Nasa spacecraft begins two-year trip home with asteroid rubble

'Osiris-Rex' reached asteroid Bennu in 2018 and spent two years flying near and around it

An artist's rendering of the 'Osiris-Rex' spacecraft at the asteroid Bennu. On Monday it began its journey back to Earth. AP

With rubble from an asteroid tucked inside, a Nasa spacecraft began the long journey back to Earth on Monday, leaving the ancient space rock in its rear-view mirror.

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

Launched in 2016, Osiris-Rex reached asteroid Bennu in 2018. It spent two years flying near and around it before collecting rubble from the surface last autumn.

The University of Arizona's Dante Lauretta, the project's principal scientist, estimates the spacecraft holds between 200 and 400 grams of mostly bite-size chunks. Either way, it easily exceeds the target of 60g.

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 this twice, but in tiny quantities.

Scientists described Monday's departure from Bennu's vicinity as a bittersweet moment.

“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.

"We have got used to being at Bennu and seeing new and exciting images and data coming back to us here on Earth," Mr Lauretta said.

Osiris-Rex was already 300 kilometres from Bennu when it fired its main engines on Monday afternoon.

Colorado-based flight controllers for spacecraft builder Lockheed Martin applauded when confirmation arrived of the spacecraft’s departure.

“We’re bringing the samples home,” announced a member of the flight control team over the public address system.

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 490 meters wide and 4.5 billion years old.

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 began on Earth. They also could improve Earth's odds against any incoming rocks.

Although the asteroid is 287 million kilometres away, Osiris-Rex will put another 2.3 billion kilometres on its odometer to catch up with Earth.

The SUV-size spacecraft will circle the sun twice before delivering its small sample capsule to the desert floor of western US state Utah, on September 24, 2023, to end the $800 million mission.

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

Scientists initially thought the spacecraft had gathered one kilogram of asteroid rubble, but more recently revised their estimate downward.

They won’t 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."

Nasa has more asteroid projects planned.

Set to launch in October, a spacecraft named Lucy will fly past swarms of asteroids out near Jupiter, while Dart will blast off in November in an attempt to redirect an asteroid as part of a planetary protection test.

Then in 2022, the Psyche spacecraft will take off for an odd, metallic asteroid bearing the same name.

None of these missions, however, involve sample return.