Scientists say new views of the massive asteroid Vesta reveal it is more like a planet.
Since slipping into orbit around Vesta in July, Nasa's Dawn spacecraft has beamed back stunning images of the second largest object residing in the asteroid belt.
Vesta's rugged surface is unique compared to the solar system's much smaller and lightweight asteroids. Impact craters dot Vesta's surface along with grooves, troughs and a variety of minerals.
"Vesta is unlike any other asteroid," said mission co-scientist Vishnu Reddy of the Max Planck Institute for Solar System Research in Germany. The new findings were presented at a meeting of the American Geophysical Union in San Francisco.
Most asteroids resemble potatoes, but Vesta is more like an avocado with its iron core, Mr Reddy said.
Asteroids are remnants from the birth of the solar system some 4.5 billion years ago around the same time as the formation of the rocky planets including Earth. Studying asteroids can offer clues about how our planetary system began.
Instead of returning to the moon, Nasa has decided to land astronauts on a yet-to-be determined asteroid as a stepping stone to Mars.
David Williams of Arizona State University considers Vesta a "transitional body" between rocky planets and the thousands of asteroids floating between Mars and Jupiter.
The mission has yielded a mystery. Before Dawn arrived at Vesta, scientists predicted that the surface would harbour a volcano. There is a hill on Vesta, but researchers said there was no evidence of lava flow or volcanic deposits.
Mr Williams said it was possible the volcanic materials were buried, so the team would keep looking.
Powered by ion propulsion instead of conventional rocket fuel, Dawn will study Vesta for several more months before cruising to an even bigger asteroid, Ceres, where it will arrive in 2015.