Update January 7th, 8:20PM ET: Around 8 minutes after launch, the first stage of the Falcon 9 successfully landed at SpaceX’s landing zone at Cape Canaveral, Florida. That brings SpaceX’s total number of successful landings to 21 and continues the company’s success streak of landing rockets on land. SpaceX did not provide coverage of the payload’s deployment, but confirmed that the nose cone surrounding the satellite did separate successfully.

Original story: 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.

“Northrop Grumman realizes that this is monumental responsibility.”

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 for the launch, and Northrop Grumman ultimately picked the Falcon 9.

“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 a statement to The Verge. Northrop Grumman has not released any further information on the spacecraft.

It’s not the first time that SpaceX has launched secretive payloads into orbit. After receiving certification in 2015 to launch military satellites, the company has already launched two classified payloads, and is slated to launch even more over the next couple of years. However, all of SpaceX’s missions for the military have known customers, such as the US Air Force or the National Reconnaissance Office. So far, no government office has claimed the Zuma mission. And the NRO, which usually announces the launches of its spy spacecraft, said that Zuma doesn’t belong to the agency.

Apart from its super unique payload, this SpaceX launch is otherwise routine. When the Falcon 9 flies, it will attempt to land at SpaceX’s ground-based Landing Zone 1, located at the Cape. If that touchdown is successful, it’ll mark the eighth ground recovery for the company, and the 21st landing SpaceX has pulled off to date.

Zuma also marks SpaceX’s debut mission of 2018, jumpstarting what could be a busy year for the company. SpaceX plans to do even more launches this year than it did in 2017, which saw a total of 18 missions — the most the company has ever done in a single year. And once this mission is complete, SpaceX’s next scheduled launch is the eagerly anticipated first flight of the Falcon Heavy — an upgraded, heavy-lift version of the Falcon 9.

Zuma’s takeoff is set for sometime between 8PM and 10PM ET on Sunday. SpaceX was originally planning for a Thursday launch, but it delayed the mission by multiple days to test out propellant loading of the rocket on the changed launchpad. The company also said the recent bout of cold weather slowed tests down at the Cape. Live coverage of the launch usually starts 15 minutes prior to takeoff, but given the flight’s secrecy, chances are the live broadcast won’t follow the satellite’s deployment. Check back on Sunday to watch as much of this mission as we can live.

Update January 5th, 2018 8:45AM ET: This post was updated to include the new January launch date and again to reflect delays in the launch.

Update November 17th, 2017 4:15PM ET : On Thursday, SpaceX stated that it had decided to stand down from the launch as it reviewed data of a fairing test the company did for another customer. SpaceX said it still had the opportunity to launch on Friday, but that the launch might not happen depending on how long it takes the company to review the test data. Now it looks like SpaceX won’t be launching in the next couple days, and the company will come up with a new launch date soon.