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Savor the Shooting Stars this Weekend

The Geminid Meteor Shower is best visible this year after midnight on December 13 and 14. You should also see meteors on nights before and after that. Best direction to look? The meteors appear to radiate from the constellation Gemini, but, like all meteors in annual showers, they will appear in all parts of the sky. The meteor numbers intensify as evening deepens into late night, with the greatest numbers falling an hour or two after midnight – when the meteor shower radiant looms highest in the sky.

The source of the Geminids is a mysterious object named 3200 Phaethon. The mystery, properly told, begins in the 19th century. Before the mid-1800s there were no Geminids, or at least not enough to attract attention. The first Geminids appeared suddenly in 1862, surprising onlookers who saw dozens of meteors appear to shoot out of the constellation Gemini. (That's how the shower gets its name, the Geminids.)  Astronomers immediately began looking for a comet. Meteor showers result from debris 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 hundred years astronomers searched in vain for the parent comet. On October 11, 1983, during a search for moving objects amidst the data gathered by the Infrared Astronomical Satellite (IRAS), Simon Green and John K. Davies found a rapidly moving asteroid in Draco. The next evening, Charles Kowal (Palomar Observatory, California) confirmed the body by photographing it with the 48-inch Schmidt telescope. The asteroid received the preliminary designation 1983 TB. As early orbital calculations were being made, the International Astronomical Union Circular for October 25, 1983, relayed the opinion of Fred L. Whipple that this asteroid possessed an orbit almost identical to that of the Geminid meteor stream. Additional observations confirmed the link and the asteroid eventually received the permanent designation of 3200 Phaethon. The intensity of the storm has been growing as Jupiter's gravity has been acting on Phaethon's debris stream, causing it to shift more and more toward Earth's orbit.

Just one problem: Meteor showers are supposed to come from comets, but 3200 Phaethon seems to be an asteroid. It is rocky (not icy, like a comet) and has no obvious tail. Officially, 3200 Phaethon is catalogued as a "PHA"—a potentially hazardous asteroid whose path misses Earth's orbit by only 2 million miles. If 3200 Phaethon is truly an asteroid, with no tail, how did it produce the Geminids? "Maybe it bumped up against another asteroid," offers Dr. Bill Cooke, head of NASA's Meteoroid Environment Office. "A collision could have created a cloud of dust and rock that follows Phaethon around in its orbit."

This jibes with studies of Geminid fireballs. Some astronomers have studied the brightest Geminid meteors and concluded that the underlying debris must be rocky. Density estimates range from 1 to 3 g/cm3. That's much denser than flakes of comet dust (0.3 g/cm3), but close to the density of rock (3 g/cm3).  So, are the Geminids an "asteroid shower"?

Cooke isn't convinced. 3200 Phaethon might be a comet after all--"an extinct comet," he says. The object's orbit carries it even closer to the Sun than Mercury. Extreme solar heat could've boiled away all of Phaethon's ice long ago, leaving behind this rocky skeleton "that merely looks like an asteroid."

In short, no one knows. It's a mystery to savor under the stars—the shooting stars.

For more information see  EarthSky.org “Everything You Need to Know” and EarthSky.org “New Meteor Shower Might Coincide”

Article by: W. Jerrold Samford, Environmental Compliance Specialist,Troutman Sanders, LLP