The Air Force's secretive X-37B space plane gets more mysterious by the day. Designed to spend up to nine months on unspecified errands in Earth's orbit, the second copy of the Boeing-made craft, known as Orbital Test Vehicle 2, has now been in space for a year and two days – and is still going strong. The endurance milestone is unqualified good news for America's space force at a time when its funding and future missions are in doubt.

There's just one thing. We still don't know exactly what the 30-foot-long X-37B is doing up there.

Since the launch of Orbital Test Vehicle 1 in April 2010, the Air Force has insisted that the X-37 program is a purely scientific endeavor. But analysts say the spacefaring craft, which launches into orbit atop a rocket but glides back to Earth like an airplane, is capable of much more than that. It could be an orbital spy – in essence, a more maneuverable satellite. Or it could be used to tamper with enemy satellites.

With its pickup-truck-size payload bay, the estimated billion-dollar craft could even haul small batches of supplies to the International Space Station. In October, Boeing program manager Art Grantz proposed to build an enlarged X-37C model that could also carry astronauts to the station, filling a gap left by the retired NASA Space Shuttle.

Though unlikely, the X-37B could even function as an orbital bomber. "You could stick munitions in there," said Eric Sterner, an analyst with the Marshall Institute, "provided they exist."

The latest rumor has the Air Force extending OTV-2's time in orbit in order to perform close passes on the new Chinese space station, which has been in orbit since September but does yet have astronauts on board. Some analysts have noted that the X-37's path nearly intersects with that of the Tiangong station. Others point out that the two spacecraft would pass each other at thousands of meters per second, making useful surveillance impossible. "If the U.S. really wanted to observe Tiangong, it has enough assets to do that without using X-37B," Brian Weeden from the Secure World Foundation told the BBC.

In any event, the space plane's impressive endurance can only boost the Air Force's space credentials at a critical moment in U.S. orbital capabilities. The Obama administration's proposed 2013-2017 budget plan cuts satellites and rockets, and entirely eliminates the office that oversaw the X-37's development. Meanwhile, Boeing is preparing to shut down its "Building 31" facility in California, where the X-37s were assembled.

Insiders believe the space plane will safely maneuver into a new line of funding, preserving it even as other space systems wither away. All the same, the Air Force has a strong incentive to demonstrate its space prowess in order to stave off deeper cuts. "We should not be surprised if the Air Force is pushing the envelope," Weeden tells Danger Room.

The X-37's efficient design means its performance limits could be farther out than even the most fervent space boosters anticipate. Deftly combining the vehicle's solar panels and rocket-fuel reserves, Air Force and Boeing operators have refined the space plane's operations to an art. "It sips fuel like a Prius," one space insider boasts. "It could be on station into April for all I know."

Doing who-knows-what.