Astronomers studying an asteroid that has a slim chance of colliding with Earth have made a mysterious discovery - it's firing rocks into space.
Bennu, which has a similar orbit to Earth, is currently being orbited by NASA's OSIRIS-REx spacecraft.
The team behind the mission have now revealed they were taken by surprise when just a week later the asteroid began spitting out rocks.
"No one has ever seen an active asteroid up close like this," said University of Arizona astronomer Carl Hergenrother. "It wasn't that long ago that the conventional wisdom was that asteroids are these dead bodies that didn't change very much."
At first the scientists believed the particles photographed were just stars in the distance, but they soon realised they were coming from Bennu itself.
Further examination revealed the rocks were up to 10cm across. Some would fly off into space, others would orbit Bennu for a while before the massive rock's gravity sucked them back in.
The biggest eruptions happened in the afternoons, when the sun's light heats up the otherwise icy-cold rock.
"As a result of this temperature change, rocks may begin to crack and break down, and eventually particles could be ejected from the surface," the scientists said. "This cycle is known as thermal stress fracturing."
Another theory is water deep inside Bennu heating up and forming pressure, pushing particles out like a geyser. A third explanation is smaller rocks crashing into Bennu outside of OSIRIS-REx's vision, shaking stuff loose.
"It could be that more than one of these possible mechanisms are at play," said Steve Chesley, scientist at NASA's Jet Propulsion Laboratory in Pasadena. "For example, thermal fracturing could be chopping the surface material into small pieces, making it far easier for meteoroid impacts to launch pebbles into space."
Scientists hope when OSIRIS-REx heads down to the surface of Bennu next year, it'll be able to pick up some of the rocks that were briefly flung into space and bring them back to Earth.
A Japanese craft that visited another asteroid last year didn't find any evidence of rubble geysers.
Bennu, which is about half a kilometre across and weighs about 73,000 tonnes, currently has a one-in-2700 chance of hitting the Earth sometime in the late 22nd century. It's due to have a few brushes with Earth before then however, and scientists say those close encounters could change its trajectory and put it on course for a collision.
Computer simulations reported in 2015 found over the next 300 years it has a 48 percent chance of falling into the sun, 26 percent chance of hitting Venus and only a 10 percent chance of colliding with Earth.
NASA chose to fly a ship to Bennu over four other asteroids on its shortlist, because of its hazardous orbit.
It's been estimated if Bennu did hit Earth, it would unleash energy equivalent to 1200 megatonnes of TNT - 80,000 times stronger than the bombs dropped in World War II, but nowhere near enough to pose a threat to civilisation. In comparison, the asteroid that finished off the dinosaurs was probably about 10km wide.
The latest findings were published in journal Science.
