NASA's robotic Dawn spacecraft drifted into orbit around Vesta on Saturday, starting a yearlong science campaign to map one of the solar system's largest unexplored worlds sandwiched in the main asteroid belt between Mars and Jupiter.

Dawn captured this view of Vesta on July 9 at a range of 26,000 miles. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
 
Launched from Earth in September 2007, Dawn spiraled away from the sun with the help of a cutting edge ion propulsion system and a gravity boost from a flyby of Mars.

"Today, we celebrate an incredible exploration milestone as a spacecraft enters orbit around an object in the main asteroid belt for the first time," said NASA Administrator Charlie Bolden. "Dawn's study of the asteroid Vesta marks a major scientific accomplishment and also points the way to the future destinations where people will travel in the coming years. President Obama has directed NASA to send astronauts to an asteroid by 2025, and Dawn is gathering crucial data that will inform that mission."

After traveling 1.7 billion miels through space, Dawn approached Vesta this spring before being captured by the asteroid's weak gravity Saturday. Engineers don't know exactly when Dawn entered orbit yet because the spacecraft didn't have to conduct a major maneuver like many other interplanetary missions.

Instead, Dawn consistently pulsed its ion engines until Vesta's gravity naturally snared the probe into a slow-moving distant orbit. The ion engines produce little thrust, but they efficiently consume xenon propellant over thousands of hours to accelerate the spacecraft to blazing speeds.

Less than 1,000 pounds of xenon gas aboard Dawn will generate 24,000 mph of velocity change over the 8-year mission, according to Bob Mays, Dawn's project manager at the Jet Propulsion Laboratory in Pasadena, Calif.

The ion system will continue to fire until early August, when the spacecraft arrives at a post nearly 1,700 miles from Vesta.

Dawn will begin its science mission there in August with mapping and imagery collections before moving closer to Vesta for more detailed observations, according to Carol Raymond, Dawn's deputy principal investigator at JPL.

The closest Dawn will get to Vesta is about 120 miles, where its cameras will get the best views of the asteroid's rocky surface, resolve Vesta's chemical make-up, and the spacecraft will probe the gravity field.

"One of the reasons we're going to Vesta is not only because it's so big, but it's also one of the earliest bodies to form in the solar system, so the surface of Vesta will hold a record of the earliest history of the solar system," said Christopher Russell, Dawn's chief scientist from UCLA.

Russell describes Vesta as an analogue of the planetary building blocks from the early solar system. As small bodies collected together to form planets, Jupiter's immense gravity disrupted the accretion process near Vesta, leaving behind thousands of leftover rocks and minor planets.

Artist's concept of the Dawn spacecraft. Credit: NASA/JPL-Caltech
 
"We believe these were examples of the building blocks," Russell said. "So we're going back and doing some investigation into our roots, the roots of the solar system."

Vesta measures 359 miles long by 285 miles wide. Discovered in 1807, it is the second-most massive object in the main asteroid belt. A day on Vesta lasts 5.3 hours and its surface likely consists of ancient lava flows from the asteroid's formation 4.5 billion years ago.

Until Dawn's arrival, the best images of Vesta came from the Hubble Space Telescope. Despite the observatory's sharp instruments, the imagery only showed faraway Vesta as a blurry disk with slight color variations.

"Until now, it's only been a fuzzy blob," said Jim Adams, the deputy director of NASA's planetary science division. "Over the next year, the Dawn team will paint a face on that fuzzy blob. The pictures will just get better and better, and we'll begin to understand this awesome new world."

Scientists know Vesta has a large crater up to 285 miles across and 8 miles deep near its south pole. Whatever crashed into Vesta to form the crater ejected one-half million cubic miles of rock into space, according to NASA.

Researchers believe about 5 percent of all the meteorites that fall to Earth originate from the cataclysmic collision with Vesta.

"We have more Vesta samples than we have of the moon and Mars," Russell said.

According to Raymond, Dawn's instruments will peer into the south pole crater to get a glimpse of Vesta's internal structure.

Dawn's framing camera was built in Germany and its visible and infrared spectrometer instrument comes from Italy. A gamma ray and neutron detector was developed by the Los Alamos National Laboratory.

The sensors will investigate the composition of Vesta's surface, study the link between Vesta and meteorites that have fallen to Earth, and create a geologic map of the asteroid.

"We're really going to get to know the surface of Vesta and decipher its geologic history," Raymond said.

Scientists will also monitor tiny changes in the probe's radio signal to Earth to measure Vesta's gravity field, yielding insights into the body's interior layers and potentially unraveling whether Vesta has a metallic core.

Once its mission at Vesta is complete, Dawn's ion engines will ignite in July 2012 to propel the spacecraft toward its second destination.

After another two-and-half years journeying through the solar system, Dawn will enter orbit around the dwarf planet Ceres in February 2015. The visit to Ceres will make the spacecraft the first to ever orbit two objects in the solar system.