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

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

Osiris-Rex 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 200 and 400 grams of mostly bite-size chunks. Either way, it easily exceeds the target of at least 60 grams.

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’s 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.

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

Osiris-Rex was already about 300 kilometres from the solar-orbiting Bennu when it fired its main engines Monday afternoon for a fast, clean get-away.

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

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 arose on Earth. They also could improve Earth’s odds against any incoming rocks.