Dec 9, 2002

Meteorsfrom the Twilight Zone




The annual Geminid meteor shower peaks this year on Saturday morning, Dec. 14th.




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December 9, 2002: The Geminid meteor shower has begun.


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It started on Saturday, Dec. 7th, when our planet entered a cloud of dusty space-debris. The meteor rate is low now, because we're still in the outskirts of the cloud, but it will increase as Earth penetrates the debris field. "We expect to see more than 100 meteors per hour when the shower peaks on Saturday, Dec. 14th," says Bill Cooke of the NASA/Marshall Space Environments Team.


Right: Amateur astronomer George Varros photographed this Geminid fireball on Dec. 7, 2002. "It was as bright as Venus," says Varros. [more]

"The Geminids are one of the best annual meteor showers," says Cooke. Geminid meteors are bright, plentiful, and sky watchers can see them from both hemispheres. And that's not all...

"I enjoy the Geminids because they come from the Twilight Zone," he laughs.




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Meteor showers are supposed to come from comets. "Comets are big chunks of ice mixed with some amount of dust and rock," explains Cooke. When they swing by the Sun, the ice evaporates; clouds of dust and gas spew into space--"that's why comets have tails," he says.

The source of the Geminid shower is an object called 3200 Phaethon. Discovered in 1983 by NASA's Infrared Astronomical Satellite, Phaethon shares its orbit with the Geminid meteoroids. Furthermore, it is about the right size to be a comet. So far, so good...

But 3200 Phaethon doesn't act like a comet, says Cooke. "3200 Phaethon doesn't sprout a tail when it comes close to the Sun. It doesn't have a halo or a coma ... in fact, based on its orbit and the way it reflects sunlight, it seems much more like an asteroid."

Perhaps, he speculates, it's a little bit of both.

"It has become quite obvious in recent years that our nomenclature in astronomy is not keeping pace with our discoveries. We like to place objects in neat well-defined categories even though Mother Nature makes no such distinctions. For instance, is Pluto a planet or a Kuiper-belt asteroid? When does a Jupiter-like planet become a brown dwarf?"


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Likewise, he asks, "after a comet has gone around the Sun so many times that its ices are exhausted, so that it no longer has a tail or a coma, is it still a comet? Or do we re-classify it as an asteroid?"


Phaethon may well be a "Twilight Zone" object--intermediate between asteroids and comets. "That's my opinion," says Cooke. "3200 Phaethon was once an active comet, but about 1000 years ago the last of its ices evaporated." Indeed, dynamical studies by Bo Gustafson and colleagues at the University of Florida show that Geminid meteoroids are about 1000 years old. "Now," he adds, "all that's left is a rocky comet corpse."

Above: Something else from the Twilight Zone? In 1996, asteroid Elst-Pizarro briefly sprouted a tail and looked much like a comet. Learn more about it in an earlier Science@NASA story "The Baffling Geminid Meteor Shower."

An asteroid, a comet, a comet corpse? The meteors are lovely no matter what.

Geminid meteors stream out of the constellation Gemini--hence their name. Try looking for them any clear night this week after 10 p.m. local time, when Gemini is well above the horizon. The best time to watch, says Cooke, is actually around 2 a.m. local time. "That's when the Moon sets and when Gemini is highest in the sky," he explains.

This year's Geminid shower is expected to peak between Friday evening, Dec. 13th, and Saturday morning, Dec. 14th. This is true no matter where you live.

Friday evening is also a good time to look for Geminid earthgrazers. Earthgrazers are disintegrating meteoroids that fly over the horizon nearly parallel to the atmosphere. They have remarkably long and colorful tails. Around 8:00 p.m. local time at mid-northern latitudes, Gemini will be hanging low above the eastern horizon--ideal geometry for earthgrazing meteors. "You might not see many, because they're rare," says Cooke. "But earthgrazers are beautiful, so it's worth a try."


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Above: The radiant of the Geminid meteor shower (denoted by a red dot) is located near Castor and Pollux, the two brightest stars of Gemini. Northern-hemisphere observers can find the constellation at 2:00 a.m. hanging high in the southern sky. Southern-hemisphere observers should look north instead. Geminid meteors can appear anywhere in the sky; their tails, however, will all point back toward the radiant in Gemini. Sky maps:




And finally, don't forget to dress warmly because mid-December nights in the northern hemisphere are likely to be cold. Bring a reclining chair, suggests Cooke, or spread a thick blanket over a flat spot of ground. Lie down and look up. Geminid meteors can appear in any part of the sky.

The shower will quickly wane after Dec. 14th. "Earth exits the debris cloud on Dec. 17th and the Geminids will vanish until next year," he says. So catch them now, while you can--the meteors from the Twilight Zone.




Credits & Contacts
Author: Dr. Tony Phillips
Responsible NASA official: John M. Horack
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
The Science and Technology Directorate at NASA's Marshall Space Flight Center sponsors the Science@NASA web sites. The mission of Science@NASA is to help the public understand how exciting NASA research is and to help NASA scientists fulfill their outreach responsibilities.
more information


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Visit on Dec. 13th and 14th for updates and images of the ongoing shower.


Right: Photographer Thad V'Soske captured this image of a Geminid meteor streaking through the belt of Orion during last year's shower. Copyright: Thad V'Soske.

The History of the Geminid Meteor Shower -- by meteor scholar Gary Kronk

3200 Phaethon: 3D orbit (JPL); 3200 Phaethon (EarthSky)


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