The U.S. Air Force launched an unmanned shuttle into space on Friday afternoon — but its exact mission remains a mystery. The 29-foot X-37B resembles a miniature version of the space shuttle, and will complete a "classified mission" before landing in California later this year. An earlier version of the X-37B spent 224 days in orbit last year. Both were funded by the Pentagon's top-secret "black" budget. What could the spacecraft be doing up there? Here, five theories:

1. Testing or launching spy satellites
Amateur astronomers who studied the earlier incarnation of the X-37B theorize that the craft is testing sensors for a new generation of secret spy satellites. The military craft, the stargazers told The New York Times, repeatedly passes over the same region — a "common feature of U.S. imaging reconnaisance satellites." Tellingly, the aircraft's "payload bay is the size of a small pickup truck bed," which suggests that it could be used to deploy or capture small satellites.

2. Taking out "enemy" satellites... with spraypaint
Some speculate that the spacecraft could contain technology to put rival countries' satellites out of orbit, or destroy them completely. "Simply by spraying a satellite black, you can put it out of action because it will overheat very quickly," says defense journalist Bill Sweetman, as quoted in TechNewsWorld. If the shuttle contained some kind of remote-controlled spraypaint machine, it could easily destroy "enemy" satellites. "Not to sound too conspiratorial," says Sweetman, "but once the satellite went down, it would look like an accidental failure."

3. Dropping "Rods from God" on enemy targets
Such a reusable space plane offers the U.S. "unheard-of" potential for quick, surprise launches, says It could be fitted with a "weapon to drop tungsten rods," nicknamed "Rods from God," on targets back on earth. The Chinese military apparently fears this scenario, adds the Los Angeles Times. Chinese professor Li Daguang wrote that the X-37B would soon be "capable of taking military actions" against the enemies of the U.S.

4. Doing things we can't even imagine
All of these theories ignore the fact that "simpler spacecraft" can already do these things "better and cheaper," says David Axe at Wired. A simple capsule and parachute is sufficient to retrieve items from space, for example, and building a disposable spacecraft is far cheaper than equipping a vehicle with the fuel and technology it needs to return. It seems the X-37B is "capable of something — or a mix of things — we outsiders haven't yet imagined."

5. Doing nothing at all
"It is difficult to find a mission for which the space plane makes sense," say Laura Grego and David Wright at All Things Nuclear. Even a space weapon would be practically useless when compared to our technology on the ground. In fact, the shuttle's "only unique capability" seems to be its ability to return from orbit and land on its own. Could it be that the Pentagon is spending billions of dollars just to "keep the space-plane concept alive," even though it is both expensive and ineffective?