The X-37B, the Air Force's unmanned "mini-space shuttle," has launched into orbit for a third time and once again what it actually does in space remains a big mystery.

One of the few things known about the space plane's classified missions is that it can stay in orbit for extended periods of time: On its previous mission, a sister craft stayed in orbit for 469 days.

The unmanned space plane lifted off from Cape Canaveral, Fla., Tuesday at 1:03 p.m. atop an Atlas V rocket. The curious could watch the launch on a live webcast that was allowed to broadcast for only 17 minutes into the mission as the space plane began its classified mission.

Officially known as the Orbital Test Vehicle, the reusable aircraft is often referred to as a mini-space shuttle, because it looks like a smaller version of NASA's now-retired space shuttles. Measuring 29 feet in length and with a wingspan of 15 feet, the X-37B is a quarter the size of the shuttles and could easily fit into two long car parking spaces.

Like the space shuttle, the X-37B lands on runways, though it does so without pilots at the helm.

The launch today marked the first time since the retirement of the space shuttle fleet that a spacecraft has gone into space more than once.

Both X-37Bs in the Air Force's inventory have been to space, but the one launched today was the original craft launched into space in April 2010 that remained in orbit for 224 days.

The space plane's orbits are often tracked by space enthusiasts who speculate as to what it might be doing on its classified missions. In the lead-up to the first launch, Air Force officials said the craft offered a platform for testing new technologies in space.

When the X-37B will return to Earth is an open guess, the robotic vehicle is designed to stay in orbit for at least 270 days.

The robotic space planes have previously landed at Vandenberg Air Force Base and it may land there again, though there has been speculation the Air Force might want to have it land at the runways built for the NASA shuttles at the Kennedy Space Center.

But when it will return to Earth remains an open question. If previous missions are any indicator it could be in space for quite a long time.