The US military has had a robotic space plane in orbit around Earth since December - and no one knows what it's doing.

The US Air Force's X-37B space craft is a reusable, currently unmanned vehicle similar - though smaller - than the retired Space Shuttle.

The Boeing-built X-37B is about 30 feet long, 15 feet wide, weighs 11,000 pounds and can carry about the same load as a delivery van.

It took off from Cape Canaveral on 11 December, carried by an Atlas V rocket.

It has now been in space for more than two months, and shows no signs of returning.

It is the same vehicle which spent 225 mysterious days in space in 2010 before landing automatically in California - and as such marks the first time the US Navy has successfully reused space hardware.

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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)

  • 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)

  • X-37 Orbital Test Vehicle

    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)

  • 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)

  • 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)

  • X-37 Orbital Test Vehicle

    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)

  • FILE - This undated file image provided by the U.S. Air Force shows the X-37B spacecraft. 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/U.S. Air Force, File)

  • FILE - This undated file image provided by the U.S. Air Force shows the X-37B spacecraft. 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/U.S. Air Force, File)

  • X-37B Orbital Test Vehicle

    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)

  • 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)


Officially named USA-240, the current mission is also known as Orbital Test Vehicle-3 since it is the third time the craft has flown.

A report by Space.com did not turn up details about what the space plane is doing - but still makes for fascinating reading.

Among the choice quotes is this by Air Force Maj. Eric Badget, who says simply: "The mission is ongoing".

Enlightening.

In a fact sheet, the Air Force says simple that the X-37B is developed "to demonstrate a reliable, reusable, unmanned space test platform for the United States Air Force".

It is commanded by the 3rd Space Experimentation Squadron, 21st Space Wing, of the Air Force Space Command in Colorado.

Read the full story at Space.com.