SpaceX is gearing up to launch a third top-secret spacecraft for the US government, a mission the company calls "Zuma" but has said little else about.

The mystery satellite is slated to lift off from Florida's Kennedy Space Center on Friday between 8 pm and 10 pm ET, and SpaceX plans to broadcast the event live via YouTube starting about 15 minutes before launch. (You can watch the video feed at the end of this post.)

SpaceX, the aerospace company founded by tech mogul and Mars-colonising hopeful Elon Musk, initially planned to launch the clandestine spacecraft on Wednesday.

However, SpaceX delayed the launch to Thursday and then to Friday, according to Florida Today.

The reason for the delay doesn't appear to be weather-related, given the current forecast. Previous delays were made "to allow engineers to complete additional mission assurance work," according to Spaceflight Now.

SpaceX has loaded the Zuma payload atop a reusable Falcon 9 rocket.

After lugging the satellite a few dozen miles above Earth, the rocket's huge booster - its largest and most expensive part - will detach and attempt to land at Cape Canaveral Air Force Station. Then the second-stage rocket will fire up and finish pushing Zuma into orbit.

A mission without a customer?

SpaceX/Flickr

The Zuma mission is more secretive than most, as public filings don't even mention the launch customer paying SpaceX.

It's not clear whether the satellite is owned by the US military or a commercial entity. The National Reconnaissance Office typically launches spy satellites, but a representative told Aviation Week that Zuma doesn't belong to the NRO.

SpaceX declined to answer questions about the Zuma mission, but Northrop Grumman - one of the largest defence contractors in the world - has acknowledged that they're playing a role.

Lon Rains, the communications director for Northrop Grumman's space systems division, sent Business Insider this statement:

"Northrop Grumman is proud to be a part of the Zuma launch. This event represents a cost effective approach to space access for government missions. The US Government assigned Northrop Grumman the responsibility of acquiring launch services for this mission. We have procured the Falcon 9 launch service from SpaceX.

As a company, Northrop Grumman realises this is a monumental responsibility and we have taken great care to ensure the most affordable and lowest risk scenario for Zuma.

The Zuma payload is a restricted payload. It will be launched into Low Earth Orbit."

Low-Earth orbit, or LEO, is considered to be less than about 1,000 miles (1,600 km) above the surface of the planet.

Rains declined to provide further detail about the mission, however, cautioning that the company is "not saying anything else or answering any other questions."

Zuma's secrecy has spurred rampant speculation

Satellite trackers found out about the launch via public yet cryptic FCC filings in October, and shared them in a NASA Spaceflight forum thread.

Since then, the vacuum of information has led to evolving speculation about the details and purpose of the launch. NASA Spaceflight's thread about Zuma and a November story from Spaceflight Now have floated a number of ideas and theories:

  • If the National Reconnaissance Office isn't behind Zuma (although Ars Technica claims it is the NRO's), the payload may be for the Central Intelligence Agency, the National Security Agency, or some other non-military US government agency.
  • Estimates of Zuma's orbit around Earth may be good for spying on China and North Korea, though its trajectory can't be confirmed until after launch.
  • The predicted orbit is similar to that if the NRO's USA-276 (or NROL-76) satellite. With Zuma launching around 8:00 pm ET, it could go into orbit less than 10 minutes behind USA-276 - very close in terms of outer space.
  • This has led some to suggest Zuma may be a refuelling mission for USA-276, or part of a spy satellite constellation or program associated with that mission.

Watch the launch live

You can watch the launch live via SpaceX's YouTube feed around 8 pm ET on Friday.

This article was originally published by Business Insider.