On March 29, 1807, German astronomer Heinrich Wilhelm Olbers spotted Vesta as a pinprick of light in the sky. Two hundred and four years later, as NASA's Dawn spacecraft prepares to begin orbiting this intriguing world, scientists now know how special this world is, even if there has been some debate on how to classify it.
Many astronomers call Vesta an asteroid because it lies in the main asteroid belt between Mars and Jupiter. But Vesta is not a typical member of that orbiting rubble patch. The vast majority of objects in the main belt are lightweights, 100 kilometers wide or smaller, compared with Vesta, which is a 530 kilometer-wide monster.
NASA's Dawn spacecraft has reached its official approach phase to Vesta and will begin using cameras for the first time to aid navigation for an expected July 16 orbital encounter. The large asteroid is known as a protoplanet -- a celestial body that almost formed into a planet.
At the start of this three-month final approach to this massive body in the asteroid belt, Dawn is 1.21 million kilometers (752,000 miles) from Vesta, or about three times the distance between Earth and the moon. During the approach phase, the spacecraft's main activity will be thrusting with a special, hyper-efficient ion engine that uses electricity to ionize and accelerate xenon. The 12-inch-wide ion thrusters provide less thrust than conventional engines, but will provide propulsion for years during the mission and provide far greater capability to change velocity.
http://www.dailygalaxy.com/my_weblog/2011/05/newsflash-ion-powered-dawn-spacecraft-in-final-approach-to-vesta-an-asteroid-the-size-of-arizona-.html#more
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