display of meteors.

      Astronomer Paul Wiegert at the University of Western Ontario has
studied the possibility:

      "We believe the cloud is expanding too slowly to reach Earth only
eleven years after the break-up," he says, "but it all depends on what
caused the comet to fly apart-and that we don't know."

      "The most likely explanation is thermal stress, with the icy nucleus
cracking like an ice cube dropped into hot soup: the comet broke apart as it
approached the Sun after a long sojourn the frigid outer solar system," he
explains. "If this is truly what happened, then the debris cloud should be
expanding slowly, and there will be no strong meteor shower."

      Right: Clouds of comet dust from Comet 73P are expected to miss Earth
in 2006. [More]

      On the other hand, 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 in 2006.

      Wiegert expects to see nothing, but he encourages sky watchers to be
alert. It wouldn't 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 says. "At least three
very intense meteor showers (3000-15000 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. Their results: dust
should reach Earth in 2022, "producing a minor meteor shower--nothing
spectacular. However," he adds, "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."

      The watch begins on May 12th.

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      Correction: An earlier version of this story stated that 73P would
pass closer to Earth than any comet in almost 80 years. That was incorrect.
In May 1983, Comet IRAS-Iraki-Alcock passed about 3 million miles from
Earth, making 76P at 6 million miles the closest comet in only 23 years.

      Author: Dr. Tony Phillips | Editor: Dr. Tony Phillips | Credit:
Science at NASA

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