Comet ISON: What's Next?   
       
   Nov. 14, 2013:  Comet ISON is now inside the orbit of Earth as it plunges   
   headlong toward the sun for a fiery close encounter on Nov. 28th.  Although   
   the comet is not yet as bright as many forecasters predicted, the comet is   
   putting on a good show for observatories around the solar system.  NASA   
   spacecraft and amateur astronomers alike are snapping crisp pictures of the   
   comet's gossamer green atmosphere and filamentary double-tail.   
       
   http://spaceweathergallery.com/indiv_upload.php?upload_id=89419   
       
   Comet ISON photographed on Nov. 10th by amateur astronomer Michael J„ger of   
   Jauerling Austria. More   
       
   Because ISON has never passed through the inner solar system before (it is a   
   first-time visitor from the distant Oort cloud), experts aren't sure what will   
   happen next. Can the comet survive its Thanksgiving Day brush with the sun?    
   Will it emerge as a bright naked-eye object?   
       
   Lowell Observatory astronomer Matthew Knight, a member of NASA's Comet ISON   
   Observing Campaign, lays out some of the possibilities.   
       
   "I've grouped the possible outcomes into three scenarios, discussed in   
   chronological order," says Knight. "It is important to note that no matter   
   what happens, now that ISON has made it inside Earth's orbit, any or all of   
   these scenarios are scientifically exciting.  We're going to learn a lot no   
   matter what."   
       
   http://hubblesite.org/newscenter/archive/releases/2000/27   
       
   The breakup of Comet LINEAR (D/1999 S4) as viewed by Hubble Space Telescope in   
   2000. More#1 Spontaneous Disintegration before Thanksgiving   
       
   The first scenario, which could happen at any time, is that ISON spontaneously   
   disintegrates. A small fraction (less than 1%) of comets have disintegrated   
   for no apparent reason.  Recent examples include Comet LINEAR (C/1999 S4) in   
   2000 and Comet Elenin (C/2010 X1) in 2011. ISON is now reaching the region of   
   space, within ~0.8 AU of the Sun where comets like these have disintegrated.   
       
   Comet ISON is being observed by a tremendous variety of telescopes on Earth   
   and beyond. If ISON does disintegrate, it would be the best-observed case of   
   cometary disruption in history and would likely contribute vast new   
   information about how comets die.   
       
   #2 Death by Sunburn around Thanksgiving Day   
       
   Assuming ISON survives the next few weeks intact, it faces an even more   
   daunting challenge: making it around the Sun. At closest approach to the sun,   
   the comet's equilibrium temperature will approach 5000 degrees Fahrenheit, hot   
   enough to cause much of the dust and rock on ISON's surface to vaporize.   
       
   While it may seem incredible that anything can survive this inferno, the rate   
   at which ISON will likely lose mass is relatively small compared to the actual   
   size of the comet's nucleus. ISON needs to be 200 m wide to survive; current   
   estimates are in the range 500 m to 2 km. It helps that the comet is moving   
   very fast so it will not remain long at such extreme temperatures.   
       
   Unfortunately for ISON, it faces a double whammy from its proximity to the   
   Sun: even if it survives the rapid vaporization of its exterior, it gets so   
   close to the sun that the suns gravity might actually pull it apart.   
       
   Destroyed comets can still be spectacular, though.   
       
   Sungrazing Comet Lovejoy, for instance, passed within 100,000 miles of the   
   sun's surface in December 2011. It disintegrated, forming a long tail of dust   
   that wowed observers on Earth.   
       
   http://tinyurl.com/n3a4axz   
       
   Sungrazing Comet Lovejoy (C/2011 W3) seen over Australia in Dec. 2011. Image   
   credit: Alex Cherney, TWAN.   
       
   #3 Survival   
       
   The final case is the most straightforward: ISON survives its brush with the   
   sun and emerges with enough nuclear material to continue as an active comet.   
   If ISON survives in tact, it would likely lose enough dust near the Sun to   
   produce a nice tail. In a realistic best-case scenario, the tail would stretch   
   for tens of degrees and light up the early morning sky like Comet McNaught   
   (C/2006 P1) did in 2007.   
       
   The best of all possible worlds would be if ISON broke up just a bit, say,   
   into a few large pieces.  This would throw out enough extra material to make   
   the comet really bright from the ground, while giving astronomers pieces of a   
   comet to study for months to come.   
       
   "I'm clearly rooting for #3," says Knight.   
       
   "Regardless of what happens, we're going to be thrilled," he predicts.   
   "Astronomers are getting the chance to study a unique comet traveling straight   
   from 4.5 billion years of deep freeze into a near miss with the solar furnace   
   using the largest array of telescopes in history."   
       
   "Hang on," he says, "because this ride is just getting started."   
       
   For updates and more information about Comet ISON as it approaches the sun,   
   visit http://isoncampaign.org.   
       
   Credits:   
       
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   Editor's note: The text of this story is closely based on a blog post by   
   Matthew Knight at the CIOC web site.  It is recommended reading for more   
   information about the three scenarios:     
   http://www.isoncampaign.org/mmk/what-might-happen   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.96   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)