Today, NASA’s Dawn spacecraft left the orbit of Vesta and is headed toward Ceres. This marks the first time an unmanned probe has orbited one body and then left for another. Dawn is scheduled to for a two-and-a-half-year journey to the dwarf planet Ceres. Dawn began its 3-billion-mile (5-billion kilometer) journey to explore the two most massive objects in the main asteroid belt in 2007. Dawn arrived at Vesta in July 2011 and will reach Ceres in early 2015 about the same time that the New Horizons probe will go past the planet Pluto (yes, I said the P word, to heck to with the IAU). Dawn’s has other firsts including her/his/its ion propulsion engine that I have discussed previously on this blog. The ion propulsion system uses electricity to ionize xenon gas to generate thrust. The 12-inch-wide ion thrusters provide less power than conventional engines, but can maintain thrust for months at a time. “Thrust is engaged, and we are now climbing away from Vesta atop a blue-green pillar of xenon ions,” said Marc Rayman, Dawn’s chief engineer and mission director, at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.
Dawn revealed that Vesta had completely melted in the past, forming a layered body with an iron core. The spacecraft also revealed the scarring from titanic collisions Vesta suffered in its southern hemisphere, surviving not one but two colossal impacts in the last two billion years. Without Dawn, scientists would not have known about the dramatic troughs sculpted around Vesta, which are ripples from the two south polar impacts. “We went to Vesta to fill in the blanks of our knowledge about the early history of our solar system,” said Christopher Russell, Dawn’s principal investigator, based at the University of California Los Angeles (UCLA). “Dawn has filled in those pages, and more, revealing to us how special Vesta is as a survivor from the earliest days of the solar system. We can now say with certainty that Vesta resembles a small planet more closely than a typical asteroid.”