The US Air Force’s mysterious robotic space plane landed at Vandenberg AFB, California, on 17 October after nearly two years in orbit.

Completion of the recent 674-day mission – the programme’s third ‑ extends the total time an X-37B orbital test vehicle (OTV) has spent outside the earth’s atmosphere to a total 1,367 days. The air force has built two examples of the reusable, unmanned spacecraft. OTV-3 was the longest the vehicle has yet spent in space at a time.

Still, the air force has given precious few details about what the program is meant to achieve because the programme remains classified.

X-37B, which was developed by Boeing, is managed by the air force rapid capabilities office, which “performs risk reduction, experimentation and concept of operations development for reusable vehicle technologies,” the service says. It is the newest and most advanced re-entry spacecraft, according to the air force.

"The landing of OTV-3 marks a hallmark event for the programme," Col Keith Balts, X-37B programme manager and commander of USAF 30th Space Wing, says in a prepared statement. "The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane."

The Air Force plans to launch the fourth X-37B mission in 2015 from Cape Canaveral Air Force Station in Florida.