Dawn Approaches Asteroid Vesta   
       
   April 7, 2011:  After 3 « years of thrusting silently through the void, NASA's   
   Dawn spacecraft is on the threshold of a new world. It's deep in the asteroid   
   belt, less than 4 months from giant asteroid Vesta.   
       
   "We're closing in," says Marc Rayman, Dawn's chief engineer and mission   
   manager. "And I'm getting more excited every day!"   
       
   Dawn will enter orbit around Vesta in July 2011, becoming the first spacecraft   
   ever to orbit a body in the asteroid belt. After conducting a detailed study   
   of the uncharted alien world for a year, the spacecraft will pull off an even   
   more impressive first. It will leave Vesta, fly to dwarf planet Ceres, and   
   enter orbit there.   
   [...]   
   This video shows the scientists' best guess to date of what the surface of the   
   protoplanet Vesta might look like. "We can't wait to see the real thing," says   
   Marc Rayman.   
       
   "This is unprecedented," says Rayman. "No spacecraft has ever orbited two   
   target bodies, much less worlds in the asteroid belt. A few probes have passed   
   through this vast region of space, but not one could stop and develop an   
   intimate portrait of its residents."   
       
   A conventional spacecraft gets a boost from a big rocket, then coasts to its   
   target. Carrying enough fuel for making significant changes in speed or   
   direction along the way would make it too heavy to launch.   
       
   Dawn is far more fuel efficient. Spanning 65 feet, its solar arrays collect   
   power from the sun to ionize atoms of xenon gas. These ions are expelled   
   silently out the back of the spacecraft by a strong electric field, producing   
   a gentle thrust. The weightless, frictionless conditions of space flight allow   
   this gossamer force effect to build up, so the spacecraft continuously gains   
   speed.   
       
   "This spacecraft ultimately achieves fantastically high velocity while   
   consuming very little propellant -- using only a kilogram of xenon every 4   
   days, though its engines are almost constantly active."   
   [...]   
   Celebrate the beginning of Dawn's year-long exploration of new worlds with a   
   Vesta Fiesta! [more] With this system Dawn has been quietly, gradually   
   reshaping its orbit around the sun, slowly spiraling out to its target,   
   getting closer and closer as it loops around.   
       
   "By the time the spacecraft is in the vicinity of Vesta, its orbit will be   
   very much like the asteroid's," explains Rayman. "So upon arrival, Dawn can   
   slip into orbit as gently as it's been moving for 3 « years."*   
       
   A conventional spacecraft screeches into orbit in a single dramatic, nail   
   biting instant. The mission team is usually gathered in the mission control   
   room with their eyes riveted on the telemetry to see that the final critical   
   maneuver goes smoothly.   
       
   "With Dawn, there is no one big maneuver, no fiery burn, no single critical   
   moment. Dawn's entry into orbit will be no different from what the spacecraft   
   does almost all the time, what it's doing as you read this article. In fact,   
   when Dawn sidles into orbit, I might be asleep. Or if it's Friday night I'll   
   be dancing, or if it's Saturday I might be out taking pictures of dragonflies."   
       
   But you can bet he'll be in mission control when the pictures start coming in.   
       
   "It will be incredibly exciting to watch Dawn close in on Vesta. We'll witness   
   the uninteresting smudge in the first distant images grow into a full-sized   
   world as we loop closer and closer, ending up just 110 miles above the   
   surface. That's closer than the ISS is to Earth! We'll be right there, and if   
   there are no tall trees we'll be safe."   
       
   After exploring Vesta for a year, Dawn will take leave of the rocky world as   
   softly as it arrived there, climbing out along a spiral, gradually getting   
   farther and farther away, the loops getting longer and longer, until the   
   asteroid's gravity gently releases the spacecraft. Dawn will again be orbiting   
   the sun on its own, just as it is now. It will complete about two thirds of a   
   lap before arriving at Ceres.   
       
   There it will once again slide gently into orbit around a new world, guided by   
   ion thrusters as silent as space itself.   
       
   "Even if we imagined a sound, it would be the faintest of whispers, the   
   softest of sighs. Yet it tells us the secret of making an interplanetary   
   spaceship that can travel to and explore distant, alien worlds, carrying with   
   it the dreams of those on Earth who long to know the cosmos."   
       
   Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Dawn -- mission home page at JPL   
       
   Is Vesta Really an Asteroid? -- Science@NASA   
       
   How Capt. Kirk Changed the World -- Science@NASA   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.59   
    * Origin: NCS BBS (1:3828/7)