What are the Geminid meteors? Scientists aren't sure.
Perhaps chips off an exotic asteroid or dust from an extinct
comet. In either case, they'll soon be here.
The answer is "now." Today Earth is entering the outskirts of a dusty debris cloud shed by a mysterious object named 3200 Phaethon. It's the beginning of the annual Geminid meteor shower, which peaks this year on Dec. 13th and 14th.
Above: This Leonid Earthgrazer, which streaked 80 degrees across the sky on Nov. 18, 2001, was too long for the camera's field of view. Sky watchers could see similar Geminid Earthgrazers after sunset on Dec. 13th. Image credit: M. Vasseur and P. Martin. [more]
The two-week long Geminid shower is barely a trickle at the moment -- only 5 to 10 meteors per hour. But soon it will intensify ten-fold or more.
"When the Sun goes down on Thursday," says Bill Cooke of the NASA Marshall Space Flight Center, "Gemini will be low but rising over the eastern horizon [as viewed from mid-northern latitudes]. You won't see many meteors then, but the ones you do will likely be beautiful Earthgrazers" -- that is, disintegrating meteoroids that fly over the horizon nearly parallel to the atmosphere. Earthgrazers are long, bright and vivid. A remarkable sight.
"After an hour or so of watching for Earthgrazers, you might want to go back inside for a few hours and warm up," adds Cooke. Meanwhile, Gemini will continue to climb higher in the sky. "Around midnight go back outside," he suggests. "Gemini will lie almost directly overhead. From midnight until dawn on Friday, Dec. 14th, you could spot as many as 100 shooting stars per hour."
Cooke's suggestions are correct for observers in any time zone of the United States or Europe. Visit SpaceWeather.com for observing tips for other parts of the world.
Above: Geminid meteors stream from a point called "the radiant" in the constellation Gemini. Denoted here by a red dot, the radiant will rise above the eastern horizon around sunset on Dec. 13th. Meteor activity will reach a maximum between midnight and dawn on Dec. 14th when the radiant is high in the sky. [more sky maps]
Nowadays the Geminids are generally regarded as one of the best annual meteor showers. But it wasn't always so. Before the mid-1800's there were no Geminids, or at least not enough of them to attract attention. The first Geminid shower suddenly appeared in 1862, surprising sky watchers who saw 15 or so shooting stars each hour.
Astronomers immediately began looking for a comet. Most meteor showers result from debris that that boils off a comet when it passes close to the Sun. When Earth passes through the debris, we see a meteor shower.
For more than a century astronomers searched in vain for the parent of the Geminids. Finally, in 1983, NASA's Infra-Red Astronomy Satellite (IRAS) spotted something. It was several-km wide and moved in much the same orbit as the Geminid meteoroids. Scientists named it 3200 Phaethon.
But rather than solving the puzzle of the Geminids parentage, the IRAS discovery simply deepened the mystery. Why? Because Phaethon appears to be an asteroid. Indeed it's cataloged as a potentially-hazardous one that skims by Earth's orbit only 8 times farther away than the Moon. Asteroids that spew debris into space like a comet are rare indeed, so astronomers were more baffled than ever.
Left: This unusual asteroid, known as Elst-Pizarro, briefly sprouted a tail in 1996 after, perhaps, a collision with another object in the asteroid belt. Is this what happened to 3200 Phaethon long ago? Brian Marsden of the Minor Planet Center discusses the possibility in a Science@NASA article about last year's Geminids.
Since then many sky watchers have come to regard the Geminids as a "weird" meteor shower -- the only one caused by an asteroid. But maybe, says Cooke, it's not so weird after all. "I don't think the Geminids come from an asteroid. They're cometary ... just like all the other meteor showers. 3200 Phaethon is indeed the parent, but it's an extinct or dormant comet."
According to Cooke, Phaethon probably looked much like other comets many centuries ago, with a fuzzy head and a glowing dusty tail. But this one was doomed to rapid extinction by its short-period sungrazing orbit. Every one and a half years Phaethon plunges sunward from the asteroid belt and swings by the Sun at a distance of 0.14 astronomical units -- closer even than the planet Mercury. Such near encounters with the Sun would have cooked Phaethon, vaporizing its ices and leaving behind a shell of asteroid-like dust and rock.
Such over-cooked comets may be abundant, says Mike A'Hearn (Univ. of Maryland), the principal investigator of NASA's Deep Impact mission. "Dynamical studies suggest that perhaps a few percent to 50% of all near-Earth objects are dormant or extinct comets masquerading as asteroids."
From a distance there's no definitive way to tell the two apart. "Both comets and main belt asteroids are very dark," says Lucy McFadden (Univ. of Maryland), a member of the Deep Impact science team. "And we don't know of any robust chemical or spectral signature to absolutely identify a comet's nucleus." Indeed, she says, "even if we were to fly to Phaethon we might not be able to tell whether it is an extinct comet" without somehow looking beneath its crust.
Above: Scientists aren't sure what the upper layers of a comet are like. These two models, for example, disagree about the depth of the mantle and its structure. [more information]
That's exactly what the Deep Impact spacecraft will do to Comet Tempel 1 when it travels there in 2005 and excavates a crater by dropping a 350 kg impactor onto the comet. A goal of the mission is to learn what the crusts of comets are made of and what lies beneath them. "Perhaps in a few years we will feel more confident of the chemical signature of comet nuclei," adds McFadden.
Meanwhile 3200 Phaethon is likely to remain a puzzle. Are the Geminids caused by old dust from an extinct comet or chips off an exotic asteroid? No one knows. But don't let that stop you from heading outdoors on Dec. 13th and 14th. This is a mystery best pondered under dark skies ... with a flurry of beautiful Geminids soaring overhead!
Editor's Note: Unless otherwise noted, all times, sunrises, and sunsets mentioned in this story are "local" -- that is, pertaining to the place where you live.Web Links & More....
History of the Geminids -- from Gary Kronk's "Comets & Meteor Showers"
The Baffling Geminid Meteors -- (Science@NASA) In this article about the 2000 Geminid meteor shower, Brian Marsden of the Minor Planet Center discusses the possibility that 3200 Phaethon is an asteroid.
Right: These two meteors streaked through Orion on December 13, 1998, during the Geminid meteor shower. The fainter of the two is a Geminid; the brighter is a sporadic meteor or perhaps a Coma Berenicid. Photo details: Astrovid 2000 CCD video camera, 12 mm f1.2 lens, 1/60 sec shutter speed. Credit: T. Phillips.
3200 Phaethon's orbit in 3D -- (JPL) This Java applet lets you explore the orbit of 3200 Phaethon.
"Comet" Elst-Pizarro -- (ESO) this strange asteroid grew a tail and, for a while, posed as a comet.
more Observing Tips -- (SpaceWeather.com) Sky maps and observing tips for the 2001 Geminid meteor shower.
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