[meteorite-list] Asteroid 3200 Phaethon Sprouts A Tail

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 28 Nov 2013 20:58:51 -0800 (PST)
Message-ID: <201311290458.rAT4wpKj003760_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/science-news/science-at-nasa/2013/27nov_rockcomet/

Rock Comet Sprouts a Tail
NASA Science News
November 27, 2013

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.

Indeed, in further STEREO observations from 2009 and 2012, Jewitt along
with colleagues Jing Li of UCLA and Jessica Agarwal of the Max Planck
Institute have spotted a small tail sticking out behind the "rock."

"The tail gives incontrovertible evidence that Phaethon ejects dust,"
says Jewitt.

Jewitt's team believes that the dust is launched by thermal fracturing
of the asteroid's crust. A related process called "desiccation fracturing"--like
mud cracks in a dry lake bed--may play a role too.

Seeing 3200 Phaethon sprout a tail, even a small one, gives researchers
confidence that Phaethon is indeed the source of the Geminids--but a mystery
remains: How can such a stubby protuberance produce such a grand meteor
shower?

Adding up all of the light STEREO saw in Phaethon's tail, Jewitt and colleagues
estimate a combined mass of some 30 thousand kilograms. That might sound
like a lot of meteoroids but, in fact, it is orders of magnitude too small
to sustain the massive Geminid debris stream.

Perhaps Phaethon experienced a "big event" in the recent past. "The analogy
I think of is a log in a campfire," says Jewitt. "The log burns, makes
a few embers, but occasionally will spit out a shower of sparks."

Continued monitoring by NASA's STEREO probes might one day catch the rock
comet spitting out a shower of dust and debris, solving the mystery once
and for all.

Until then, it's a puzzle to savor under the stars. This year's Geminid
meteor shower peaks on the nights of Dec. 13-14 with dozens of "rock comet
meteors" every hour. Bundle up and enjoy the show.

Credits:
Author: Dr. Tony Phillips
Production editor: Dr. Tony Phillips
Credit: Science at NASA
Received on Thu 28 Nov 2013 11:58:51 PM PST