The first close-up pictures of the asteroid Vesta, a protoplanet that dates back to the early days of the solar system, revealed a surprisingly diverse terrain and several unexplained geologic features, NASA scientists said on Monday.

The images were taken by the U.S. space agency's Dawn robotic probe, which is two weeks into a planned year-long survey of the second largest object in the main asteroid belt, located between Mars and Jupiter.

"These photos already have been a great revelation to the team about what the surface (of Vesta) is like. We did not imagine the detail that we're seeing," Dawn lead scientist Chris Russell, with the University of California at Los Angeles, told reporters.

Scientists believe Vesta grew from a clump of gas and dust left over after the sun's formation some 4.65 billion years ago. About twice the size of California, Vesta is remarkably diverse, with grooves around its equatorial belt, bright spots, dark pits and craters filled with unexplained streaks of black and white debris.

"I haven't seen anything like that before," Russell said. "It's really a beautiful and exciting small world sitting there in the middle of the asteroid belt."

Scientists believe that as Vesta was coming together about 5 million years after the sun's birth, a supernova exploded, which added radioactive materials to the growing body.

The extra heat would have caused Vesta to melt and eventually form an inner core of iron and an outer lava crust. That may explain the myriad of surface features seen in Dawn's first close-ups of Vesta, which were released on Monday.

"This is not a uniform body. Different things were happening at different regions of the surface. That indicates to me that the interior was being very active," Russell said. "We're going to learn a lot from this body."

ION PROPULSION

Dawn will spend about a year circling Vesta, tweaking its orbit and altitude using an innovative ion propulsion system, a technology that chief engineer Marc Rayman, with NASA's Jet Propulsion Laboratory in California, said he first heard about in an episode of the TV show "Star Trek."

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 (142,400 kph) escape into space.

The force of the expelled gas causes the spacecraft to move in the opposite direction.

The motion, about equal to the pressure of a sheet of 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.

The ion propulsion system will enable Dawn to leave Vesta's orbit after a year of study and head off to a second destination, the dwarf planet Ceres, which is the largest object in the asteroid belt.

NASA California Los Angeles NASA