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Rock comet sprouts a tail

Posted November 28, 2013
Rock comet sprouts a tail
This STEREO image of 3200 Phaethon reveals a stubby but distinct tail.
Astronomers have long been puzzled by a certain meteor shower.

Every year in mid-December the sky fills with flashes of light shooting out of the constellation Gemini. The Geminids are fast, bright, and reliable. They never fail to show up and many observers count them as the finest meteors of the year.

But where do they come from? That is the puzzle.

Meteor showers are supposed to come from comets, yet there is no comet that matches the orbit of the Geminid debris stream. Instead, the orbit of the Geminids is occupied by a thing called “3200 Phaethon.” Discovered in 1983 by NASA’s IRAS satellite, Phaethon looks remarkably like a rocky asteroid. It swoops by the sun every 1.4 years, much like a comet would, but it never sprouts a dusty tail to replenish the Geminids.

That is, until now.

A group of astronomers led by Dave Jewitt of UCLA have been using NASA’s STEREO probes to take a closer look at 3200 Phaethon when it passes by the sun. The twin spacecraft were designed to monitor solar activity, so they get a good view of sungrazing comets and asteroids. In 2010 one of the STEREO probes recorded a doubling of Phaethon’s brightness as it approached the sun, as if sunlight were shining through a cloud of dust around the asteroid. The observers began to suspect 3200 Phaethon was something new:

“A rock comet”, says Jewitt. A rock comet is, essentially, an asteroid that comes very close to the sun—so close that solar heating scorches dusty debris right off its rocky surface. This could form a sort of gravelly tail.

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