The plane, known as the X-37B, resembles a mini space shuttle.
Just what the plane was doing has been the subject of sometimes spectacular speculation.
Several experts have theorized it carried a payload of spy gear in its cargo bay.
Other theories sound straight out of a James Bond film, including that the spacecraft would be able to capture the satellites of other nations or shadow China’s space lab.
The X-37B program has bounced between several federal agencies, NASA among them, since 1999.
The plane has been in space for a total of 674 days, far more than its two previous flights which lasted 225 and 469 days.
The program’s first mission launched in April 2010 and landed in December that year. The second space plane took off on March 2011 and came back to Earth in June 2012.
Nasa last week said it has entered into an agreement with the Air Force’s X-37B program for use of the Kennedy Space Center’s Orbiter Processing Facility (OPF) Bays 1 and 2 — former space shuttle hangars.
Boeing is performing construction upgrades in those facilities that are targeted to be complete in December. All missions so far have launched from Cape Canaveral Air Force Station, Florida. While the airplane looks like NASA’s retired space shuttles, it has its own identity.
An infrared view of the X-37B unmanned spacecraft landing at Vandenberg Air Force Base. The purpose of the U.S. military’s space plane is classified, only fueling speculation about why it has been orbiting Earth for nearly two years on this, its third mission.
Like a shuttle, it is blasted into orbit by a rocket. However, it lands using a runway like a normal aircraft. The X-37B is too small to carry people onboard, but does have a cargo bay similar to that of a pickup truck, which is just large enough to carry a small satellite.
‘I’m extremely proud of our team for coming together to execute this third safe and successful landing,’ Col. Keith Balts, commander of the 30th Space Wing that is headquartered at Vandenberg, said in a statement.
‘Everyone from our on-console space operators to our airfield managers and civil engineers take pride in this unique mission and exemplify excellence during its execution.’