US Air Force's Secret X37-B Space Plane Launches on Fourth Mission
What the U.S. Air Force's aircraft will be doing this time in space.
— -- 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.
![](https://s.abcnews.com/images/Technology/HT_space_plane_launch_01_jef_150520_16x9t_992.jpg)
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.
![](https://s.abcnews.com/images/Technology/HT_space_plane_launch_02_jef_150520_16x9_992.jpg)
Like the space shuttle, the X-37B lands on runways, though it does so without pilots at the helm.
![](https://s.abcnews.com/images/Technology/HT_space_plane_launch_04_jef_150520_16x9_992.jpg)
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.