Sky Show Tonight: Lyrids Kick Off Meteor Season

John Roach
for National Geographic News
April 21, 2004
Beginning late tonight and stretching into the predawn hours of Thursday morning, more than 20 meteors an hour will streak across the sky, bringing a burst of spring joy to meteor shower enthusiasts.

Known as the Lyrids, the shower appears to emanate from a point in the sky, or radiant, just west of the constellation Lyra, which contains the bright star Vega. (Lyra is found near the more familiar constellation Cygnus, also known as the Northern Cross.) (See star chart.)

In nearly all the world's time zones, the Lyrids' radiant rises above the horizon at around 11 p.m. If the skies are clear and dark, sky watchers should begin to see a steady stream of shooting stars about an hour later, as Earth turns into the meteor stream, according to astronomers.

"If you are brave enough or hardy enough to stay awake to the wee hours of the morning, the higher the radiant gets, the better," said William Cooke, a meteor-shower expert with the Space Environment Group at NASA's Marshall Space Flight Center in Huntsville, Alabama.

The radiant reaches maximum altitude just before dawn on Thursday morning. The anticipated peak for meteor activity is 11 p.m. ET tonight, but stargazers in the United States will not see much until the radiant is higher, Cooke said.

Peter Jenniskens is an astronomer and meteor-shower expert with the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California. He said this year should be an excellent one in which to see the Lyrids, since the moon will be absent during the shower, making for a dark sky.

"It's not a very intense shower, not as intense as the Perseids. But it happens after a long time of very low [meteor-shower activity] in the spring. This sort of starts the meteor season," Jenniskens said.

Shooting Stars

Though named the Lyrids because the meteors appear to radiate from the constellation Lyra, the meteors have nothing to do with Lyra. Rather, they are bits of dusty debris shed by comet Thatcher.

Thatcher is a so-called long-period comet, because it takes about 450 years to make a single trip around the sun. Any comet with an orbit longer than 200 years is considered a long-period comet. Thatcher last made its closest approach to the Sun in 1861.

When Thatcher makes a close approach to the Sun, debris is cooked off. Initially, this debris forms a cloud of dust that travels along with the comet.

But when the comet reaches the furthest point in its orbit from the sun and then rounds the bend on its return, the extra kick that some dust particles received during their prior ejection makes them move even farther out. Thus, these dust particles lag behind the comet when they fall back to the sun, forming a trail in the comet's path.

When Earth's annual orbit around the sun crosses this trail, bits of debris enter the atmosphere from the direction of the constellation Lyra. Stargazers see them as fiery streaks of light, or shooting stars.

When Earth hits a fresh dust trail, a meteor outburst can be seen. In 1803 stargazers were treated to a storm of about 700 Lyrids per hour. The Lyrids were also intense in 1922 and 1982. Based on an analysis of comet Thatcher's debris trail, Jenniskens predicts that there will also be heightened Lyrid activity in 2040 and 2041.

"Sadly, the debris streams from all long-period comets usually stick around for just one revolution, becoming dispersed perpendicular to the comet orbit by the second revolution," Jenniskens said.

This rapid spreading occurs due to the gravitational tug exerted by Jupiter, which causes dust trails to wag in and out of Earth orbit. Jupiter's gravitational tug can also dramatically alter the orbital period of the debris. Different sections of dust trails catch up on each other during their second revolution, creating a much broader but less dense shower.

Comet Impact?

Many astronomers consider long-period comets a huge impact risk to Earth. But since these comets sweep through the inner solar system so infrequently, very little is known about them.

This dearth of knowledge may be problematic. By the time a comet bound for Earth enters the inner solar system, it could be too late for anyone to take effective action against it.

When astronomers detect an unanticipated meteor shower, much can be learned if they record when the shower occurs and the location of the shower's radiant, Jenniskens notes. With such information, astronomers can predict when the next outburst will occur, roughly calculate the comet's orbit, and get a better feel for whether or not the comet poses a risk of impact with our planet, Jenniskens said.

"It's very important that people watch on any night of the year for this type of meteor shower," he said. "When rates coming out of one area in the sky [go up], that's when people should report it."

Jenniskens notes that Thatcher is "perhaps the shortest long-period comet" that comes near Earth orbit. Because of that, Thatcher is also the only long-period comet with a strong annual meteor shower. The debris streams of other long-period comets are too thinly spread out to produce a noticeable annual shower.

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