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Two Magical Mornings

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Two Magical Mornings

Before dawn on Tuesday, Oct. 21st, and Wednesday, Oct. 22nd, sky watchers can see two bright planets, the crescent moon and a meteor shower.

NASA

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see captionOct. 20, 2003: This week, Jupiter will have a close encounter with the crescent moon. Dozens of meteors will shoot out of the constellation Orion. And Saturn will shine down over it all.

You can see these things on two magical mornings: before dawn on Tuesday, Oct. 21st, and Wednesday, Oct. 22nd.

Right: The last time Jupiter and the crescent moon appeared together before dawn, this is how they looked. Andy Skinner took the picture at Mono Lake, California, on Sept. 24, 2003. Click for labels.

"The meteors come from Halley's comet," says Bill Cooke, a member of the Space Environments Team at the Marshall Space Flight Center. "Every year in October Earth glides through a stream of dusty debris from the comet. The bits of comet dust, most no larger than grains of sand, disintegrate in the atmosphere and become shooting stars."

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"It's not an intense shower, but it is a pretty one."

The shower, which lasts for days, peaks on Tuesday morning, Oct. 21st. The best time to look is shortly before local sunrise--around 5:00 or 5:30 in the morning. Sky watchers with clear dark skies will spot a meteor every five minutes or so.

Meteors will streak out of the constellation Orion. That's why they're called the Orionids. The radiant point is near Orion's left shoulder; it is also, coincidentally, near the bright planet Saturn. Simply look south and up--you can't miss it.

But don't stare too long at the radiant, advises Cooke. 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.

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Above: Looking southeast from mid-northern latitudes on Tuesday, Oct. 21st at 5:30 in the morning.

In fact, a great place to look is in the direction of Jupiter rising in the eastern sky. Jupiter and the crescent moon will be pleasingly close together on Oct. 21st--a wonderful backdrop for a meteor shower. They'll be close together again on Wednesday morning, Oct. 22nd. The Orionids will still be active, too, only a little diminished from the day before.

Two mornings in a row: Jupiter, Saturn, the crescent moon and a sprinkling of meteors. It doesn't get much better than that.

More about the Orionids

The orbit of Halley's cometThe 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 Uranus now--Halley's dusty debris constantly moves through the inner solar system and causes the two regular meteor showers.

In 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.

Below: 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]

see captionAt 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.

Web Links

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

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

Cold Comet Halley -- (APOD) a faraway picture of Halley's comet

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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