Saturn's Transit of Venus on Dec. 21, 2012   
       
   Dec. 20, 2012: Last June, astronomers urged sky watchers to observe the   
   transit of Venus.  It was a once in a lifetime opportunity, they said. The   
   black disk of the second planet wouldn't crawl across the face of the sun   
   again for more than 100 years.   
       
   http://spaceweather.com/gallery/indiv_upload.php?upload_id=67524   
       
   A transit of Venus seen from Earth on June 6, 2012. Photo credit: Bum-Suk Yeom   
   of Daejeon, South Korea In fact, it's happening again this week--not on Earth,   
   but Saturn.   
       
   "On Friday, Dec. 21st, there will be a transit of Venus visible from Saturn,   
   and we will be watching it using  the Cassini spacecraft," says Phil   
   Nicholson, a Cassini science team member from Cornell University. "This will   
   be the first time a transit of Venus has been observed from deep space."   
       
   Because Saturn is 10 times farther from the sun than Earth, this transit of   
   Venus won't be so easy to see.   The silhouette of the second planet will be   
   just a tiny black speck on the shrunken disk of a sun 10 times farther from   
   Saturn than Earth.  Cassini won't be beaming back any "beauty shots."   
   Nevertheless, the spacecraft will be conducting potentially ground-breaking   
   science.   
       
   "As Venus crosses the face of the sun, we will see if we can detect chemical   
   compounds in the planet's atmosphere by looking at the spectrum of sunlight   
   filtered by Venus," explains Nicholson.   
       
   This is, essentially, an experiment in exoplanet studies.  NASA's Kepler   
   spacecraft routinely discovers new planets around distant stars by looking for   
   the minuscule reduction in starlight that occurs during a planetary transit.    
   Watching Venus transit the sun from the faraway orbit of Saturn is a good   
   analog.   
       
   "We already know what Venus's atmosphere is made of," says Nicholson.  "But   
   this will give us a chance to see if we can pull this information out of a   
   faint, distant planetary transit."   
       
   The research team will be using Cassini's VIMS instrument.  VIMS is an   
   infrared spectrometer designed to tease out the chemical composition of Saturn   
   and its moons.  It isn't designed for planetary transits, but with a little   
   ingenuity Nicholson and colleagues have figured out how to gather useful data.   
       
   http://www.nasa.gov/mission_pages/cassini/multimedia/venustransit20121220.htm   
       
   The transit of Venus as seen from Saturn on Dec. 21, 2012. [more information]   
       
   "VIMS has a heavily-filtered 'solar port' 20 degrees off the main axis of the   
   spectrometer.  We use it to occasionally observe the sun for calibration   
   purposes--or to watch the sun set in the atmosphere of Saturn's moon's Titan,"   
   says Nicholson.  "On Dec. 21st we'll be using the solar port to monitor the   
   transit of Venus."   
       
   The images won't be very impressive.  Only a few pixels will fit across the   
   entire solar disk.  But the researchers aren't looking for images. "We want   
   spectra," says Nicholson.  "Carbon dioxide, the main constituent of Venus's   
   atmosphere, has several absorption bands squarely inside our 1 to 5 micron   
   observing window."   
       
   VIMS will gather data for the entire 9 hours of the transit--as well as many   
   hours before and after for comparison.  "Even with so much observing time, we   
   still might not detect any chemical signatures," cautions Nicholson. "The   
   signals are going to be faint--only a few parts in a million--so this is an   
   extremely difficult observation."   
       
   Nevertheless, Nicholson is looking forward to Friday.  "While most people have   
   to wait a hundred years for the next transit of Venus, we get to experience   
   one right away.  And if we make any discoveries at the same time... so much   
   the better."   
       
   Stay tuned for updates from Science@NASA.   
       
       
   Author: Dr. Tony Phillips |Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More Information   
   The Cassini-Huygens mission is a cooperative project of NASA, the European   
   Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a   
   division of the California Institute of Technology, Pasadena, manages the   
   mission for NASA's Science Mission Directorate, Washington, D.C. The visual   
   and infrared mapping spectrometer team is based at the University of Arizona,   
   Tucson.   
       
   For more information about Cassini, visit: http://www.nasa.gov/m   
   ssion_pages/cassini/main/index.html .   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.82   
    * Origin: NCS BBS (1:3828/7)