A mysterious robotic ‘space plane’ stayed in orbit for 780 days - more than two years - before finally landing on Sunday.
The unmanned space plane touched down at NASA’s Kennedy Space Center in Florida on Sunday.
The Boeing X-37B craft - also known as Orbital Test Vehicle 4 - was on a classified mission, testing new technologies.
It launched on its mission on September 7, 2017, Space.com reports.
Observers have speculated that the Space Shuttle-esque vehicle might be designed to destroy satellites, or spy on other countries.
The X37-B first went into space in 2010 and has completed five missions. The craft can carry cargo loads into space and return them to engineers on Earth.
The Air Force has said it performs “risk reduction, experimentation, and concept of operations development for reusable space vehicle technologies”.
When it landed from a previous mission, it was greeted by ground crew in biohazard suits - sparking speculation that there was something radioactive on board.
The aircraft has a wingspan of less than 15ft and is taken into space on a rocket, but glides back to Earth like a space shuttle.
NASA has previously said that there is a materials experiment on board. But its main payload remains a mystery.
During the aircraft’s previous missions, amateur astronomers were able to detect the orbital pattern of the first X-37B which included flyovers of North Korea, Iraq, Iran, Pakistan and Afghanistan, heightening the suspicion that the vehicle was being used for surveillance.