Leonid Meteor Shower Most "Spectacular" for Decades
for National Geographic News
|November 18, 2002|
Set the alarm and get out your deck chair, your long johns, and your sleeping bag; tonight is the night for watching the sky's own light show.
The Leonid meteor showers occur every year in mid-November, but some years are far better than others. Astronomers say the next truly spectacular display after tonight won't occur until 2098 or 2131.
In ordinary years, sky gazers watching a meteor shower might see about ten meteors per hour. This year, however, scientists expect a full-fledged meteor storm. Astronomers at the European Space Agency (ESA) are predicting that Europeans might see 3,000 per hour. Night owls in North America can expect to see a few hundred an hour, estimate astronomers at NASA.
The moon will be nearly full, which is not ideal, but depending on where you arehigh, dry desert is best; inner city is worstvisibility may be reduced by only about 10 percent, scientists estimate.
Streams of Dust
Comets are essentially dirty snowballs, composed of dust and ice that circle the sun. When they get close to the sun, they start to melt and shed debris.
The Leonids are small particles of dust, ranging in size from a grain of sand to a pebble, that have been cast off by Comet Tempel-Tuttle on its trip past the sun. Comet Tempel-Tuttle hurtles through space on an oval orbit that extends to the planet Uranus and takes 33 years to complete. Trailing in its wake is a perpetual debris stream of particles, called meteoroids. They're called the Leonids because they seem to radiate out of the constellation Leo.
"Think of it as an Indianapolis 500 speedway of dust racing around an oval track," said Don Yeomans, an astronomer at NASA's Jet Propulsion Laboratory. "The Earth's orbit is circular, so we cross the dust trail every year in mid-November. How much dust we encounter at that point varies from year to year."
When the dust particles collide with Earth's upper atmosphereat speeds of about 44 miles per second (71 kilometers)they heat up, glow, and become meteors. We see them as streaks of light in the night sky, sometimes calling themmistakenlyshooting stars.
The particles usually vaporize at between 56 to 62 miles (90 to 100 kilometers) above the Earth's surface. They don't present a danger to those of us on the ground, but they can ding up any of the more than 500 satellites in orbit around the planet. Many satellites recovered by manned spacecraft show pits in their metal skins caused by meteoroids.
Crashing through the Past
The Earth is actually passing through two debris streams tonight, and this is old, old comet dust we're encountering.
"Europeans this year are running through the 1767 comet trail," said Yeomans. "North Americans will see meteors from the 1866 orbit."
The first encounter should cause a flurry of meteors over Europe and Africa around 0430 UT. The second encounter favors North Americans who are likely to see an outburst around 5:30 a.m. EST or 10:30 UT. Visit NASA's web site for estimates of best viewing time by location, http://leonid.arc.nasa.gov/estimator.html
Particles shed when the comet's orbit was closest to the sun this trip, which occurred on February 28, 1998, won't be seen for a long, long time, said Yeomans.
When the comet comes closest to the sun, particles are being released at a variety of velocities, leaving a new trail of dust. The trail will be affected by gravity and other factors, and each dust stream evolves somewhat independently. The 1998 particle stream is still hot on the tail of its parent comet, he said.
This is definitely the year to watch. When the comet next passes the sun in 2031 and 2065, the Earth isn't likely to pass through concentrated dust streams because Jupiter's gravitational pull will push the comet farther away from the Earth's orbit.
"We won't see another major Leonid show until the end of the 21st century," said Peter Brown, an astronomer specializing in meteors at The University of Western Ontario.
"This is the last chance for everyone alive now to see the Leonids in their full glory."
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