Sen— After a year in orbit around the asteroid Vesta, NASA's Dawn spacecraft is about to say goodbye and head off to its next asteroid target.
The space probe is programmed to depart on September 5 to begin a two and a half year journey to the largest of the asteroids, Ceres, which is now classified as a dwarf planet.
Both giant rocky bodies are interplanetary records of the earliest days of the Solar System. Space scientists hope to learn more about the origin of the planets by studying them.
Dawn, which was launched in 2007 on its 5 billion km journey, will reach Ceres in early 2015. Ceres, the largest asteroid with a diameter of 950 km, was the first asteroid to be discovered in 1800 by Sicilian monk Giuseppe Piazzi.
It was promoted to the class of dwarf planet in 2006 alongside newly relegated Pluto which lost its former full planet status.
Planetary scientists are delighted at the close-up views that Dawn has returned from Vesta. They learned that the asteroid had completely melted in the past, forming a layered body with an iron core. The spacecraft also revealed that Vesta has suffered major collisions including 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.
Christopher Russell, Dawn's principal investigator, from the University of California, said: "We went to Vesta to fill in the blanks of our knowledge about the early history of our Solar System. 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 now can say with certainty that Vesta resembles a small planet more closely than a typical asteroid."
Dawn will spiral away from Vesta as gently as it arrived, using a special, hyper-efficient system called ion propulsion. This "fuel" system uses electricity to ionize xenon to generate thrust. The 12-inch-wide ion thrusters provide less power than conventional engines but can maintain thrust for months at a time.
A simulated flyover of Vesta produced from Dawn images. Credit: Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Marc Rayman, Dawn's chief engineer and mission director, at NASA's Jet Propulsion Laboratory (JPL) in California, said: "Thrust is engaged and we now are climbing away from Vesta atop a blue-green pillar of xenon ions.
"We are feeling somewhat wistful about concluding a fantastically productive and exciting exploration of Vesta, but now we have our sights set on dwarf planet Ceres."
There has been renewed interest in the asteroids. Japan's space agency JAXA is preparing to send a new mission, Hayabusa 2, to take samples of one, and NASA has been set the target of sending astronauts to a space rock by 2025.
A new company, Planetary Resources, has also recently been set up with the aim of mining asteroids for materials including rare metals and water that could provide supplies of drink and be converted to fuel for space missions.