The launch window for the Falcon 9 rocket opens at 8 p.m.
After launch, SpaceX returned the tall portion of the Falcon 9 rocket to an upright landing at Cape Canaveral.
"Northrop Grumman realizes that this is monumental responsibility and have taken great care to ensure the most affordable and lowest risk scenario for Zuma", Lon Rains, communications director for Northrop Grumman's Space Systems Division, said in an official statement.
The mission, code named "Zuma", is a Falcon-9 rocket launch, but it is unclear what the rocket is carrying. And we don't know what Zuma's exact orbit is (though it's a safe bet that amateur satellite trackers will be working hard over the coming days and weeks to figure that out). The launch also comes after a long delay, with the first attempts going way back in November.
After a record 18 launches in 2017, this mission will be SpaceX's first flight of the new year.
The first Falcon Heavy, made up of three Falcon 9 core stages and a single upper stage, will be erected atop historic pad 39A at the nearby Kennedy Space Center next week for a main engine test firing.
The Zuma mission was first revealed to the public in October, when NASA reported that SpaceX has requested authorization for the mysterious "Mission 1390".