NASA chief acknowledges more trouble with SLS rocket

NASA's administrator said Wednesday its next heavy-lift rocket, the SLS, is running into additional development delays, and the space agency now wants to use private launchers for its next Moon mission.

Boeing has spent years developing the SLS, or Space Launch System, which would be the most powerful rocket ever built, with the goal of resuming manned space missions to the Moon. A capsule for astronauts, Orion, is being built by Lockheed-Martin and Europe for the missions.

After a three year delay, a first unmanned mission around the Moon and back, dubbed EM-1, has been scheduled for June 2020.

But NASA administrator Jim Bridenstine told a Senate hearing that the "SLS is struggling to meet its schedule" and won't be ready in time for the EM-1 mission.

"Last week it came to our attention that we're not going to be able to maintain the schedule," he told the Senate Commerce, Science and Transportation Committee.

It is the first time that NASA has clearly acknowledged the extent of the .