The Air Force's X-37B Orbital Test Vehicle is back on the ground after completing its latest record-breaking unmanned mission in space.
The experimental, clandestine space plane landed at NASA's Kennedy Space Center in Florida on Sunday after more than two years in orbit, the service said in a release. This was the X-37B's fifth space mission. Its last orbit ended in May 2017 after 718 days in space.
"The safe return of this spacecraft, after breaking its own endurance record, is the result of the innovative partnership between government and industry," Air Force Chief of Staff Gen. David Goldfein said in a statement. "The sky is no longer the limit for the Air Force and, if Congress approves, the U.S. Space Force."
This was the second time the X-37B has landed at the Kennedy Space Center Shuttle Landing Facility. It took off for its fifth mission on Sept. 7, 2017.
While its payloads and most of its activities are classified, the Air Force said at the time that the mission would carry "the Air Force Research Laboratory Advanced Structurally Embedded Thermal Spreader (ASETS-II) payload to test experimental electronics and oscillating heat pipe technologies in the long-duration space environment."
The space plane "conducted on-orbit experiments for 780 days during its mission, recently breaking its own record by being in orbit for more than two years," the release said. That brings the total number of days spent on-orbit for the X-37B to 2,865, officials said.
The last two missions have pushed the boundaries for a test vehicle, originally designed to spend up to 270 days circling the Earth.
What the X-37B does is literally a matter of rocket science. According to the service, the X-37B is exploring the practicalities and risks of "reusable space vehicle technologies" while also experimenting with space technology.
Under the purview of the Air Force Rapid Capabilities Office, the test vehicle can autonomously reenter the atmosphere and eventually land horizontally on a flight line.
"This program continues to push the envelope as the world's only reusable space vehicle," said Randy Walden, director of the Rapid Capabilities Office.
"With a successful landing today, the X-37B completed its longest flight to date and successfully completed all mission objectives. This mission successfully hosted Air Force Research Laboratory experiments, among others, as well as providing a ride for small satellites," Walden said Sunday.
In July, the service's former top civilian gave a glimpse into the space plane's mission.
Speaking about space awareness and deterrence at the Aspen Security Forum, Heather Wilson described the vehicle as a "small version of the [NASA space] shuttle."
It "can do an orbit that looks like an egg and, when it's close to the Earth, it's close enough to the atmosphere to turn where it is," she said at the time.
"Which means our adversaries don't know -- and that happens on the far side of the Earth from our adversaries -- where it's going to come up next. And we know that that drives them nuts. And I'm really glad about that," she added.
Jonathan McDowell, an astronomer and astrophysicist at the Harvard-Smithsonian Center for Astrophysics, told Military.com that Wilson's comments on its movement may shed light on "a previously secret orbit-related capability," and explained that the aircraft's movement likely throws an adversary off, even if just for a short time.
"The dip into the atmosphere causes a change in the timing of when it next comes overhead. So [trackers'] predictions are off, and [they] have to search for it all over again," McDowell said at the time.
The Air Force is preparing to launch the sixth X-37B mission from Cape Canaveral Air Force Station, Florida, in 2020.