NASA probe enters orbit around asteroid Vesta

HOUSTON (Reuters) - A robotic NASA science probe slipped into orbit around the potato-shaped asteroid Vesta on Saturday to begin a yearlong study of the second largest object in the asteroid belt.

NASA's Dawn spacecraft relayed a signal to NASA's Jet Propulsion Laboratory in Pasadena, California, to confirm that it had entered Vesta's orbit, about 117 million miles (188 million km) from Earth, the agency said early on Sunday.

Dawn is the first probe to orbit an object in the main asteroid belt between Mars and Jupiter, and is on track to be the first spacecraft to orbit two solar system destinations beyond Earth, NASA said.

"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 coming years," NASA Administrator Charles Bolden said in a statement.

Scientists could not immediately determine the exact time that Dawn entered Vestas' orbit on Saturday, NASA said.

Dawn was dispatched to Vesta in 2007, the first stop in a $466 million quest to learn more about how the solar system formed 4.5 billion years ago. It will study Vesta for a year and then depart for its second destination in July 2012 -- a dwarf planet called Ceres.

Vesta and Ceres are two of the largest surviving protoplanets -- rocky bodies that nearly had enough mass to become full-fledged planets -- in solar system. Both reside in the main asteroid belt, located between Mars and Jupiter.

"These are two of the last unexplored worlds in our inner solar system," said Dawn project manager Robert Mase.

With its iron core and possible lava flows, scientists believe Vesta is more similar to Earth or the moon than most of its other asteroid neighbors.

Ceres, the largest object in the asteroid belt, is relatively close to Vesta, but it formed under vastly different circumstances. The so-called dwarf planet more closely resembles the icy moons of Jupiter and Saturn. Ceres has water-bearing minerals and possibly a weak atmosphere.

The goal of the Dawn mission is to collect enough information about Vesta and Ceres to understand conditions and processes of the early solar system. The spacecraft has three scientific instruments to study surface features and determine chemical composition.

"We are exploring backward in time as far as we can," said lead scientist Christopher Russell, with the University of California at Los Angeles.

ASTEROIDS EXPLORED

Several spacecraft have flown by asteroids before, including NASA's Galileo probe, which encountered three asteroids on its way to Jupiter. Japan's Hayabusa spacecraft touched down on a near-Earth asteroid named 25143 Itokawa to pluck a few grains off its surface and return the sample to Earth.

Dawn, however, is the first probe to go into orbit around an asteroid for a long-term study. It also would be the first spacecraft to rendezvous with more than one object in the solar system, a feat made possible by what is known as an ion propulsion system.

Rather than chemical rocket thrusters, Dawn's engines work by pumping electrically charged ions of xenon gas through an electric field, which accelerates the particles and prepares them for an 89,000 mph escape into space. The force of the expelled gas causes the spacecraft to move in the opposite direction.

The motion, which is about equal to the pressure of a sheet of notebook paper on the palm of your hand, is so gentle it would be useless on Earth.

But in space, where there is no counteracting gravitational force, momentum builds up over time.

Dawn traveled 1.7 billion miles (2.8 billion km) to reach Vesta. It is expected to depart in July 2012 to begin the three-year, 930 million mile (1.5 billion km) trek to Ceres. The mission at Ceres is expected to last six months.

(Additional reporting by Chris Baltimore, Editing by Philip Barbara)