OTV-4 Was Launched May 20, 2015

The latest mission of the U.S. Air Force X-37B mini-shuttle has passed the 700-day mark ... and the nature of the mission is still top secret.

Space.com reports that the OTV-4 mission was boosted into orbit by a United Launch Alliance Atlas V rocket on May 20, 2015 from Cape Canaveral Air Force Station in Florida.

The payload and mission activities are classified. The missions of the C-37B have increased in length from OTV-1, which spent 224 days in space, to the current OTV-4 mission which is at 700 days and counting. OTV-3 spent 675 days in space.

Air Force Officials say there is nothing untoward going on with the spacecraft, but they won't say what it is doing. On the X-37B fact sheet on the Air Force website, the primary objectives for the spacecraft are described as twofold: "reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth."

Boeing built the X-37B spacecraft for the Air Force. There are believed to be two of the mini-shuttles in existence. 

When it does land, it may return to Cape Canaveral. The Air Force has modified the former Shuttle Landing Facility at the Kennedy Space Center to accommodate the X-37B. The previous three mission have ended at Vandenberg Air Force Base in California.

(Images from file)

FMI: www.af.mil/About-Us/Fact-Sheets/Display/Article/104539/x-37b-orbital-test-vehicle