Comet ISON to fly by Mars   
       
   August 23, 2013:  Around the world, astronomers are buzzing with anticipation   
   over the approach of Comet ISON.  On Thanksgiving Day 2013, the icy visitor   
   from the outer solar system will skim the sun's outer atmosphere and, if it   
   survives, could emerge as one of the brightest comets in years.   
       
   First, though, it has to fly by Mars.   
       
   "Comet ISON is paying a visit to the Red Planet," says astronomer Carey Lisse   
   of the Johns Hopkins University Applied Physics Lab. "On Oct 1st, the comet   
   will pass within 0.07 AU from Mars, about six times closer than it will ever   
   come to Earth."   
       
   http://www.youtube.com/watch?v=4uJFvkJzBVA   
       
   A new ScienceCast video anticipates Comet ISON's close flyby of Mars on Oct.   
   1, 2013. Play it   
       
   Mars rovers and satellites will get a close-up view.  It's too early to say   
   whether Curiosity will be able to see the comet from the surface of Mars-that   
   depends on how much ISON brightens between now and then. Lisse says the best   
   bet is NASA's Mars Reconnaissance Orbiter.  The MRO satellite is equipped with   
   a powerful half-meter telescope named HiRISE that is more than capable of   
   detecting the comet's atmosphere and tail. Observations are planned on four   
   dates: August 20th, Sept 29th, and Oct 1st and 2nd.   
       
   HiRISE wasn't sent to Mars to do astronomy, notes the telescope's principal   
   investigator Alfred McEwen of the University of Arizona. "The camera is   
   designed for rapid imaging of Mars. Our maximum exposure time is limited   
   compared to detectors on other space telescopes. This is a major limitation   
   for imaging comets. Nevertheless, I think we will detect Comet ISON."   
       
   The Mars flyby comes at a key time in Comet ISON's journey. It will have just   
   crossed the "frost line," a place just outside the orbit of Mars where solar   
   heating is enough to start vaporizing frozen water.   
       
   "The volatiles in a comet are 80% to 90% water ice," notes Lisse. "Right now   
   in August almost all the water is still frozen, and the outgassing we see in   
   ISON is driven by carbon dioxide and other lesser constituents. Probably only   
   isolated patches of the comet's nucleus are active."   
       
   But when ISON crosses the frost line, "the whole comet could erupt in geysers   
   of gas," says Lisse. "Mars orbiters will have a ringside seat."   
       
   http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2012%20S1;orb=1   
       
   Click to view an interactive orbit of Comet ISON The amount of outgassing at   
   Mars will give researchers clues to the size of ISON's nucleus, which is   
   hidden from view deep within the comet's dusty atmosphere.   
       
   "If ISON's nucleus is much bigger than 0.5 km, it will probably survive its   
   Thanksgiving Day brush with the sun," says Lisse.  "It could turn into one of   
   the most spectacular comets in many years."   
       
   McEwen sees this as a tune-up for another comet encounter next year. "The   
   science value of observing Comet ISON is hard to predict. We've never tried   
   such a thing before. However, this is good practice for Comet Siding Spring,   
   which will pass much closer to Mars in 2014."   
       
   For now all eyes are on Comet ISON.  An unprecedented number of NASA   
   spacecraft - 16 - will be observing the comet.  Astronauts on board the   
   International Space Station will be watching, too.   
       
   Meanwhile back on Earth, Lisse is working with NASA to organize a worldwide   
   observing campaign for Comet ISON.  "Our goal is to have every telescope on   
   Earth pointed at the comet when it emerges from the sun," says Lisse.  "The   
   Mars flyby will give us a sneak preview, providing data we need to predict   
   what we might see."   
       
   Credits:   
       
    Author: Dr. Tony Phillips |Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   Related Links:   
       
   Comet of the Century?  -- ScienceCast video   
       
   Comet ISON Meteor Shower -- ScienceCast video   
       
   Collision Course?  A Comet Heads for Mars -- ScienceCast video   
       
   NASA Comet ISON Observing Campaign -- get involved!   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.94   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)