SpaceX is poised to launch on Thursday a secretive payload known as Zuma for the US government, though the nature of the mission and the agency behind it remain a mystery.
The launch atop a Falcon 9 rocket is scheduled from Cape Canaveral, Florida sometime between 8:00 pm and 10:00 pm (0100 GMT and 0300 GMT Friday).
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 US government and would be delivered to low-Earth orbit, but offered no other details, according to Space.com.
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 US 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.
Explore further: SpaceX launches, lands recycled rocket