NASA Gravity Probes Prepare to Hit the Moon   
       
   Dec. 13, 2012: A pair of NASA spacecraft that have been studying the Moon's   
   gravitational field are being prepared for a controlled descent into a   
   mountain near the Moon's north pole.  Impact is expected at about 2:28 p.m.   
   PST (5:28 p.m. EST) on Monday, Dec. 17.   
       
   "It is going to be difficult to say goodbye to our little robotic twins," says   
   MIT professor Maria Zuber, principal investigator of the Gravity Recovery and   
   Interior Laboratory (GRAIL) mission. "Planetary science has advanced in a   
   major way because of their contributions."   
       
   http://www.nasa.gov/multimedia/videogallery/index.html?media_id=156978841   
       
   This animation shows the final flight path for NASA's twin GRAIL probes, which   
   will hit the Moon on Dec. 17, 2012, around 2:28 p.m. PST. Play it   
       
   The two probes, named Ebb and Flow, are being sent purposely into the lunar   
   surface because their low orbit and low fuel levels preclude further   
   scientific operations.   
       
   Ebb and Flow's successful mission to the Moon has yielded the hi   
   hest-resolution gravity field map of any celestial body. The map will provide   
   a better understanding not only of the Moon, but also of how Earth and other   
   rocky planets in the solar system formed and evolved.   
       
   The spacecraft have been flying in formation around the Moon since Jan. 1,   
   2012. They were named by elementary school students in Bozeman, Mont., who won   
   a contest.   
       
   http://www.nasa.gov/mission_pages/grail/multimedia/pia16493.html   
       
   These 3-D comparisons depict the unnamed lunar mountain targeted by the NASA's   
   Gravity Recovery and Interior Laboratory (GRAIL) mission for controlled impact   
   of the Ebb and Flow spacecraft. Image credit: NASA/JPL-Caltech/MIT/GSFC The   
   first probe to reach the Moon, Ebb, also will be the first to go down, at   
   2:28:40 p.m. PST. Flow will follow Ebb about 20 seconds later. Both spacecraft   
   will hit the surface at 3,760 mph (1.7 kilometers per second). No imagery of   
   the impact is expected because the region will be in shadow at the time. The   
   impact site is located near a crater named Goldschmidt.   
       
   Ebb and Flow will conduct one final experiment before their mission ends. They   
   will fire their main engines until their propellant tanks are empty to   
   determine precisely the amount of fuel remaining in their tanks. This will   
   help NASA engineers validate fuel consumption computer models to improve   
   predictions of fuel needs for future missions.   
       
   "Our lunar twins may be in the twilight of their operational lives, but one   
   thing is for sure, they are going down swinging," said GRAIL project manager   
   David Lehman of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Even   
   during the last half of their last orbit, we are going to do an engineering   
   experiment that could help future missions operate more efficiently."   
       
   Because the exact amount of fuel remaining aboard each spacecraft is unknown,   
   mission navigators and engineers designed the depletion burn to allow the   
   probes to descend gradually for several hours and skim the surface of the moon   
   until the elevated terrain of the target mountain gets in their way.   
       
   The burn that will change the spacecrafts' orbit is scheduled to take place   
   Friday morning, Dec. 14.   
       
   "Such a unique end-of-mission scenario requires extensive and detailed mission   
   planning and navigation," said Lehman. "We've had our share of challenges   
   during this mission and always come through in flying colors, but nobody I   
   know around here has ever flown into a Moon mountain before. It'll be a first   
   for us, that's for sure."   
       
       
   Production editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   During their prime mission, from March through May 2012, Ebb and Flow   
   collected data while orbiting at an average altitude of 34 miles (55   
   kilometers). Their altitude was lowered to 14 miles (23 kilometers) for their   
   extended mission, which began Aug. 30 and sometimes placed them within a few   
   miles of the moon's tallest surface features.   
       
   JPL manages the GRAIL mission for NASA's Science Mission Directorate in   
   Washington. The mission is part of the Discovery Program managed at NASA's   
   Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems   
   in Denver built the spacecraft. JPL is a division of the California Institute   
   of Technology in Pasadena.   
       
   For more information about GRAIL, visit: http://www.nasa.gov/grail .   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.82   
    * Origin: NCS BBS (1:3828/7)