Dawn Probe Set for July 16 Asteroid Encounter

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SEATTLE — NASA’s Dawn probe should enter into orbit around the asteroid Vesta July 16, becoming the first spacecraft to visit the  530-kilometer-wide space rock — the second-largest object in the main asteroid belt between Mars and Jupiter.

Dawn is expected to spend a year studying Vesta from above, marking the first time a spacecraft has ever made an extended visit to a large asteroid.

Scientists hope Dawn’s mission will help them learn about the early days of the solar system and the processes that formed and shaped rocky planets like Earth and Mars.

“Bodies like Vesta are building blocks,” Dawn principal investigator Christopher Russell of the University of California, Los Angeles, told reporters in a recent briefing. “So we’re going back and doing some sort of investigation into our roots, the roots of the solar system.”

Dawn is now in the home stretch of a nearly four-year cosmic chase. The probe launched in September 2007 and has logged about 2.7 billion kilometers during its travels. As of July 1, Dawn had closed to within 86,000 kilometers of Vesta, researchers said.

Dawn will be just 16,000 kilometers from Vesta when the space rock’s gravity captures the probe July 16. At that point, Dawn and Vesta will both be about 188 million kilometers from Earth.

The capture is not expected to be a dramatic, nail-biting affair punctuated by last-minute thruster burns. Dawn has been using its low-thrust ion propulsion system to close in on Vesta slowly but surely, and mission planners expect it to slide nicely into orbit July 16.

The spacecraft will begin its science operations in early August, researchers said.

Vesta is so large that many scientists classify it as a protoplanet. The object was well on its way to becoming a full-fledged rocky planet long ago, scientists said, but circumstances intervened.

“The formation of Jupiter started stirring up that region of the asteroid belt and preventing materials from coming together any longer,” Russell said. So Vesta is a sort of time capsule, preserving some record of how the solar system came together 4.5 billion years ago.

“As we explore Vesta, we take a virtual journey back in time to the beginning of the solar system,” said Carol Raymond, Dawn deputy principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

During its year at Vesta, Dawn will map the giant asteroid’s cratered surface fully, study its composition and investigate its geological history. It will do this from several different orbits, ranging from 2,700 kilometers above Vesta to just 200 kilometers above, researchers said.

Dawn should wrap up its science work at Vesta in July 2012. But at that point, the probe’s mission will only be half-finished. It will then head off for another asteroid encounter, this time with the dwarf planet Ceres — the largest object in the asteroid belt.

Dawn should arrive at Ceres in early 2015. The spacecraft’s observations will allow scientists to compare the two giant bodies, which have been shaped by different forces, researchers said.