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Why would the American government put an unmanned military space plane in a constant orbit around our planet without a public mission plan?

Experts have suggested the United States Air Force's orbiting X-37B space plane could function as an astronaut ambulance in case of a medical emergency aboard the International Space Station.

At present, astronauts have to rely on a Russian Soyuz rocket to taxi them to the space station and back. Each rocket is launched from the Baikonur Cosmodrome in Kazakhstan, and relies on a parachute-aided descent to the same location.

(Image: Getty)

"If somebody needs to come home soon from space, coming back on the Soyuz is going to be a pretty challenging ride - at least 4.5 Gs, fairly violent landing, and then you might not be very close to the medical care you need out in Kazakhstan," said former astronaut Stephen Robinson.

Robinson flew four shuttle missions and currently chairs the mechanical and aerospace engineering department at the University of California.

He made the remarks last month during a presentation with NASA's Future In-Space Operations (FISO) working group.

Etan Halberg, one of Robinson's graduate students at the University of California-Davis, suggested that a space plane would fare better than a capsule.

(Image: Getty)

And the mysterious X-37B, which blasted off from Earth on May 20, 2015, could fit the bill.

Robinson and Halberg suggested that a new version, with a bigger payload bay, could be developed to accommodate a pilot, patient and medical crew member.

The pair believe that space capsules, like SpaceX's Dragon capsule, wouldn't be as effective as "astronaut ambulances" as the plane. The plane, they argue, can land closer to hospitals and allow the patient to be transferred while lying down.

According to Space.com , Robinson believes this modified version of the X-37B could stay docked to the ISS for long periods of time. It would only be needed if there was a medical emergency. And no-one is sure what the space plane is doing in orbit at the moment anyway.

"Exactly what the X-37B does on orbit for so long is unclear; the space plane's specific activities, and many of its payloads, are classified," explains the site.

"Some people have speculated that the X-37B may be a space weapon of some sort, but the Air Force has always denied that idea, maintaining that the vehicle is just testing a variety of technologies for future spacecraft."

The space plane had already spent a massive 1,367 days - almost four years - in space during three previous missions.

Several articles have been published in the Russian press suggesting it is actually part of a space warfare mission designed to allow America to destroy satellites.

Meanwhile, America has remained tight-lipped about what the space plane is doing.