Orionid Meteor Shower to Peak Wednesday Night

Brian Handwerk
for National Geographic News

October 18, 2004

Halley's comet won't return until 2061, but pieces of the celestial body are streaking across the sky. The heavenly show, known as the Orionids meteor shower, peaks Wednesday night, when sky-watchers may observe two dozen meteors per hour.

Though the comet remains distant, Earth is passing through the comet's ancient debris field—with dramatic results.

"Over time comets leave a trail of debris along their orbits," explained Kelly Beatty, executive editor of Sky and Telescope and editor of Night Sky magazine.

Each time a comet orbits the sun, the star's heat strips comets of dust and ice. Scientists believe that Halley's comet sheds some 20 feet (6 meters) of dust and ice particles on each pass.

"For a select few [comets], the Earth goes through their orbits at the same time every year," Beatty said. "The analogy I like to use is a garbage truck full of sand. As it barrels down the road, the sand billows out the back end. And that's what Earth plows through."

Tiny Particles Light Up Night Sky

Earth passes near Halley's cigar-shaped orbit debris field twice each year: the Orionids shower fall in October, the Aquarids shower in May.

The tiny particles of ice and rock, some as small as a grain of sand, put on quite a show as shooting stars or meteor showers.

"What we see is not the particle burning up," Beatty said. "What we're really seeing is the particle transferring all that energy to the air molecules along its path and causing them to become superheated to the point that they are incandescently hot."

Meteor particles are among the smallest celestial objects that can be seen by the human eye.

"For anyone who has eaten a bowl of Grape Nuts, the little nuggets in there are a pretty good match in size, shape, density, and even color of what a typical meteor particle looks like from space," Beatty said.

The particles haven't yet had time to drift very far from their orbit. "Given enough time they will wander from the orbit," said Scott Sandford, an astrophysicist at NASA's Ames Research Center at Moffett Field, California. "When thousands of years have passed they will have spread out quite a bit, and after hundreds of thousands of years many particles won't be obviously associated with the orbit."

Sandford notes that scientists are always interested in tracking the infall rates of meteor showers—and that amateur observation can be of great help.

"Infall rates rise and fall depending on how centrally the Earth's orbit passes through the [comet's] debris trail," he said. "The count rate can tell you something about how the particles are evolving in their obits."

The fiery meteor tails can also yield clues to the scientific eye.

"By measuring the spectra of the light given off [by meteors] we can try to learn something about the composition of the meteors," Sandford added. "It has been clear for some time that most of these particles seem to be relatively fragile. They may be size of a grain of sand but they aren't like a grain of sand, they aren't like a hard little lump of rock. A grain of that size probably consists of hundreds, thousands, maybe millions of smaller grains loosely stuck together."

A Celestial Show

The action for this week's Orionid shower is taking place some 50 to 70 miles (80 to 110 kilometers) above the Earth. The view should be good from terra firma.

Sky-watchers should look to the constellation Orion when it rises after midnight. Meteor showers are named from the area of the night sky from which they appear to originate, known as the radiant.

Thus the Orionids are named after the constellation Orion, because they appear to originate above the mythical hunter's left shoulder. The appearance is actually a trick of perception over the great distances of space.

Orion is centered within a triangle formed by Sirius (the sky's brightest star), Jupiter, and Saturn.

Earth and Halley orbit in opposite directions. Thus, the comet's meteors approach Earth head-on. This accounts for the meteor's tremendous speed.

Traveling at 42 miles (67 kilometers) a second, the Orionids are among the fastest known meteors. It also means that, from North America, the shower will be most visible after midnight, and especially in the hour before dawn.

"After midnight we're on the side of Earth that's facing forward along our orbit, so it's in that direction that we get those particles hitting us head on," Beatty said. "In the evening, we're on the 'back side' of the planet where these meteors can't hit us head-on."

To see as many of the Orionid meteors as possible, find the darkest spot you can and scan the whole sky. The meteors' distinctive tails can appear anywhere in the sky. Viewing requires no special equipment, and, in fact, telescopes are of little use.

"The best place to look is anywhere you can get where the sky is darkest," Beatty said. "For every very-bright meteor, there are lots of little faint ones. If you're swamped by urban light pollution, you'll miss the faint ones. And give your eyes ten minutes or so to adjust to the darkness. We tell people, Get one of those lounge chairs that you've just put away for the winter, dress warmly, and find a dark spot—even just away from streetlights."

"When you see one of these meteors flash across the sky it's a calling card from Comet Halley," Beatty said, "left behind during the thousands of years that it's been orbiting."

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