US Air Force's Secret X37-B Space Plane Launches on Fourth Mission

— -- A secretive space plane owned by the United States Air Force launched this morning on its fourth mission to space.

Strapped atop an United Launch Alliance Atlas V rocket, the X-37B space plane had a smooth ascent into space, quickly reaching Mach 1 -- the speed of sound -- as it traveled to its place in low Earth orbit.

While it is not known how long the plane's fourth mission will last, Wright-Patterson Air Force Base said last month the plane will be testing a Hall thruster, a type of propulsion system that could one day be used to power spacecraft on long-haul missions.

Measuring 29 feet in length and having a 15-foot wingspan, the unmanned re-usable orbital test vehicle looks like a miniature version of NASA's now retired space shuttles.

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

The plane touched down last October at Vandenberg Air Force Base in California, marking the end of its third mission and 674 consecutive days in space.