Rock Comet Sprouts a Tail
 
Nov. 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.
 
https://www.youtube.com/watch?v=4I80ZXrXucI
 
A new ScienceCast video explores the mysteries of "rock comet" 3200 Phaethon. 
Play it
 
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.
 
Auroras Underfoot (signup)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."
 
http://tinyurl.com/kqabv4e
 
This STEREO image of 3200 Phaethon reveals a stubby but distinct tail.
More"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@NASA
 
More information:
 
Phaethon Confirmed as Rock Comet by STEREO Vision -- press release
 
 
Regards,
 
Roger

--- D'Bridge 3.96
 * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)