Elon Musk's SpaceX to launch the US military's ultra-secretive X-37B spaceplane in its longest-ever flight

• US Space Force is teaming up with SpaceX to launch its X-37B spaceplane on Sunday.
• The vehicle carries secretive cargo and can fly in orbit for hundreds of days at a time.

The US Space Force's top-secret X-37B spaceplane is gearing up to launch on board a SpaceX rocket this weekend.

The pilotless vehicle is scheduled to launch from the Kennedy Space Center in Florida on Sunday 10th December, starting at around 8:14 p.m. ET, per Space.com.

X-37Bs are reusable spaceplanes that can fly for hundreds of days at a time, carrying civilian and military experiments on board. They launch vertically on top of rockets, but glide back down to Earth to land on a runway.

Their long-duration missions are shrouded in secrecy, prompting speculation that they could be used for spying by the US military.

Statements released by the US Space Force suggest this flight could be longer and reach greater heights than its previous mission when it stayed in orbit for 908 days.

First run by NASA, the project became classified when it was taken over by the Defense Advanced Research Projects Agency (DARPA) in 2004.

Since 2020, the missions have been run by the US Space Force — the government's space-defense branch of the military formalized by former President Donald Trump.

A statement from the US Space Force noted that X-37B will be "operating in new orbital regimes" during this mission.

X-37B will be riding to space on top of SpaceX's Falcon Heavy for the first time. Falcon Heavy is the world's second most powerful operational rocket. It offers three times more lift than the Falcon 9 and Atlas 5, the crafts that previously launched the X-37B missions, New Scientist reported.

Altogether, this suggests that X-37B could attempt to reach higher and fly longer than ever before during its seventh flight, New Scientist reported.

"We are excited to expand the envelope of the reusable X-37B's capabilities, using the flight-proven service module and Falcon Heavy rocket to fly multiple cutting-edge experiments for the Department of the Air Force and its partners," said Lieutenant Colonel Joseph Fritschen, the X-37B Program Director, in a statement.

If all goes to plan, this mission will add to the spaceplanes' impressive flight time. Since first launched in April 2010, X37B have accrued more than 3,770 days in space, per the Space Force's statement.

There are two known X-37Bs in operation, developed in partnership with Boeing.

The upcoming flight, dubbed OTV-7, X-37B will carry "groundbreaking" experiments that will "equip the United States with the knowledge to enhance current and future space operations," said General B. Chance Saltzman, Chief of Space Operations at Space Force.

That includes a NASA experiment to study the effect of long-term cosmic radiation exposure on seeds brought in flight.

Another experiment will test "future space domain awareness technologies," per the statement. The exact nature of this cargo was not disclosed, but it is likely to relate to the detection and monitoring of crafts floating around the Earth that could threaten national or commercial space systems.

During X-37B's last flight, which touched down in November 2022, the plane successfully beamed solar energy from space.