A Nasa spacecraft has begun the long journey back to Earth after taking rubble from a distant asteroid.

The trip home for the robotic prospector, Osiris-Rex, will take two years.

It reached asteroid Bennu in 2018 and spent two years flying near and around it, before collecting rubble from the surface last autumn.

The University of Arizona’s Dante Lauretta, the principal scientist, estimates the spacecraft holds between 8oz and 16oz of mostly bite-size chunks, easily exceeding the target of at least 2oz. It will be the biggest cosmic haul for the US since the Apollo moon rocks.

While Nasa has returned comet dust and solar wind samples, this is the first time it has gone after pieces of an asteroid. Japan has accomplished it twice, but in tiny amounts.

Scientists described Monday’s departure from Bennu’s neighbourhood as bittersweet.

“I’ve been working on getting a sample back from an asteroid since my daughter was in diapers and now she’s graduating from high school, so it’s been a long journey,” said Nasa project scientist Jason Dworkin.

Mr Lauretta added: “We have gotten used to being at Bennu and seeing new and exciting images and data coming back to us here on Earth.”

Osiris-Rex was already nearly 200 miles from the solar-orbiting Bennu when it fired its main engines on Monday afternoon for a fast, clean getaway.

Colorado-based flight controllers for spacecraft builder Lockheed Martin applauded when confirmation arrived of the spacecraft’s departure: “We’re bringing the samples home!”

Space Asteroid Grab
The asteroid Bennu (Nasa/Goddard/University of Arizona/ CSA/York/MDA/AP)

Scientists hope to uncover some of the solar system’s secrets from the samples vacuumed last October from Bennu’s dark, rough, carbon-rich surface.

The asteroid is an estimated 1,600ft wide.

Bennu — considered a broken chunk from a bigger asteroid — is believed to hold the preserved building blocks of the solar system.

The returning pieces could shed light on how the planets formed and how life arose on Earth. They also could improve Earth’s odds against any incoming rocks.

Although the asteroid is 178 million miles away, Osiris-Rex will put another 1.4 billion miles on its odometer to catch up with Earth.

The SUV-sized spacecraft will circle the sun twice before delivering its small sample capsule to Utah’s desert floor on September 24 2023, to end the more than 800 million dollar (£566 million) mission. It launched from Cape Canaveral in 2016.

The samples will be housed at a new lab under construction at Nasa’s Johnson Space Centre in Houston, already home to hundreds of pounds of lunar material collected by the 12 Apollo moonwalkers from 1969 to 1972.

Scientists initially thought the spacecraft stored 2lb of asteroid rubble, but more recently revised their estimate downwards. They will not know for certain how much is on board until the capsule is opened after touchdown.

“Every bit of sample is valuable,” Mr Dworkin said. “We have to be patient.”