Mini-Comets Approaching Earth

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TEXT-ALIGN: justify"> 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. 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.

Roman"> 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.) 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. http://aquarid.physics.uwo.ca/%7Epbrown/taus.pdf 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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