A top-secret robotic space plane owned by the US military and with an unidentified payload and mission has now been in orbit above Earth for five months.

The unmanned 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 - though what this is a mystery.

The craft, known as Orbital Test Vehicle-3 (OTV-3), took off from Cape Canaveral on 11 December using an Atlas V rocket.

It's not the first time the X-37B has been into space - the same one spent 225 mysterious days in space in 2010 before landing automatically in California.

Whatever its mission, having a US military craft in the skies above us all is a slightly unnerving thought.

Loading Slideshow...
  • 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)