US Air Force Secret X37-B Space Plane Has Been In Orbit 500 Days

America's Secret Space Plane Has Been In Orbit 500 Days
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A secret robotic space plane developed by the US Air Force has just recorded its 500th day in orbit around Earth.

And nobody knows what it's for.

The X37-B mini space shuttle is a relatively small reusable space plane, designed to orbit the Earth and feed back to military commanders on the ground.

The mysterious craft is unmanned, obviously, but its actual functions are still heavily classified.

The most recent craft launched on 11 December 2012 and has not yet returned to Earth, smashing its previous records for long-term flight above the atmosphere.

"Clearly, and understandably, the Air Force initially set the expected parameters [of the X-37B] conservatively, not really knowing what it could do until it was tested — which is what they are doing now," said Joan Johnson-Freese, professor of national security affairs at the Naval War College, to Space.com.

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X-37B Orbital Test Vehicle(01 of08)
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FILE - This Feb. 8, 2011 file image provided by the U.S. Air Force shows the X-37B during encapsulation within the United Launch Alliance Atlas V 5-meter fairing in Titusville, Fla. The unmanned Air Force space plane steered itself to a landing early Saturday, June 16, 2012, at a California military base, capping a 15-month clandestine mission. (AP Photo/US Air Force, File) (credit:AP)
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A United Launch Alliance Atlas V rocket, carrying an X-37B experimental robotic space plane, lifts off from launch complex 41 at the Cape Canaveral Air Force Station, Tuesday, Dec. 11, 2012, in Cape Canaveral, Fla. Air Force officials said the unmanned space plane, which resembles a miniature space shuttle, provides a way to test technologies in space.(AP Photo/John Raoux) (credit:AP)
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This photo released by Vandenberg Air Force Base Monday June 18,2012, shows the X-37B Orbital Test Vehicle, the Air Force's unmanned, reusable space plane, after it landed at Vandenberg Air Force Base early Saturday June 16, 2012. The test vehicle which launched from Cape Canaveral March 5, 2011, conducted on-orbit experiments for 469 days during its mission. The X-37B is the newest and most advanced re-entry spacecraft. (AP Photo/Vandenberg Air Force) (credit:AP)
X-37 Orbital Test Vehicle(04 of08)
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This March 30, 2010 photo made available by the U.S. Air Force via NASA shows the X-37B Orbital Test Vehicle during testing at the Astrotech facility in Titusville, Fla. On Tuesday, Dec. 11, 2012, the Air Force launched the top-secret, unmanned mini-space shuttle from Cape Canaveral, Fla. (AP Photo/U.S. Air Force via NASA) (credit:AP)
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This Saturday, June 16, 2012 image from video made available by the Vandenberg Air Force Base shows an infrared view of the X-37B unmanned spacecraft landing at Vandenberg Air Force Base. The spacecraft, which was launched from Cape Canaveral Air Force Station in Florida in March 2011, conducted in-orbit experiments during the 15-month clandestine mission, officials said. It was the second such autonomous landing at the base. (AP Photo/Vandenberg Air Force Base) (credit:AP)
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A United Launch Alliance Atlas V rocket stands ready for launch on the Complex 41 pad at the Cape Canaveral Air Force Station, Monday, Dec. 10, 2012, in Cape Canaveral, Fla. The Atlas V rocket, scheduled to launch on Tuesday, will deploy the U.S. military's X-37B, a prototype spaceplane also called the Orbital Test Vehicle.(AP Photo/John Raoux) (credit:AP)
X-37 Orbital Test Vehicle(07 of08)
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This April 5, 2010 photo made available by the U.S. Air Force via NASA shows the X-37B Orbital Test Vehicle at the Astrotech facility in Titusville, Fla. Half of the Atlas V five-meter fairing is in the background. On Tuesday, Dec. 11, 2012, the Air Force launched the top-secret, unmanned mini-space shuttle from Cape Canaveral, Fla. (AP Photo/U.S. Air Force via NASA) (credit:AP)
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This Saturday, June 16, 2012 image from video made available by the Vandenberg Air Force Base shows the X-37B unmanned spacecraft landing at Vandenberg Air Force Base. The spacecraft, which was launched from Cape Canaveral Air Force Station in Florida in March 2011, conducted in-orbit experiments during the 15-month clandestine mission, officials said. It was the second such autonomous landing at the base. (AP Photo/Vandenberg Air Force Base) (credit:AP)

The craft looks just like the retired Space Shuttle, but is just 29 feet long and 10 feet tall. It launches vertically on a rocket and returns via a robotic horizontal landing.

The US Air Force has so far sent three missions into orbit with the solar-powered plane, and there is no telling how long this one will last. The best guess from most experts is that the plane is being used to test new reconnaissance equipment.

Flying at a low altitude of 218 miles and a range of around 43.5 degrees north and south latitudes, it is thought the plane is mostly able to provide surveillance of the Middle East, rather than Europe, Russia or China.