SpaceX postponed the launch of a secretive U.S. government payload known as Zuma, a mission whose nature, and the agency behind it, is a mystery. SpaceX and the Pentagon did not respond to requests for comment about the nature of the mission.

Northrup Grumman, the maker of the payload, said it was for the U.S. government and would be delivered to low-Earth orbit, but offered no other details. SpaceX is no stranger to national security launches.

Earlier this year, the California-based company headed by billionaire entrepreneur Elon Musk launched a spy satellite for the National Reconnaissance Office and an X-37B space plane for the U.S. air force. After liftoff, SpaceX will attempt to return the first stage portion of its Falcon 9 rocket to Earth for a controlled landing on solid ground near Cape Canaveral.

If successful, it will be the 20th upright touchdown for a Falcon 9 since the company began its effort to recycle costly rocket parts and make spaceflight more affordable.