space planes stories

The Air Force's X-37B robotic space plane made its landing at California's Vandenberg Air Force Base Saturday morning successfully completing more than 15 months in space. The video, shot partly in infrared, showed the vehicle coming in for its automated landing glowing in parts, due to the heat from re-entry.
Not a lot is known about the U.S. Air Force's X-37B space plane. The military branch runs two of them, and one has been in orbit for a full year now, as of March 5. Is the spacecraft just checking the weather in orbit? Is it spying from space? Is it carrying anything in its cargo hold? Just how long will it be up there, anyway?