[meteorite-list] Anticipating the Perseids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:36 2004
Message-ID: <200107312229.PAA22695_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2001/ast31jul_1.htm

Anticipating the Perseids
NASA Science News

The 2001 Perseid meteor shower peaks on August 12th. Will it be an
extraordinary sky show like last year -- or a moonlit disappointment? Wake
up early and find out.

July 31, 2001: Summer nights and shooting stars. They're an unbeatable
combination. That's why northern sky watchers look forward so to the annual
Perseid meteor shower in August. During the shower's peak, which happens
this year on August 12th, dozens and sometimes hundreds of meteors shoot
across the sky every hour. Dark-sky observers who stay out all night
sometimes count more than a thousand!

The much-anticipated Perseids, as reliable as clockwork, rarely fail to
please. Around this time last year, however, enthusiasts were bracing for
something unusual: Perseid disappointment. On the night of the shower's 2000
maximum, the Moon was to be nearly Full. It would outshine all but the
brightest shooting stars.

Undaunted, sky watchers went outdoors in great numbers. Perhaps it was
simply habit. After all, the Perseids have been putting on their annual show
for more than two thousand years. Veteran meteor watchers call the shower
"Old Faithful" and many figured it might be worth watching in spite of the
Moon's glare.

They were right. Indeed, it was a night that no sky watcher was likely to
forget -- ever!

Just as hopeful observers were settling in for a long spell of meteor
watching, the sky erupted in color. A billion-ton cloud of electrified gas
from the Sun (a "coronal mass ejection") had crashed into Earth's magnetic
field and ignited widespread auroras. All across North America sheets of red
and green and purple light danced across the sky, stunning onlookers.

That's when the Perseids showed up.

"I couldn't believe my eyes," said an observer from Bishop, CA. Bright
Perseid meteors, plentiful after all, were streaking through seldom-seen
Northern Lights. "The moonlight hardly mattered," he added.

It just goes to show, you never know what might happen during a night under
the stars!

This year experts are again predicting a meager display of Perseids, and the
reason sounds familiar. A swollen quarter Moon, bright and shining, will
rise at midnight on Sunday, August 12th. By dawn -- just when the shower
should be at its best -- the Moon will lie high in the sky. Bright moonlight
will likely reduce apparent meteor rates to no more than 20 or 30 per hour.

"That's still a wonderful meteor shower and I recommend trying to see it,"
says astronomy professor George Lebo, a Summer Faculty Fellow at the NASA
Marshall Space Flight Center. "No matter where you live, the best time to
watch will be between local midnight and dawn on Sunday morning, August
12th." (Local midnight means midnight in your time zone.) "The Perseids are
primarily a northern hemisphere shower," he added. Sky watchers south of the
equator won't see nearly as many shooting stars as their northern cousins.

Watching for Perseids is easy. Simply find a dark observing site, away from
city lights if possible, and look up toward an unobstructed part of the sky.
Meteors will streak overhead. If you trace their tails backwards, most will
lead to the constellation Perseus, which contains a point in the sky called
"the Perseid radiant."

The Perseid radiant lies near the northeastern horizon at midnight, but it
will climb higher as the night wears on. By dawn it will lie about 70
degrees above the northeastern horizon. The Moon -- bright and beautiful,
but a real nuisance -- will be in the neighboring constellation Taurus. Like
an unwanted guest, the Moon will follow Perseus as it ascends the night sky.

"Don't look in the direction of the Moon," advises Lebo. Instead, face
somewhat away from the Perseid radiant with the Moon at your back. Perseid
meteors travel far across the sky, so you won't need to look directly toward
Perseus to see them. An old meteor watcher's trick is to view a moonlit
shower from the shadow of a tall building. The building will block some of
the sky, but it will also reduce the Moon's glare.

Typical Perseid meteors are about as bright as the stars in the Big Dipper
-- and many are considerably brighter. If you stay out all night on August
12th, don't be surprised to see several colorful fireballs rivaling the
brilliant planet Venus. Such meteors can be seen even from urban areas with
light pollution.

Every Perseid meteor that you spot this month is a tiny piece of periodic
comet Swift-Tuttle. The comet swings by the Sun at 135 year intervals and
leaves behind a trail of dusty debris. Its dust particles strike Earth's
atmosphere traveling 59 km/s (132,000 mph). Their extreme speed is the
reason that tiny comet particles, most no larger than grains of sand, can
produce such dazzling streaks of light. Typical dust specks burn up entirely
about 100 km above our planet's surface.

Earth entered the outskirts of Swift-Tuttle's debris stream in late July,
and already sky watchers are spotting 3 to 5 Perseids per hour in the
predawn sky. Meteor rates will gently climb in the days ahead as Earth
approaches the heart of the debris stream. Sometime on August 12th we'll
encounter a concentrated filament of comet dust and meteor rates will soar.
The shower that everyone has been waiting for will finally be underway.

But will it be worth the wait? Will the 2001 Perseids rival last year's
extraordinary show? Or will the Perseids, out of character, disappoint? The
best way to find out is to be outdoors that Sunday morning and see for
yourself!

Editor's note: On July 23, 2001, a spectacular fireball streaked over the US
east coast. It was not an early-arriving Perseid meteor, but rather a small
solitary asteroid that probably had nothing to do with comet Swift-Tuttle.
Received on Tue 31 Jul 2001 06:29:44 PM PDT