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)