Meteor Show: "Better" Leonid Shower Starts Tonight

John Roach
for National Geographic News
November 17, 2003
Shooting stars should be visible tonight, the traditional climax of the Leonid meteor shower. Sky watchers in the Americas, however, will observe peak activity early Wednesday morning.

The shower climaxed for viewers in eastern Asia late last week, but the nearly full moon washed out some meteors that would have otherwise been visible in a dark sky.

The shower's peak early Wednesday morning "will be better" than last week's, said Bill Cooke, an expert on meteor showers with the Space Environment Group at NASA's Marshall Space Flight Center in Huntsville, Alabama.

Weather permitting, residents in the northeastern United States, for example, may witness 17 meteors every 15 minutes as the shower peaks at around 2:30 a.m. ET on Wednesday.

While still impressive, Leonid activity is returning to normal after several years of spectacular displays, said Michael Bakich, an associate editor with Astronomy Magazine in Waukesha, Wisconsin.

"This year will not be one of the super showers as it has been over the last few years," he said. "It's coming back to normal." Bakich said sky watchers won't discern much difference in shower activity tonight and early Tuesday morning, the traditional Leonid peak, versus tomorrow night and early Wednesday morning.

Cooke maintains that while the 2003 Leonids will be less intense than in recent years, tomorrow night's peak will still see a ten-fold increase over normal shooting star activity, making that the optimal night to catch sight of a meteor.

Debris Streams

The Leonids are caused by the comet Tempel-Tuttle, which swings through the inner solar system every 33 years. With each pass, the sun cooks off the comet a bit of dusty debris that trails in the comet's wake.

Earth orbits through this debris zone every year. When the planet passes through a stream, or trail, of debris, meteor showers grace the skies as the dusty particles vaporize in Earth's atmosphere.

"We always intersect multiple streams of Leonids. For the past few years, we've been intersecting up to six streams," said Cooke. "What makes this [year] unusual is the streams are so spread out so far in time."

In 2001, Earth directly hit a dense stream, sparking a meteor storm. The occasional phenomenon occurs when Earth encounters at least 1,000 meteors per hour. However, this year's peaks are separated by six days and will produce no more than 100 per hour, said Cooke.

"The Earth will not go through one of the highest density regions in the Leonid[s]," said David Asher, a Leonid expert at Ireland's Armagh Observatory. "There is no hard and fast rule of where the highest density regions are … but one can say the densest regions are associated with younger trails."

On late Tuesday night and early Wednesday morning, astronomers expect Earth to intersect a stream of debris cooked off the Tempel-Tuttle in 1533. The meteors on November the 13 were from a stream cooked in 1499.

Faint or Bright?

While astronomers have improved the accuracy with which they predict when the Earth will intersect debris of comet Tempel-Tuttle that produces the Leonids, they are less sure in their predictions of how many meteors will be seen and how bright they will be.

The aid of newer, bigger, and faster computers has allowed astronomers to chart where and when Earth is likely to intersect a debris stream and correlate any particular debris stream (or trail) to the year it was cooked off Tempel-Tuttle by the sun.

"But the number of bright meteors and intensity requires knowing how particles are being ejected from the comet in more detail than we actually can do," said Cooke.

Asher said that the heavier particles tend to stay closer to the comet as the smaller particles are pushed further back by radiation pressure from the sun.

"The parts of the 1499 and 1533 trails that will be encountered by Earth this month are not the parts of those trails that contain the largest particles," he said. Since smaller particles produce fainter meteors, Asher expects this year's Leonids to be faint.

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