US: Mysterious robotic spacecraft launched for fourth secretive mission
May 21 , 2015
A mysterious US space plane has been launched for its fourth secretive mission.
The mission by the X-37B robotic plane will involve an orbital flight, the fourth in five years.
The reusable craft’s third mission went on for a record 675 days in space, following which it landed on a California air base last October.
The plane’s mission and objectives remain secret, but the US air force did reveal that it would test a new type of electric engine that “produces thrust by ionizing and accelerating a noble gas.”