NASA's OSIRIS-REx spacecraft has been assembled and is now going through environmental testing. From the looks of it, its rendezvous with asteroid Bennu in 2018 is right on schedule.

Aerospace company Lockheed Martin announced the completed assembly of the spacecraft for the first American mission to return asteroid samples to Earth and its current testing at its Space Systems facilities near Denver, Colorado.

The $800 million mission - which means Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer - will be making the journey to Bennu, a carbon-rich space rock that could give scientists insight into the solar system's origin.

The spacecraft is expected to get at least 2.1 ounces of material from 1,840-foot-wide Bennu and send the sample back to Earth in 2023. Apart from providing clues about the early days of the solar system, the sample will help determine future efforts to mitigate an asteroid impact.

There is, for instance, a 0.04 percent risk of Bennu slamming into the planet any time from 2175 to 2199.

Rich Kuhns, program manager of the space mission, called it an exciting time to complete the spacecraft and test-drive it for flying to Bennu. He said the environmental testing stage is vital to "reveal any issues with the spacecraft and instruments" while on Earth and before embarking on space travel.

Mike Donnelly, who is overseeing the program at the Goddard Space Flight Center in Maryland, added that the environmental testing is on time and on budget, with schedule reserves for flexibility.

OSIRIS-REx will undergo a series of tests in the next five months to simulate the vacuum, vibration and extreme temperatures it will be subjected to during the mission. Some of the areas of focus are thermal vacuum, vibration, launch acoustics, and electromagnetic interference.

The assembled spacecraft will be shipped from the Lockheed Martin facility to the Kennedy Space Center of NASA in May 2016, where it will be prepared for launch.

While OSIRIS-REx is NASA's first asteroid hunt mission, the first to do so worldwide is Japan's Hayabusa mission, which returned tiny parts of asteroid Itokawa to Earth in 2010. Hayabusa 2 was launched December last year and is expected to return samples from asteroid 1999 JU3 in late 2020.