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A Nasa probe sent to collect rock from an asteroid several hundred million kilometres from Earth has grabbed so much that samples are spilling out.

Officials behind the Osiris-Rex probe, which landed on Bennu earlier this week, say the collection operation may have performed too well.

Pictures beamed back to Earth show a rock has wedged open the door of a container and a fraction of the sample is leaking, Nasa says.

Nasa is now trying to stow it safely.

"A substantial fraction of the required collected mass is seen escaping," head of mission Dante Lauretta said.

The craft is believed to have collected some 400g (14oz) of fragments, he said.

The probe could not have done better, he added. "My big concern now is that the particles are escaping because we were almost a victim of our own success here."

"Time is of the essence," Thomas Zurbuchen, Nasa's associate administrator for science, told reporters as the space agency focuses on making sure no more is lost.

The collection container will now be stowed within the spacecraft, which means it will not be possible to measure exactly how much sample has been taken.

"Although we may have to move more quickly to stow the sample, it's not a bad problem to have," Mr Zurbuchen said. "We are so excited to see what appears to be an abundant sample that will inspire science for decades beyond this historic moment."

Osiris-Rex touched down on Tuesday on 500m-wide Bennu, some 320 million kilometres (200 million miles) from Earth.

It kicked up debris and dust when it took the samples from the asteroid's surface. "We really did kind of make a mess," Mr Dante said on Tuesday.

Scientists hope the mission will throw light on how the Solar System began 4.5 billion years ago, once the samples are examined when the spacecraft returns home in 2023. Asteroids contain debris from the formation of the Solar System.

The spacecraft launched in 2016 and begins its journey back to Earth next March.