The pilotless X-37B Orbital Test Vehicle lifted off from California Saturday aboard an Atlas 5 rocket.
Air Force officials have kept details of the craft secret, saying only that it provides a way to test new outer space technologies.
The mystery has fueled speculation that the X-37B is some kind of weapon or spy device, a charge denied by Air Force officials.
The craft is almost 9 meters long and looks like a miniature space shuttle. It is designed to stay in orbit for up to 270 days, and operates completely autonomously.
This is the second mission for the X-37B, which first launched in April 2010 and landed in December.
It was originally developed by the U.S. space agency NASA, but was later taken over by the military.
