U.S. scientists say the first close-up pictures of the massive asteroid called Vesta reveal a mysteriously diverse surface that they say will keep them busy studying for years.

NASA's Dawn space probe is on a year-long mission orbiting Vesta — the second-largest object in the solar system's asteroid belt.

Until now, scientists could only take pictures of Vesta from afar. The first-ever close up images reveal a surface covered with numerous craters and long black grooves, which the experts say they are surprised to see.

The asteroid belt is a region between the planets Mars and Jupiter. Asteroids fascinate scientists. They are rocks that orbit the sun, but for some reason, failed to develop into planets.

The experts believe the asteroid belt formed about 5 billion years ago.