SpaceX is launching its secret payload today: Here's what we know

SpaceX plans to launch a secret payload known as Zuma on Thursday evening (Nov. 16), from NASA's Kennedy Space Center (KSC) on Florida's Space Coast.

And when I say secret, I mean secret; everybody involved with the mission is pretty tight-lipped about it. Here's a brief rundown of what we know. (Spoiler alert: It isn't much.)

Zuma is a U.S. government payload

Aerospace and defense company Northrop Grumman has confirmed that it procured Zuma's launch atop a SpaceX Falcon 9 rocket for the U.S. government. But it's unclear which agency is in charge of the Zuma project. [The Rockets and Spaceships of SpaceX (Photos)]

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Such secrecy is atypical, even if Zuma happens to be a sensitive national-security satellite. (And we don't know that it is; Northrop Grumman has simply described Zuma as a "restricted payload.")

For example, SpaceX has two national-security launches under its belt, and in both cases basic details about the mission were announced. One flight, in May 2017, lofted a satellite for the National Reconnaissance Office, which builds and operates the nation's fleet of spy satellites. The other, which launched in September, launched the robotic X-37B space plane for the U.S. Air Force.

Zuma has a two-hour launch window

One thing SpaceX has announced is the launch window for Zuma. The payload will launch some time during a two-hour window that opens at 8 p.m. EST (0100 GMT) and closes at 10 p.m. EST (0300 GMT). This suggests that getting Zuma to its destined orbit does not require the instantaneous launch window some satellites and spacecraft have needed in the past. So SpaceX may have some flexibility to handle any simple glitches or temporary weather issues that pop up late in the countdown.

If SpaceX is unable to launch Zuma Thursday night, the company does have a backup window available on Friday. That backup window is also two hours long and opens at 8 p.m. EST.

Zuma had an earlier launch date

SpaceX was initially expected to launch Zuma today (Nov. 15) during the same two-hour launch window mentioned above. But early this morning, the company announced a 24-hour delay for the mission.

While an exact reason for the delay was not revealed, SpaceX representatives did say in a statement that they would "use the extra day to conduct some additional mission assurance work in advance of launch."

Zuma is headed for low-Earth orbit

Northrop Grumman did reveal that the Falcon 9 will deliver Zuma to low-Earth orbit (LEO), a range of altitudes that extends up to about 1,240 miles (2,000 kilometers) above the planet. Objects in LEO zoom around Earth frequently, completing one lap roughly every 90 to 120 minutes, depending on their altitude.

This destination doesn't tell us much about what Zuma may be doing, however, for the denizens of LEO are a varied lot. Spy satellites rub shoulders there with communications spacecraft and weather satellites (which often zoom low over the planet's poles), as well as the International Space Station and the X-37B (when it's aloft).

Launching Zuma will not use all of the Falcon 9's fuel

Shortly after liftoff, SpaceX plans to bring the first stage of the two-stage Falcon 9 down for a soft touchdown at Landing Zone 1, a facility at Cape Canaveral Air Force Station, which is next door to KSC. Such landings are part of SpaceX's effort to develop and fly reusable rockets and spacecraft. So far, the company has successfully landed Falcon 9 first stages 19 times and reflown a booster on three separate occasions.

Falcon 9 first stages need to have a bit of fuel left to maneuver back for a landing, so we can assume that the Zuma launch won't tap out the booster completely. You can read a full breakdown of the launch countdown and post-liftoff timeline for Zuma in SpaceX's press kit here.

But, again, this doesn't tell us much.

SpaceX generally forgoes Falcon 9 landing attempts only during missions that haul very heavy payloads to distant geostationary transfer orbit (GTO), and Zuma is going to the much closer LEO. (During some GTO missions from the Space Coast, the first stage does have enough fuel left to land — but on "drone ships" in the Atlantic Ocean, not all the way back at Landing Zone 1.)

So, we can't really make any informed guesses about Zuma's mass.

You can watch Zuma launch online

Here's perhaps one of the most exciting things we just learned Wednesday: SpaceX will webcast the Zuma launch.

Despite all of the secrecy around Zuma, SpaceX has set up a webcast for the mission and added a YouTube stream on the company's YouTube feed. The webcast will begin about 15 minutes before launch, which would be 7:45 p.m. EST (0045 GMT) for an 8 p.m. EST launch target.

You can also watch the launch live here, courtesy of SpaceX's webcast feed.

Well, that's about the extent of it. If we learn anything else, we'll be sure to let you know!

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