The X-37B vehicle and its cargo bay packed with a classified payload is set to make the third mission of the program. Also called Orbital Test Vehicle-3, or OTV-3, the unpiloted craft is slated to be hurled into Earth orbit by an Atlas 5 rocket from Florida's Cape Canaveral Air Force Station (CCAFS) at 1:03 p.m. EST (1803 GMT).

There's an interesting angle to the upcoming mission. This third flight will use the same X-37B spacecraft that flew the first test flight, the OTV-1 mission, back in 2010.

That maiden voyage of the miniature space plane lasted a little over 224 days, orbiting Earth from April 22, 2010 to Dec. 3 of that year, and finally landing on autopilot at a specially prepared runway at Vandenberg Air Force Base in California. [Photos: U.S. Military's X-37B Space Plane]

A different X-37B vehicle made a similar Vandenberg touchdown this past June 16, after staying in orbit 469 days on its OTV-2 mission.