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The U.S. Air Force's secretive X-37B space plane is set to launch on its fifth classified mission today — the only question is exactly when.

Weather permitting, the space plane will lift off aboard a SpaceX Falcon 9 rocket from Kennedy Space Center in Florida sometime between 9:50 a.m. EDT and 2:55 p.m. EDT (1350-1855 GMT), and you can watch the launch live online.

SpaceX will broadcast the launch live, and you can also watch it live on Space.com, courtesy of SpaceX. But the launch window spans hours, and the webcast will start just 10 minutes before the launch — so stay on your toes. Shortly after the launch, the Falcon 9's first stage will land SpaceX's Landing Zone 1 at the nearby Cape Canaveral Air Force Station. [The  Mysterious X-37B Space Plane: 6 Surprising Facts]

A SpaceX Falcon 9 rocket carrying the U.S. Air Force's X-37B space plane moves to Launch Pad 39A at NASA's Kennedy Space Center in Cape Canaveral, Florida for a Sept. 7, 2017 launch. It will be the fifth classified X-37B space plane mission.
A SpaceX Falcon 9 rocket carrying the U.S. Air Force's X-37B space plane moves to Launch Pad 39A at NASA's Kennedy Space Center in Cape Canaveral, Florida for a Sept. 7, 2017 launch. It will be the fifth classified X-37B space plane mission.
Credit: Ken Kremer/KenKremer.com
The U.S. Air Force's robotic X-37B space plane is a miniature space shuttle capable of long, classified missions in orbit. <a href="http://www.space.com/75-x-37b-spaceplane.html">See how the X-37B space plane works in this Space.com infographic</a>.
The U.S. Air Force's robotic X-37B space plane is a miniature space shuttle capable of long, classified missions in orbit. See how the X-37B space plane works in this Space.com infographic.
Credit: By Karl Tate, Infographics Artist

This flight marks SpaceX's second military launch; in May, a Falcon rocket carried a classified spy satellite into orbit for the Department of Defense.

The Air Force's 45th Space Wing posted an airspace closure advisory spanning a slightly more than a 5-hour window for launch.

Of course, this launch all depends on the weather. According to SpaceNews, the weather is 50 percent "go" for a launch tomorrow but will grow increasingly worse over the following days as Hurricane Irma approaches the area. If SpaceX is unable to launch the X-37B today, it has a backup launch day of Sept. 8.

Though Irma will be 950 miles southeast of the spaceport and won't affect a Thursday launch, a schedule slip to Friday could mean winds of up to 20 mph (32 km/h) by the end of the launch window, Emre Kelly reported for Florida Today.

X-37B space plane  also called the Orbital Test Vehicle (OTV), is one of two known reusable vehicles in the military fleet.

"The fifth OTV mission continues to advance the X-37B's performance and flexibility as a space technology demonstrator and host platform for experimental payloads," Air Force officials said in a statement. "This mission carries small satellite ride shares and will demonstrate greater opportunities for rapid space access and on-orbit testing of emerging space technologies."

They added that the mission will host a payload to test experimental electronics and oscillating heat pipe technologies for use in long-duration missions. It will also be launched into, and landed from, a higher-inclination orbit than in previous missions.

The robotic minishuttle is no stranger to civilian uncertainty; the last X-37B mission finally landed May 7 after 718 days in space, with no prior warning. Each of the four OTV missions, starting in 2010, has stayed in space longer than the previous one.

So there are a few question marks with this mission: Not only are the precise launch time and details of the mission nebulous, but the landing date is also uncertain — although it's safe to guess it won't be anytime soon.

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.