Two Comets to Fly By Mercury   
       
   Nov. 15, 2013:  What are the odds? On Nov. 18th and 19th not one but two   
   comets will fly by the planet Mercury.   
       
   "This is a unique coincidence," says Ron Vervack an astronomer at the Johns   
   Hopkins University Applied Physics Lab and a member of the science team for   
   NASA's MESSENGER spacecraft, "and a golden opportunity to study two comets   
   passing close to the sun."   
       
   On Nov. 18th Comet Encke will pass within 0.025 AU of Mercury, followed a day   
   later by Comet ISON at 0.24 AU (1 AU is the distance between the sun and   
   Earth, 150 million km).  The MESSENGER spacecraft, which is orbiting Mercury,   
   will turn its sensors toward the passing comets for a point-blank   
   investigation of both.   
       
   http://www.youtube.com/watch?v=v9Wzlu5quTU   
       
   A new Sciencecast video previews a rare double encounter between Mercury and   
   two comets. Play it   
       
   The double flyby is exciting, says Vervack, but "it makes things a little   
   crazy. We have to rush to complete our observations of Comet Encke, then do it   
   all over again for Comet ISON. Everything is happening at more or less the   
   same time."   
       
   MESSENGER was designed to study Mercury, not comets, "but it is a capable   
   spacecraft with a versatile instrument package," he adds. "We hope to get some   
   great data." Onboard spectrometers will analyze the chemical makeup of the two   
   comets while MESSENGER's cameras snap pictures of atmospheres, jets and tails.   
       
   Comet ISON is already a media favorite. Astronomers have been tracking it   
   since Sept. 2012 when it was discovered on a trajectory that would take it   
   perilously close to the sun.  On Nov. 28th of 2013, Thanksgiving Day in the   
   USA, Comet ISON will pass through the sun's atmosphere little more than a   
   million kilometers above the sun's fiery surface.  If the icy comet survives,   
   it could emerge as a beautiful naked-eye object for observers in the northern   
   hemisphere.  MESSENGER's glimpse of Comet ISON as it plunges inward could give   
   astronomers the data they need to predict the comet's fate.   
       
   Comet Encke is less well known, but no less interesting.  For one thing, it is   
   the source of the Taurid meteor shower, a slow display of midnight fireballs   
   that occurs every year in early- to mid-November. Comet Encke dips inside the   
   orbit of Mercury every 3.3 years, so it is regularly exposed to solar   
   activity.  In 2007, NASA's STEREO spacecraft watched as a solar storm ripped   
   off Encke's tail--which promptly grew back: movie.   
       
   "We'll be catching Comet Encke just days before its closest approach to the   
   sun (0.3 AU)," Vervack  says, "so we get to see it at its most active."   
       
   http://tinyurl.com/lo4zqfo   
       
   MESSENGER's first images of the approaching comets. Larger image, details   
       
   Ironically, the fact that MESSENGER is designed to study a rocky planet could   
   prove advantageous for the icy comets.  MESSENGER's x-ray spectrometer, in   
   particular, could detect signs of `comet dirt'.   
       
   "We hope to obtain the first definitive detections of x-ray emissions from   
   silicon, magnesium and aluminum," he explains. "If you think of a comet as a   
   dirty snowball, these are elements that make up the dirt.  Close to the sun is   
   where we expect the dirt to be vaporized."   
       
   In total, Vervack expects MESSENGER to gather 15 hours' worth of data on Comet   
   Encke and another 25 hours on Comet ISON.  With that kind of observing time,   
   discoveries are a distinct possibility.   
       
   Vervack says the first images will be beamed back and released to the public   
   within days of the flybys.  "There are no guarantees," he cautions, "but I   
   can't wait to see the pictures."   
       
   Credits:   
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   Comet ISON: What's Next?  -- Science@NASA   
       
   MESSENGER -- home page   
       
   The Sun Rips off a Comet's Tail -- Science@NASA   
       
   MESSENGER's First Images of Comets Encke and ISON   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.96   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)