The US Air Force launched its robotic space plane into orbit for a fourth flight on Wednesday aboard an Atlas 5 rocket, in a mission aimed at testing a new engine to steer satellites, officials said.
The rocket carrying the X-37B successfully lifted off from Cape Canaveral, Florida and officials said the scheduled return of the unmanned plane had yet to be determined.
The mini-shuttle has been shrouded in secrecy and military officers have refused to discuss its purpose. But defense experts have speculated it might be meant for spying from space, fixing broken satellites or even as a space "bomber."
Captain Chris Hoyler, a spokesman for the US Air Force, told AFP the latest flight was part of efforts looking at the "technical parameters for an affordable, reusable space vehicle."
The X-37B will be testing a new orbital "thruster system" -- which uses electricity and xenon -- that could be employed to maneuver satellites in space, officials said.
Asked if the plane could be used for surveillance, Hoyler declined to comment.
The X-37B payload also includes a NASA experiment, which will study how a range of materials can endure conditions in space. The results could help scientists working on the possible design of future spacecraft.
The last mission for the X-37B in 2014 extended over 674 days but officials never said what the plane was up to.