Hello All!   
      
   Strange but True: Curiosity's Sky Crane    
      
   July 30, 2012:  On August 5th at 10:31 p.m. Pacific Time, NASA will gently   
   deposit their new, 2000-pound Curiosity rover on the surface of Mars,   
   wheels-first and ready to roll. Quite a feat - because it will come screaming   
   through the Martian atmosphere at 13,000 mph.    
      
   Curiosity, aka the Mars Science Laboratory, will be the largest mission ever   
   to land on another planet. It's big because it has a big mystery to solve: was   
   Mars ever or is it still capable of harboring life?    
      
   During its grand entrance, the lander must slow to 1 1/2 mph to touch down   
   safely. That kind of braking action for a one-ton payload demands the   
   nail-bitingly precise unfolding of an intricately choreographed sequence of   
   events. Key players: a red-hot heat shield, a huge parachute, 76 explosive   
   bolts -- and a Sky Crane.    
      
   http://www.youtube.com/watch?v=QrxCA1leQyY   
      
   The now-famous "Seven Minutes of Terror" video dramatically documents   
   Curiosity's descent to the surface of Mars. Play it   
      
   "The whole ballgame transpires within 7 minutes, from atmospheric entry to   
   touch-down," says Jet Propulsion Laboratory's Steve Sell, Deputy Operations   
   Lead for Entry, Descent, and Landing. "The onboard computer calls the shots.   
   And if any one maneuver fails, it's game over."    
      
   Here's the game plan.    
      
   "Atmospheric friction slows the capsule containing Sky Crane -- an   
   eight-rocket jetpack attached to the rover -- from 13000 to 1000 mph. [Mars'   
   atmosphere is too thin to slow it more.] The friction burnishes the capsule's   
   heat shield to a glowing 3800 degrees Fahrenheit (2100 degrees Celsius). Then   
   a 60-foot diameter parachute deploys and inflates above the capsule on   
   160-foot lines. What's left of the heat shield jettisons, giving Curiosity its   
   first look at its new home below."    
      
   This is the largest, strongest parachute ever flown to another world. It has   
   to be a super-chute to handle the 65000 pounds of force produced when the   
   rover snaps to attention below it.    
      
   http://www.nasa.gov/mission_pages/msl/multimedia/gallery/pia14839.html   
      
   The Sky Crane in action. Larger image"After the payload slows to about 200   
   mph, explosive bolts free the chute and Sky Crane free-falls for a second.   
   Then its retrorockets fire."    
      
   The rockets slow the descent to 1 « mph and power a sideways parry to avoid   
   the faster falling chute. As Sky Crane descends to 60 feet above Mars'   
   surface, the rover inches down from underneath it on three nylon ropes like a   
   spider spinning strands of its web. With Curiosity dangling 20 feet below, Sky   
   Crane continues its downward progress until the rover is resting on the   
   surface. Explosive bolts cut Curiosity's last physical attachments to the   
   outside world, and Sky Crane flies away to death-plunge into the red sands,   
   its incredible job done.    
      
   It might sound frighteningly complicated, "but what appears to be a complex   
   system actually simplifies the landing greatly," explains Sell.    
      
   Previous missions such as Vikings I and II and the Mars Phoenix Lander used   
   retrorockets to lower spacecraft all the way to the surface atop a legged   
   lander. Others have used airbags. Neither method is feasible for Curiosity.    
      
   "With a payload this size, the rockets could kick up enough dust to compromise   
   the rover and its instruments," explains Sell. "And the rockets could excavate   
   craters Curiosity would have to avoid as it drives away. Add to that the risk   
   of a big, heavy vehicle driving down off the lander via an exit ramp to reach   
   the surface."    
      
   Pathfinder, Spirit, and Opportunity used airbags to eliminate these concerns.   
   But Curiosity is too large for airbags.    
      
   "Bags big enough to soften its landing would be too heavy or too costly to   
   launch. Besides, you'd have to drop the payload so slowly for the bags to   
   survive the load, you may as well place the rover right on its wheels."    
      
      
   Three generations of Mars rovers. Curiosity (pictured right) is more massive   
   than its predecessors, which is why NASA had to develop an innovative landing   
   system.    
   Sky Crane, says Sell, makes sense for Curiosity. But it still keeps him up at   
   night.    
      
   "I leave myself voicemails in the middle of the night about things to check in   
   the morning. We've run thousands of tests and simulations, thinking of ways to   
   'break' the system so we can build in comfortable performance margins. We're   
   still testing. There's always one more test we can run. We're always afraid we   
   missed something."    
      
   In the control room at JPL the night of August 5th, it will be too late. It   
   takes 14 minutes for signals to travel from Mars to Earth. When the team   
   receives the signal 'I am entering the atmosphere,' Curiosity will be alive or   
   dead on the surface.    
      
   Says Sell: "I'm already holding my breath."    
      
   Author: Dauna Coulter| Editor: Dr. Tony Phillips | Credit: Science@NASA   
      
      
   Regards,   
      
   Roger    
   --- timEd/386 1.10.y2k+   
    * Origin: NCS BBS - Houma, LA - (1:3828/7)