[meteorite-list] Return of the Leonids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Nov 14 12:53:19 2006
Message-ID: <200611141753.JAA10483_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2006/14nov_leonids.htm

Return of the Leonids
NASA Science News
November 14, 2006

Nov. 14 , 2006: On Sunday, Nov. 19th, Earth will pass through a stream
of debris from comet 55P/Tempel-Tuttle. The result: a shower of Leonid
meteors.

"We expect an outburst of more than 100 Leonids per hour," says Bill Cooke,
the head of NASA's Meteoroid Environment Office in Huntsville, AL. This
pales in comparison to the Leonid storms of 2001 and 2002, when sky watchers
saw thousands of meteors. Even so, a hundred per hour would make the Leonids
one of the best showers of 2006.

The problem is, you have to be in the right place at the right time to
see them.

Earth's encounter with the comet dust is going to be brief - "possibly no
more than a few hours long," says Cooke. Forecasters differ on when the
outburst will occur. Estimates range from 0445 UT to 0630 UT on Nov.
19th (11:45 p.m. on Nov. 18th to 1:30 am EST on Nov. 19th). The timing
favors western Europe, Africa, Brazil and eastern parts of North
America: map http://science.nasa.gov/headlines/y2006/images/leonids/worldmap.gif.

Cooke urges observers to find the darkest possible skies. "These Leonids
are going to be faint." Why? "The stream contains very small grains of
comet dust. Small grains make faint meteors--it's as simple as that."

The mid-November region of Earth's orbit is littered with debris from
Comet Tempel-Tuttle. Every time the comet visits the inner solar system
(once every 33 years), it lays down a new stream of dust, pebbles and
rock. This creates a sort of "minefield" for Earth to navigate every
November.

Not all of these debris streams are alike. For example: "A Leonid stream
we hit in 1998 was full of rock-sized debris. They made brilliant
fireballs when they hit the atmosphere," recalls Cooke. "The stream
we're hitting this year is just the opposite. It's mostly fine dust."

Debris streams are segregated "dusty vs. rocky" by the force of
sunlight. Consider the stream directly ahead of us: "It was ejected from
the comet in 1933," says Cooke. "At first, the debris was a mixture of
many sizes." But as years passed, the smaller particles diverged from
the larger ones. Radiation pressure - the delicate pressure of sunlight
itself - pushed the light dust onto a collision course with Earth. Heavier
rock-sized fragments resisted the pressure and lagged behind.

Perhaps in some future year we'll encounter the larger debris from 1933
and receive an overdue display of fireballs. How would they get here?
"Nudged by Jupiter," suggests Cooke. Jupiter's gravity is strong enough
to alter the course of heavier fragments. Indeed, by guiding debris
toward us, Jupiter is indirectly responsible for many bright Leonid
displays in the past.

But this is 2006. So prepare for an outburst of faint Leonids.

Extra: Don't believe everything you read. While meteor forecasters have
done a splendid job predicting Leonid outbursts in recent years - sometimes
"nailing the peak within minutes" - they could be wrong in
2006. The outburst might happen at an unexpected time or it might be
better than expected. Cooke urges enthusiasts everywhere to keep an eye
out for Leonid meteors the nights of Nov. 17th - 19th. "The best time to
look," he says, "is just before local dawn when the constellation Leo is
high in the sky."
Received on Tue 14 Nov 2006 12:53:15 PM PST


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