Conspiracy theorists on alert as X-37B plane returns from 674-day secret space mission

The U.S. Air Force’s unmanned X-37B space plane touched back down on Earth after a two-year mission — but conspiracy theorists are still wondering what the craft was doing all that time.

A military spokesman said the “X-37B Orbital Test Vehicle mission 3 (OTV-3)” returned to Vandenberg Air Force Base in California, on the heels of a 674-day mission to conduct experiments in orbit, CNN reported. But how exactly it spent those 674 days isn’t really known.

Conspiracy theorists have been trying to figure out for months what the Pentagon has been doing with its “newest and most advance re-entry spacecraft,” CNN reported. One guess: The spacecraft is a space bomber, CNN reported. Other say it’s a spy plane.

The X-37B took off from Cape Canaveral in Florida on Dec. 11, 2012. Then, the Air Force said its mission would last nine months, CNN said. The government has only released the most general of statements about its mission.

“Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems and autonomous orbital flight, re-entry and landing,” the Air Force said in a statement, CNN reported.