x37b space plane 2009
The X-37B Orbital Test Vehicle on a runway at Vandenberg Air Force Base, in California, in June 2009.
US Air Force
A classified unmanned space plane has been in orbit for over 500 days, but no one is telling the public what it's doing.

Drones are in the news nearly every day. Tiny toys snapping exciting photos for our Instagram accounts. Commercial drones working for farmers and municipal agencies. Missile-armed drones performing strikes on enemy locations. Unmanned craft, in the air, on the ground, and in the sea, are conducting more missions for more people all the time.

Operated by the United States Air Force, the X-37B is at the top of the drone pyramid and is pushing the outside edge of the envelope as you read this.

The X-37B, sometimes called the Orbital Test Vehicle, is a small unmanned reusable spacecraft built by Boeing that looks a lot like a small space shuttle. It's 29 feet-long with stubby wings and angled tail fins. Unlike the famous shuttle orbiters that it resembles, however, it lacks a vertical stabilizer. It launches atop an Atlas V 501 launch vehicle inside the booster's payload fairing and, at the end of its mission, returns to earth and lands on a runway like the 80s-era space shuttle.

What the space plane does while it's up there, though, is mostly a mystery.

The first X-37B mission, OTV-1, flew in 2010 and lasted for 224 days — close to the X-37B's designed 270-day endurance. The second mission, flown by a second X-37B, flew in 2011 and lasted 468 days. The third mission, performed by the first spacecraft, lasted an astounding 674 days. The current mission, dubbed "OTV-4," was launched on May 20, 2015, and recently passed 500 days in orbit.

The Air Force is tight-lipped about how long OTV-4 will last, though it must last until at least March 25, 2017, if it's going to break OTV-3's record.

The X-37B program began as a NASA project. One of the X-37's primary missions was to have been satellite rendezvous for refueling and repair, and the small space plane would have been carried to orbit inside a space shuttle's cargo bay.

A graphic rendering of the X-37B.

As the program progressed, however, the plan shifted to launching atop an expendable booster and, in 2004, the Defense Advanced Research Projects Agency took over the program. DARPA continued development of the X-37, resulting in the current X-37B for the US Air Force.

Once the project transferred to the military, it became classified.

The spacecraft itself is not terribly secret. For the past six years, numerous news stories have been run about the "secret" space plane and many photographs have been published.

What the X-37B does while it's up there on missions lasting well over a year, though, is the source of much conjecture. The fact sheet for the X-37B on the official Air Force web site describes the mission as "an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force. The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth."

That's an explanation that doesn't explain much, however, and doesn't seem to justify the enormous expense of building, launching, and operating the spacecraft.

Air Force via Boeing

Of course, theories on the internet abound. Some have claimed that the X-37B is an advanced spy satellite. Some wonder if the X-37B could be an experimental space bomber. Others believe that the original mission of satellite rendezvous for maintenance could easily have been adapted to more nefarious purposes, such as interfering with or destroying satellites operated by nations such as Russia or China.

Ground observers have watched what they believe to be the X-37B make significant changes to its orbit, a maneuver in line with the space saboteur theory.

In 2012, during the OTV-2 mission, the editor of Spaceflight magazine claimed that the X-37B was probably spying on the Chinese space station Tiangong-1. Other experts, however, publicly doubted and dismissed the idea as improbable at best.

If you spend enough time online, you're certain to find any number of wild ideas. One of the most outlandish theories about the X-37B is that it's not unmanned at all. The idea is that a hibernating astronaut is onboard the space plane and that experiments are being conducted to prepare for long-duration manned missions to Mars or, perhaps, to station a quick-reaction force of soldiers in orbit for secret missions anywhere on the globe. Or above it.

Whatever the X-37B is up to, it seems to be doing a good job of it. Work is being done to make it possible for the drone to land at Kennedy Space Center on the space shuttle landing strip. Part of the shuttle Orbiter Processing Facility, without a mission since the shuttle program ended, is being modified to process X-37Bs.

A recent NASA presentation discussed the potential to develop a space ambulance which could service the ISS from the X-37B. An X-37C proposal, more than 50 percent larger than the current model, would possibly be able to carry astronauts.

The classified little space plane is a bit of a mystery but certainly one of the most exciting drones in use today. Even if we aren't sure what's it doing.