Mini-Comets Approaching Earth (Comet 73P/Schwassmann-Wachmann 3)

http://science.nasa.gov/headlines/y2006/24mar_73p.htm

Mini-Comets Approaching Earth
NASA Science News
March 24, 2006

A cometary "string-of-pearls" will fly past Earth in May closer than
any
comet has come in almost 80 years.


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 almost eighty
years.

"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

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.

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."

On the other hand, what if "the comet was shattered by a hit from a
small interplanetary boulder?" A violent collision could produce
faster-moving debris that would 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.