2pm ET Thursday update: The launch has been postponed a day, until Friday. The launch window opens at 8pm ET. No immediate reason was given for the delay.

Original post: Since earning certification from the Air Force to launch national security payloads—typically spy satellites and military communications satellites—SpaceX has flown two military missions. It launched a classified payload for the National Reconnaissance Office in May and an Air Force space plane in September.

On Thursday, SpaceX will attempt to launch its third and most secretive payload for the military. The mission was not even made public until a launch license was released a month ago, and the company didn't acknowledge the launch until this last week. The payload was developed by Northrup Grumman and has the mysterious name "Zuma," which sounds similar to a carbonated, alcoholic beverage briefly popular in the 1990s, Zima. There are few clues about the name in the launch's mission patch.

Although Ars understands the payload is being launched for the National Reconnaissance Office, the mission's press kit offers absolutely no details about the payload. It simply refers to the payload as the "Zuma spacecraft," which is bound for low-Earth orbit. (And indeed, the NRO has denied that Zuma is its satellite).

There is a general feeling within the company that this is an important launch for SpaceX, as it will be the third national security mission, and if successful it would validate the company as a major player to meet the country's military launch needs at a time when the US Congress is pushing the Air Force to rely on US made rocket engines. Overall, this will be the company's 17th launch of 2017, and it would more than double the company's launch totals in any previous year.

Zuma has a two-hour launch window for its flight from Kennedy Space Center. The window opens at 8pm ET, and a back-up window opens on Friday at the same time. Weather conditions appear to be decent for a launch Thursday. The first stage of the Falcon 9 rocket will attempt a land-based return after sending the payload into orbit. The webcast below should begin about 15 minutes before the launch window opens.