All that is publicly known about the Zuma payload is that it is a satellite manufactured for the USA government by Northrop Grumman, and it is bound for low-Earth orbit.
"Three, two, one, ignition and liftoff", said a SpaceX commentator as the Falcon 9 rocket launched under cover of darkness from Cape Canaveral, Florida, at 8:00 pm (0100 GMT Monday).
SpaceX ended launch commentary five minutes into the flight, due to the classified nature of the USA satellite. The first stage of the Falcon 9 rocket then returned for a vertical landing at Landing Zone 1 at Cape Canaveral. The company has said it plans to launch roughly 30 missions in 2018 after completing a record 18 past year.
Originally planned for a November launch, the mysterious Zuma mission may finally go to space on Sunday evening. That would place Zuma near the others once in orbit, perhaps not by coincidence.
SpaceX on Sunday night picked up where it left off in 2017, its best year yet with 18 launches and no failures.
Both of those will launch on used Falcon 9 boosters, as SpaceX continues to re-fly rockets it has landed on previous missions. It's likely the feed will go live around 7:45pm ET, a few minutes prior to the launch window.
The Falcon 9 lifted off at 8 p.m. EST (0100 GMT on January 9) from Cape Canaveral Air Force Station, on Florida's Space Coast. But Sunday's launch is perhaps the company's most secretive yet.