The Pentagon’s X-37B space plane has finally touched down to Earth after orbiting the planet for two years on a top-secret mission – but we still have no idea what it was actually doing up there. 

The uncrewed space plane landed at the Kennedy Space Center Shuttle Landing Facility in Florida in the early hours of Sunday, October 27, according to a statement from the US Air Force. The X-37B was launched into its fifth mission with the help of a SpaceX Falcon 9 rocket on September 7, 2017. It's been orbiting Earth for 780 days, marking its longest mission to date.

The US Air Force put out a press release and a few photographs of the mysterious space vehicle after it landed, however, as ever, they remain thin on details.

“This spacecraft is a key component of the space community. This milestone demonstrates our commitment to conducting experiments for America’s future space exploration,” Lt. Col. Jonathan Keen, X-37B program manager, said in a statement. “Congratulations to the X-37B team for a job well done.”

The official line from the military is that the X-37B is “an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force.”  

Boeing has built two X-37B vehicles for the US Air Force, both of which have performed numerous flights. The pair are reusable robotic space planes and are powered using solar panels and lithium-ion batteries. 

Measuring 8.8 meters (29 feet) in length and with a wingspan of 4.6 meters (nearly 15 feet), the craft looks a lot like a shrunken-down version of the iconic Space Shuttle that was operated from 1981 to 2011. By no coincidence, the X-37B is said to have been designed in the early 1990s when NASA was studying cheaper alternatives to the Space Shuttle orbiter.

There have been hints that the space plane also plays a non-scientific, perhaps even adversarial, role. Heather Wilson, former US Secretary of the Air Force, recently explained that the X-37B spacecraft can also turn and changes its course when it’s at a lower altitude.

"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," Wilson told a panel at the Aspen Security Forum in July 2019.

Needless to say, all this secrecy has also fueled a lot of speculation. One of the most entertained theories is that its job is to spy on China. According to a report in Spaceflight magazine in 2012, X-37B's orbit closely followed that of China's former space lab, Tiangong-1, suggesting that it could have been used for space-to-space surveillance.