Orionids Meteor Shower Starts This Weekend

Victoria Jaggard
National Geographic News
October 16, 2009

Earth is currently plowing through space debris left behind by a visitor that last swung by during the Reagan Administration.

Spawned by Halley's comet, which last buzzed the planet in 1986, tiny space rocks are the seeds of the annual Orionid meteor shower.

Beginning Friday, sky-watchers should see some of the year's first Orionids. And thanks to a moonless midnight sky, the shooting stars should be fairly simple to spot.

At its peak on Wednesday night, the Orionids shower should produce 20 to 25 meteors an hour—a "relatively decent show," according to astronomer Anita Cochran, of the University of Texas at Austin's McDonald Observatory.

While the Orionids are not as flashy as some other meteor showers, she said, "it's a known shower that comes along regularly, … and the moon will be down, so that will help."

A big, bright moon can make it hard to spot streaking meteors. But the moon will be new during this year's Orionids peak, she said, meaning it'll be dim and will dip below the horizon not long after sunset.

Orionids' "Very Recognizable" Region

The Orionids are so named because the meteors appear to radiate from near the constellation Orion, aka the Hunter.

This easily spotted constellation "kind of looks like an hourglass with a very recognizable belt of stars," said astronomer Mark Hammergren of the Adler Planetarium in Chicago.

(Related: "Famous Star Is Shrinking, Puzzling Astronomers.")

In addition, "the constellation is visible from pretty much anywhere in the world, because it appears along a line of sight close to the Equator," he said.

How to See the Orionids

At this time of year, Orion rises at about 11 p.m. local time worldwide, so the best time to view the Orionids will be after midnight, Hammergren said.

For the best views, Hammergren and Cochran both recommend going to a dark site away from city lights and allowing enough time for your eyes to adjust to seeing fainter objects in the sky.

"You don't need binoculars," Cochran added. "Just lie back in a reclining chair or on a blanket and enjoy the show."

But dress warmly, Hammergren advised: "You always cool off more than you think you will just lying there—that's a lesson novice astronomers learn real fast!"

Orionid Meteor Shower's Famous "Parent"

The Orionid meteors are created by a band of small particles that circle through the solar system in the orbit of Halley's comet.

The comet is visible to the naked eye, and in the 1700s astronomer Edmond Halley was the first to correctly predict its return, calculating that the comet comes back every 76 years.

Later studies revealed historical sightings of Halley's comet—in circa-240 B.C. Chinese records and in medieval England's 11th-century Bayeux Tapestry, for example.

(Related pictures: "Sun Probe Spies New Periodic Comet.")

But it wasn't until the 1800s that astronomers noticed that some large groupings of "shooting stars" also appeared at regular intervals. Still later, those groupings were linked to comets.

The mass of a comet is basically a fifty-fifty combination of dirt and ice left over from when the planets were forming about 4.5 billion years ago, the McDonald Observatory's Cochran explained.

When a comet comes into the inner solar system—which includes Mercury, Venus, Earth, Mars, and the asteroid belt—the sun's heat turns some of the comet's ice into gas. Without the ice to hold them to the comet, some rocky fragments fall away.

"That stuff doesn't just disappear," Cochran said.

"When Earth's orbit intersects with those dust streams, the particles hit the upper atmosphere, creating meteors."

No Orionid Clump

A fragment shed by a comet is usually no larger than a grain of sand. But sometimes those grains travel together in groupings, or clumps, which can be held together by gravity for hundreds of years, Adler's Hammergren added.

Since these clumps form when the comet is near the sun, Earth is most likely to hit them in years when the comet is once again close to the planet—resulting in a more spectacular meteor shower.

For example, next month astronomers expect Earth to pass through a clump of material left behind by comet Tempel-Tuttle, which might boost the annual Leonid meteor shower to produce as many as 500 meteors an hour.

But Halley's comet is now almost at its farthest point away from the sun, Hammergren said, so "I wouldn't expect a clump [for the Orionids] this year."

Still, he said, for fans of the famous space rock, the Orionids in any year are "a great chance to see a piece of Halley's comet in the sky."





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