A rock has mysteriously appeared in the path of the veteran Mars rover Opportunity, which has been exploring the planet's surface for over a decade.

News of the mysterious rock was announced by NASA Mars Exploration Rover lead scientist Steve Squyres on Thursday night.

Squyres displayed before-and-after photographs of the spot where the rock suddenly appeared. In the earlier photo, captured by the rover’s panoramic camera, only bare bedrock can be seen. But a fist-sized rock is clearly visible in the later shot.

“It’s about the size of a jelly doughnut,” Squyres told Discovery News. “It was a total surprise, we were like ‘wait a second, that wasn’t there before, it can’t be right. Oh my god! It wasn’t there before!’ We were absolutely startled.”

Two options have been advanced to explain the rock's appearance. The first is that the rover “flipped” the object while maneuvering, and the second is that it landed there, right in front of the rover, after a nearby meteorite impact.

“My best guess for this rock … is that it’s something that was nearby,” said Squyres. “I must stress that I’m guessing now, but I think it happened when the rover did a turn in place a meter or two from where this rock now lies.”