The United Launch Alliance Atlas V rocket carrying NASA's Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft lifts off from Space Launch Complex 41 at Cape Canaveral Air Force Station, Florida, US in this September 8, 2016 handout photo. [Photo/Agencies]

The main spacecraft will remain in orbit around the Sun after the sample return and Earth flyby.

Scientists have previously studied pieces of asteroids for decades in the form of meteorites, but all of them were quickly contaminated when they fell to Earth, Lauretta said, noting that the sample returned from a primitive asteroid would enable precise analyses that can't be duplicated on the ground.

"We expect these samples will contain organic molecules from the early solar system that may give us information and clues to the origin of life," he said.

The seven-year journey will also help explore how Bennu's orbit is affected by the so-called Yarkovsky effect, the slight push created when the asteroid absorbs sunlight and re-emits that heat as infrared radiation, in the hope of better understanding the hazards of near-Earth space rocks to Earth.

Bennu, with a diameter of 492 meters, is classified as a potentially hazardous object, with a 1 in 2700 chance of impacting Earth in the 22nd century. Lauretta said its orbit has changed by over 160 kilometers due to the Yarkovsky effect since it was discovered in 1999.

"We really want to understand this phenomenon, so that we can better understand asteroid Bennu and apply that understanding to all asteroids not only in near-earth space but throughout our solar system," he said.

The world's first asteroid sample-return spacecraft was Japan's Hayabusa 1, which successfully brought back thousands of dust grains from the asteroid 25143 Itokawa after a seven-year space trip.