(CNN) NASA's OSIRIS-REx mission arrived at near-Earth asteroid Bennu a year ago, and the spinning top-shaped space rock has been full of surprises. The latest findings now classify it as an active asteroid with observable events happening on the surface.

OSIRIS-REx and Bennu got to meet face-to-face on December 3 of last year. OSIRIS-REx has been orbiting the asteroid, which is 70 million miles from Earth, since December 31, 2018. It's a "rubble pile" asteroid, a grouping of rocks held together by gravity rather than a single object.

The greatest surprise of the mission happened only a few days in, when an unexpected observation occurred that signaled activity on Bennu, the mission researchers said. They detected particle plumes ejecting off the surface on January 6, followed by additional smaller plumes on January 19 and February 19.

Before the close approach of the spacecraft a year ago, they couldn't be sure if there was any activity on Bennu. Now they know for certain, which makes Bennu an active asteroid that is regularly ejecting material into space. This is not only rare, but it's also the first time scientists have had close-up observations of particle plumes erupting from an asteroid's surface, NASA said.

Particles can be seen releasing from the asteroid.

The particles range from single-digit centimeters to tens of centimeters in length, and some of them are slow-moving while others are quickly making their way through interstellar space. Some fall back onto the asteroid's surface, while others go into orbit around it, like small satellites.

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