2:30pm ET Tuesday update: The Zuma launch attempt has slipped 24 hours, to Friday evening.

Original post: Fresh off a year with a record 18 orbital launches, SpaceX is set to try its first mission of the new year on Thursday. Less than two weeks since its last flight, the company returns to Space Launch Complex-40 at Cape Canaveral Air Force Station in Florida for the launch of the secretive "Zuma" mission for the US government.

Originally planned for November, an undisclosed issue with the Falcon 9 rocket's fairing caused SpaceX to delay the launch for several weeks and eventually move the date forward to January 4. The fairing is the cap at the top of the rocket that protects its payload during the dynamic launch environment; it is then jettisoned once the spacecraft climbs above Earth's atmosphere.

All that is publicly known about the Zuma payload is that it was manufactured for the US government by Northrop Grumman and bound for low-Earth orbit. One source told Ars that SpaceX is launching the payload for the National Reconnaissance Office, but the NRO has denied that Zuma is its satellite. (But really, would you expect anything less from the spy satellite agency?)

Whatever the payload entails, weather conditions appear to be fine for Thursday's launch attempt. The rocket's launch window opens at 8pm ET and closes at 10pm ET, and forecasters predict a 90 percent chance of "go" conditions at the Cape. The first stage will then attempt a land-based landing along the Florida coast.

Success Thursday 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 two. Although a launch date has not been set, it is likely to occur in late January or early February.