On Sunday, SpaceX is set to launch perhaps its most secretive payload yet: a classified government satellite built by defense contractor Northrop Grumman. The purpose of the mission, codenamed Zuma, is essentially unknown. It’s unclear what kind of spacecraft is going up, or which government agency the launch is for. All we really know is that Zuma is scheduled to go into lower Earth orbit on top of a Falcon 9 rocket out of Cape Canaveral, Florida.

The launch has had some trouble getting off the ground, though. The mission was originally supposed to launch in late November, but was repeatedly delayed for unknown reasons. Then on November 17th, SpaceX decided to stand down from the mission for a while as it reviewed data from a test that the company did for another customer. Now, it seems that review is complete and SpaceX is ready to try again, though this time the company is switching up launchpads for the mission. Zuma will launch from SpaceX’s recently renovated pad at Cape Canaveral Air Force Station instead of the company’s nearby pad at NASA’s Kennedy Space Center like originally planned in November.

The Zuma mission became public in October 2017, when NASASpaceflight.com reported on documents that SpaceX had filed with the Federal Communications Commission, requesting authorization for a mysterious “Mission 1390.” A few days later, several news outlets confirmed that the flight, also called Zuma, would launch a Northrop Grumman-made payload. The contractor had been assigned by the US government to find a rocket…