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Halley's Comet Returns ... in Bits and Pieces

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Halley's Comet Returns
... in Bits and Pieces

The annual Orionid meteor shower peaks this weekend on October 21st.

NASA
Marshall Space Flight Center

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see captionOct. 17, 2001: Next Sunday, try doing something out of the ordinary: Wake up at 3 a.m. Put on warm clothes. Step outside. Look up.

If the skies are cloudy, go back to bed. Otherwise keep looking. Before long you'll spot something that makes the trip outside worthwhile: a bright shooting star -- and a genuine piece of Halley's comet!

"It's the annual Orionid meteor shower," explains Bill Cooke, a member of the Space Environments team at the Marshall Space Flight Center (MSFC). "Every year in October Earth passes through a stream of dusty debris shed long ago by Halley's comet." When bits of comet dust -- most no larger than grains of sand -- strike Earth's atmosphere and disintegrate, they become "shooting stars."

Above: Artist Duane Hilton created this picture of an Orionid meteor streaking over a clump of golden aspens near Bishop, California.

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The Orionids -- so named because they appear to streak out of a point (called the radiant) in the constellation Orion -- will peak on Sunday morning, October 21st. Sky watchers north of the equator with dark clear skies will spot 15 to 20 meteors each hour before dawn. Observers south of the equator will see almost as many: 10 to 15 per hour.

Finding the Orionid radiant is easy. It lies near the left shoulder of Orion the Hunter, roughly centered within an eye-catching triangle consisting of Sirius -- the brightest star in the sky -- and the giant planets Jupiter and Saturn. (These stars and planets are in the southeastern sky before dawn, as viewed from mid-northern latitudes.)

But don't stare directly at the radiant, say experienced meteor watchers. Orionids that appear there will seem short and stubby -- a result of foreshortening. Instead, look toward any dark region of the sky about 90 degrees away. You'll see just as many Orionids, but they will seem longer and more dramatic. The tails of all Orionid meteors, no matter where they appear, will point back toward the radiant in Orion.

sky map -- see caption for details

Above: Observers at mid-northern latitudes can find the radiant of the Orionid meteor shower in the southeastern sky at 3 a.m. local time on October 21st. Click here for a southern hemisphere sky map.

The October Orionids are cousins of the eta Aquarids -- a mostly southern hemisphere meteor shower in May. Both spring from Halley's comet.

"Earth comes close to the orbit of Halley's comet twice a year, once in May and again in October," explains Don Yeomans, manager of NASA's Near-Earth Object Program at the Jet Propulsion Laboratory. Although the comet itself is rarely nearby -- it's beyond the orbit of Saturn now -- Halley's dusty debris constantly moves through the inner solar system and causes the two regular meteor showers.

The orbit of Halley's CometIn 1986, the last time Comet Halley swung past the Sun, solar heating evaporated about 6 meters of dust-laden ice from the comet's nucleus. That's typical, say researchers. The comet has been visiting the inner solar system every 76 years for millennia, shedding dust each time.

At first, newly-liberated dust specks simply follow the comet -- which means they can't strike our planet. Earth's orbit and Halley's orbit, at their closest points, are separated by 22 million km (0.15 AU). Eventually, though, the dust spreads out and some of it migrates until it is on a collision course with Earth.

"Particles that leave the nucleus evolve away from the orbit of the comet for two main reasons," explains Yeomans. "First, gravitational perturbations caused by encounters with planets are different [for the dust and for the comet]. Second, dust particles are affected by solar radiation pressure to a far greater extent than the comet itself."

"The orbital evolution of Halley's dust is a very complicated problem," notes Cooke. No one knows exactly how long it takes for a dust-sized piece of Halley to move to an Earth-crossing orbit -- perhaps centuries or even thousands of years. However, one thing is certain: "Orionid meteoroids are old."

And fast. "These meteoroids strike Earth's atmosphere traveling 66 km/s or 148,000 mph," he continued. Only the November Leonids (72 km/s) are faster. Such meteors often leave glowing "trains" (incandescent bits of debris in the wake of the meteor) that last for several seconds to minutes.

see captionLeft: The European Space Agency's Giotto probe captured this close-up image of the nucleus of Halley's comet. The bright jets are spewing debris that might one day return as Orionid or eta Aquarid meteors. [more]

Cooke and a group of his colleagues, led by Rob Suggs of the MSFC Engineering Directorate, will be observing the Orionids this weekend from Huntsville, Alabama, using an array of image-intensified cameras that can detect stars as faint as 8th magnitude. (For comparison, the unaided human eye can see stars of 6th magnitude against a very dark sky. An 8th magnitude star is 6.3 times dimmer than a 6th magnitude star.)

"This is our tune-up for the Leonid meteor storm next month," says Suggs. "We plan to station these cameras, which were developed at the University of Western Ontario, all around the world to monitor meteor activity on November 18th." That's when Earth will pass through a series of debris streams from periodic comet Tempel-Tuttle, perhaps unleashing a meteor shower of thousands of shooting stars per hour.

"The Orionids won't produce nearly as many meteors as the Leonids," added Suggs, "but, like the Leonids, the Orionids are fast, so they'll provide a good test for our system."

Next week Science@NASA will feature the results of Suggs' weekend meteor filming and explain more about the upcoming Leonid meteor shower. But why wait? You can enjoy a Leonid tune-up of your own this weekend. Simply go outside, look up, and watch as Halley's Comet returns ... in bits and pieces.

Editor's Note: Unlike most annual meteor showers the Orionids do not exhibit a sharp maximum. Meteor rates are elevated for a few days centered on October 21st. If Sunday morning is not convenient, try watching the Orionids before dawn on Saturday or Monday instead.

Web Links

The Orionids -- more information including a detailed history of the shower from Gary Kronk. The Orionids are cousins of the eta Aquarids.

Leonid Forecasts for 2001 -- prepared by Bill Cooke of the NASA/MSFC Space Environments and Effects program

Comet Halley -- history's best-known snowball, from The Nine Planets.

Comet Halley's Orbit in 3D -- find out where Halley is now using this Java applet from JPL's Near Earth Object Program web site.


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