A
cometary "string-of-pearls" will fly past Earth
in May 2006 giving astronomers a fantastic view of a dying
comet.
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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.
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 Hyakutake 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. (Note added April
10, 2006: Astronomers are currently tracking 19 fragments.)
![see caption](images/73p/guido_strip.jpg)
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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Author: Dr. Tony
Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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