On July 16, NASA's Dawn spacecraft will begin a prolonged encounter with the asteroid Vesta, making the mission the first to enter orbit around a main-belt asteroid.

The main asteroid belt lies between the orbits of Mars and Jupiter. Dawn will study Vesta for one year, and observations will help scientists understand the earliest chapter of our solar system's history.

As the spacecraft approaches Vesta, surface details are coming into focus, as seen in the most recent image below, taken from a distance of about 41.000 km.

Engineers expect the spacecraft to be captured into orbit at ~7AM UT Saturday morning, July 16. They expect to hear from the spacecraft and confirm that it performed as planned during a scheduled communications pass that starts at approximately 5:30 AM UT Sunday July 17. When Vesta captures Dawn into its orbit, engineers estimate there will be ~16.000 km between them. At that point, the spacecraft and asteroid will be ~188 million km from Earth.

"It has taken nearly four years to get to this point," said Robert Mase, Dawn project manager at NASA's JPL "Our latest tests and check-outs show that Dawn is right on target and performing normally."

Engineers have been subtly shaping Dawn's trajectory for years to match Vesta's orbit around the sun. Unlike other missions, where dramatic propulsive burns put spacecraft into orbit around a planet, Dawn will ease up next to Vesta. Then the asteroid's gravity will capture the spacecraft into orbit. However, until Dawn nears Vesta and makes accurate measurements, the asteroid's mass and gravity will only be estimates. So the Dawn team will need a few days to refine the exact moment of orbit capture.

Launched in September 2007, Dawn will depart for its second destination, the dwarf planet Ceres, in July 2012. The spacecraft will be the first to orbit two solar system destinations beyond Earth.

Source:: http://www.nasa.gov/dawn
Image is available at: http://www.nasa.gov/mission_pages/dawn/multimedia/dawn-image-070911.html .