SpaceX is preparing to send into space a satellite for the US government that is so secret the public cannot know even which branch of the administration commissioned the launch.
A weather report ahead of the launch released on Tuesday described the conditions as excellent.
Unlike the private aerospace company's previous classified launches for the military's National Reconnaissance Office and the super-secret space-plane it took into orbit for the Air Force, there is almost no information available about the "Zuma" payload.
Zuma is known to be a low Earth orbit satellite (orbiting within 2,000km) which is an orbit necessary both for spy and military communication satellites.
The secrecy surrounding the launch and the involvement of defence contractor Northrup Grumman in building and operating the spacecraft has led many to speculate that it is defence related.
The National Reconnaissance Office has denied that Zuma belongs to them.
The launch window opens at 8pm local time on Thursday at Cape Canaveral in Florida.
SpaceX rocket landing
Video:
SpaceX rocket lands itself minutes after launch
After delivering the satellite into orbit, the Falcon 9 rocket is planned to return to a different launchpad at Cape Canaveral in Florida.
Where the company's competitors are forced to build new rockets for every launch, SpaceX has managed to achieve an economy of business by reusing previously developed craft.
This has helped the company to be valued at $21bn (£16bn), making it one of the most valuable privately-owned companies in the world - and drives down the launch cost for companies such as Northrup Grumman.
According to Northrup Grumman's communications director, Lon Raid: "This event represents a cost effective approach to space access for government missions.
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"As a company, Northrop Grumman realises that this is a monumental responsibility and has taken great care to ensure the most affordable and lowest risk scenarios for Zuma."
The launch had been pushed back despite a US Government desire to launch Zuma before November of last year.
