The payload is from the National Reconnaissance Office, and is called NROL-76, Phys.org reports. A new Falcon 9 rocket carrying the mysterious cargo is set to take off sometime between 7 a.m. and 9 a.m. from Cape Canaveral, Florida.
No other details on the size of the load, where it will be delivered or what it will do have been released. The NRO describes itself as the US's "eyes and ears" on its website.
The company, owned by tech entrepreneur Elon Musk, is scheduled to launch GPS satellites into orbit for the US Air Force sometime in 2018, after a fight to win government contracts that included filing suit against the government for the right to compete for such deals.
The US military had previously only used United Launch Alliance, a joint venture of Boeing and Lockheed Martin, Phys.org points out. NASA, however, has used SpaceX to help resupply the International Space Station nine times since 2012.
Only minutes after the launch, the 14-story first stage of SpaceX's rocket will power on and make a controlled landing at the same space center. SpaceX is attempting to make much of the hardware for space exploration reusable, and has so far landed rockets on solid ground successfully several times.
The launch on Sunday will be the first since SpaceX's historic launch of a reused Falcon 9 booster at the end of March. That launch, which put into orbit a satellite for Luxembourg-based SES, was the first time a booster rocket had been sent successfully back into space, The Verge reports. That particular rocket had been launched and had come back home in April 2016. Tomorrow's launch will use a new rocket, however.
Creating workable reusable rockets is crucial to a sustainable space program: CNN Money reports that the first stage of a Falcon 9 rocket represents about 70 percent of the roughly $62 million price tag for the device.
SpaceX is planning six or more used rocket missions this year.