NASA's OSIRIS-REx spacecraft begins its 2-year trip home with asteroid debris

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 rearview 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 itbefore collecting rubble from its surface last fall.

The University of Arizona's Dante Lauretta, the principal scientist, estimates the spacecraft holds between 200 grams and 400 gramsof 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 U.S. 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.

WATCH | NASA's OSIRIS-REx spacecraft collects asteroid sample:

NASA's Osiris-Rex spacecraft grabs sample from asteroid

4 years ago

Duration 18:44

A NASA spacecraft descended to an asteroid and momentarily touched the surface to collect a handful of cosmic dust. Michael Daly, lead scientist for the instrument that mapped the asteroid's surface, answered some questions from two special guests.

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," said NASA project scientist Jason Dworkin."Now she's graduating from high school, so it's been a long journey."

OSIRIS-REx was already nearly 300 kilometresfrom the solar-orbiting Bennu when it fired its main engines 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!"

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 metreswide.

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 287 million kilometresaway, OSIRIS-REx will put another 2.3 billion kilometreson 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 Sept. 24, 2023, to end the more than $800 million-mission. It launched from Cape Canaveral, Fla.in 2016.

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

Scientists initially thought the spacecraft stored 1 kilogramof 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," Dworkin said. "We have to be patient."