We were treated to the latest Leonid meteor storm last month, and now it?s time for the annual Geminid meteor shower, which peaks this year on December 13th and 14th. You can see it best beginning just after sunset today, Thursday, Dec. 13th.

?When the Sun goes down on Thursday,? says Bill Cooke of the NASA Marshall Space Flight Center, ?You won?t see many meteors then, but the ones you do will likely be beautiful Earthgrazers.? Earthgrazers are long, vivid meteroids that fly over the horizon nearly parallel to the atmosphere. ?Around midnight go back outside,? he suggests. ?From midnight until dawn on Friday, Dec.14th, you could spot as many as 100 shooting stars per hour.? Cooke?s suggestions will work for observers in any time zone of the United States or Europe.

The Geminids are regarded as one of the best annual meteor showers. But before the mid-1800?s there were no Geminids. The first Geminid shower suddenly appeared in 1862, surprising sky watchers who saw about 15 shooting stars each hour.Astronomers immediately began looking for a comet, since 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 that was several miles wide and moved in the same orbit as the Geminid meteoroids. They named it 3200 Phaethon.

Phaethon appears to be an asteroid, and is thought to be potentially hazardous, since it flies by Earth?s orbit only 8 times farther away than the Moon. Asteroids that send debris into space like a comet are very rare, so astronomers are baffled by this object.

But Cooke says, ?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.?

Cooke believes Phaethon looked like other comets many centuries ago, with a fuzzy head and a glowing dusty tail. But it was doomed to rapid extinction by its short-period orbit close to the sun. Every one and a half years Phaethon plunges towards the sun 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 have cooked Phaethon, vaporizing its ices and leaving behind a shell of asteroid-like dust and rock.

Over-cooked comets may be common, says Mike A?Hearn of the University 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,? he says.

From a distance there?s no way to tell the two apart. ?Both comets and main belt asteroids are very dark,? says Lucy McFadden of the University 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. 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.

That?s 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 explosive onto the comet. The goal of the mission is to learn what the crusts of comets are made of and what lies beneath them.

Meanwhile, enjoy the sky show.

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