It then set the launch for January 5th, but had to push it back due to harsh weather.
It's also SpaceX's third such launch for the U.S. government, with previous payloads including a spy satellite and the X-37B space plane. SpaceX plans to land the first stage of the Falcon 9 rocket at Cape Canaveral Air Force Station.
A series of engine firings to flip it around and slow its plunge through the atmosphere culminated in SpaceX's 21st rocket landing in just over two years.
This video shows the 229-foot-tall (70-meter) Falcon 9 rocket taking off from Cape Canaveral's Complex 40 launch pad at 8 p.m. EST Jan. 7 (0100 GMT Jan. 8) and climbing through mostly clear skies over Florida's Space Coast.
SpaceX successfully sent something into space for the U.S. government, and that may be all we'll ever know about the so-called "Zuma" mission that Elon Musk's commercial space company completed from NASA's Kennedy Space Center in Florida Sunday evening.
One theory floated was that the mission might be connected to one SpaceX launched into a similar orbit last May for the National Reconnaissance Office.
It's still unclear where the Zuma mission will be heading.
"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 launch was originally scheduled for late November, but was repeatedly delayed. The exact time of the launch has yet to be set, though SpaceX has set a two-hour window beginning at 12 AEDT.
It will be carrying a secret payload for the USA government, code-named Zuma.
CEO Elon Musk says the heavy-lift launcher's debut test flight - carrying his own Tesla Roadster sports vehicle into space - is possible from KSC before the end of this month.