Sunday night update: SpaceX provided no additional details about the launch after it delivered the Zuma spacecraft into orbit on Sunday night. As of 10:15pm ET, more than two hours after the launch occurred, it still hadn't even provided official confirmation of mission success. However, the launch appeared to proceed nominally, and shortly after liftoff the first stage of the Falcon 9 rocket safely returned to Earth.

The launch clears the way for SpaceX to proceed with the inaugural launch of the Falcon Heavy rocket, perhaps within as few as three or four weeks.

Original post: Originally planned for a November launch, the mysterious Zuma mission may finally go to space on Sunday evening. SpaceX has confirmed that its rocket, and the undisclosed national security payload, are ready for launch, and weather conditions appear to be generally favorable. The two-hour launch window opens at 8pm ET.

An undisclosed issue with the Falcon 9 rocket's fairing caused SpaceX to delay the launch for several weeks in November and eventually move the date forward to January 4. Earlier this week additional propellant loading tests contributed to further delays, as did "extreme weather" at Cape Canaveral Air Force Station in Florida—mostly gusty winds.

But now conditions for the mysterious mission are 80-percent go, weather-wise, in Florida. This is SpaceX's third classified mission, and arguably its most secretive flight for the US military. All that is publicly known about the Zuma payload is that it is a satellite manufactured for the US government by Northrop Grumman, and it is bound for low-Earth orbit.

After a record 18 launches in 2017, this mission will be SpaceX's first flight of the new year. After the launch, the rocket's first stage will attempt to make a return to a landing site along the Florida coast for what would be the company's 21st rocket landing. (SpaceX has had 17 consecutive successful landings, so they're getting pretty good at this).

Success on Sunday will clear the way for SpaceX's next launch from Florida, the much anticipated test flight of the Falcon Heavy booster. The rocket was rolled to the launch pad for tests in late December, and a critical static firing of its 27 engines should come within a week or so of the Zuma mission. Although a launch date has not been set, it is likely to occur in late January or early February.

As usual, SpaceX will stream the launch of the Falcon 9 mission, and the webcast should begin about 15 minutes before the launch window opens.