NASA says it has been issued a go-ahead for construction of a spacecraft designed to land on and study an asteroid named Bennu when it makes a close approach to Earth in 2018.

After spending a year in exploring the asteroid, the OSIRIS-REx spacecraft will return to Earth bringing about two ounces of asteroid material back with it, the space agency said.

The Mission Critical Design Review, a board of experts from both NASA and several outside organizations, has issued clearance for NASA to begin constructing the spacecraft, its flight instrumentation and the mission support facilities for the Origins Spectral Interpretation Resource Identification Security Regolith Explorer.

"This is the final step for a NASA mission to go from paper to product," says Gordon Johnston, OSIRIS-REx program executive at NASA Headquarters in Washington, D.C. "This confirms that the final design is ready to start the build-up towards launch."

OSIRIS-REx would launch in late 2016, arrive at the asteroid in 2018 and voyage back to Earth in 2023 with asteroid samples aboard.

The review clearance is a major step, NASA managers say, but only a first hurdle in a long process.

"Successfully passing mission CDR is a major accomplishment, but the hard part is still in front of us -- building, integrating and testing the flight system in support of a tight planetary launch window," says Mike Donnelly, OSIRIS-REx project manager at the space agency's Goddard Space Flight Center in Greenbelt, Maryland.

A major goal of the mission is to answer questions regarding the very early solar system's composition and the possible part asteroids may have played in bringing to Earth the water and organic material that made life on our planet possible.

Ten years of planning have already been invested in the project, principle mission investigator Dante Lauretta of the University of Arizona says.

One of the planning milestones was the selection of the best target asteroid.

"There's something like 700,000 asteroids in the solar system," Lauretta told, "and we had to pick one to send a spacecraft to."

"The mission will be a proof-of-concept -- can you go to an asteroid, get material, and bring it back to Earth," says Lauretta.

With the university acting as head investigator institution, NASA's Goddard Space Flight Center will provide overall mission management, while the spacecraft itself will be build by Lockheed Martin Space Systems.

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