A mysterious American space plane, codenamed X-37B, has returned from space after 15 months in orbit.

The 'mini space shuttle', which is also known as the Orbital Test Vehicle-2, was unmanned and made an automatic landing from space.

The American military has never said what the plane was intended to do, or what its ultimate mission objectives may be.

The plane launched in 2010 atop an Atlas V rocket from Cape Canaveral, and landed at the Vandenberg Air Force Base, north of Los Angeles after more than 400 days in space - much longer than was originally intended.

The mission was called a "spectacular success" by General William Shelton, commander of Air Force Space Command.

"Although I can't talk about mission specifics, suffice it to say this mission has been a spectacular success," he said in May.

The plane has caused an inevitable tide of speculation and conspiracy theories about its mission to erupt. Some have said it could be monitoring Chinese progress in space, or else being used to target secret military satellites.

The US military will not say what the plane is for, but a third mission is now on the cards to launch in the autumn.

The next version of the craft - the X37-C - is said to be about twice as big as the previous craft, with space to carry astronauts if needed.

Loading Slideshow...
  • X-37B Unmanned Space Plane

    This NASA image obtained on December 1, 2010 shows an artist's rendition of the X-37B as it might look like orbiting Earth. The purpose of the Space Plane's mission has been kept a secret by the US Air Force. The plane is scheduled to return to Earth as early as December 3,2010 Air Force officials said on November 30, 2010. The X-37B, also known as the Orbital Test Vehicle 1, was launched atop an Atlas 5 rocket on April 22, 2010. It has been circling the Earth since then and performing a mission that has been covered in secrecy. (GETTY)

  • 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-37B Unmanned Space Plane

    The second Boeing X-37B Orbital Test Vehicle, built for the U.S. Air Force, is shown here during encapsulation within the United Launch Alliance Atlas V 5-meter fairing at Astrotech in Titusville, Fla., on Feb. 8, 2011. (Boeing)

  • 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 Unmanned Space Plane

    This picture provided by the US Air Force shows personnel inspecting the X-37B, the Air Force's first unmanned re-entry spacecraft, after landing on December 3, 2010 at Vandenberg Air Force Base in California. The X-37B, named Orbital Test Vehicle 1 (OTV-1), conducted on-orbit experiments for more than 220 days during its maiden voyage, and fired its orbital maneuver engine in low-earth orbit to perform an autonomous re-entry before landing, according to the Air Force. (Getty)

  • 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)

  • X-37B Unmanned Space Plane

    This picture provided by the US Air Force shows personnel inspecting the X-37B, the Air Force's first unmanned re-entry spacecraft, after landing on December 3, 2010 at Vandenberg Air Force Base in California. The X-37B, named Orbital Test Vehicle 1 (OTV-1), conducted on-orbit experiments for more than 220 days during its maiden voyage, and fired its orbital maneuver engine in low-earth orbit to perform an autonomous reentry before landing, according to the Air Force. (Getty)

  • This picture provided by the US Air Force shows personnel inspecting the X-37B, the Air Force's first unmanned re-entry spacecraft, after landing on December 3, 2010 at Vandenberg Air Force Base in California. The X-37B, named Orbital Test Vehicle 1 (OTV-1), conducted on-orbit experiments for more than 220 days during its maiden voyage, and fired its orbital maneuver engine in low-earth orbit to perform an autonomous reentry before landing, according to the Air Force. (Getty)

  • X-37B Unmanned Space Plane

    This picture provided by the US Air Force shows personnel inspecting the X-37B, the Air Force's first unmanned re-entry spacecraft, after landing on December 3, 2010 at Vandenberg Air Force Base in California. The X-37B, named Orbital Test Vehicle 1 (OTV-1), conducted on-orbit experiments for more than 220 days during its maiden voyage, and fired its orbital maneuver engine in low-earth orbit to perform an autonomous reentry before landing, according to the Air Force. (Getty)

  • X-37B Unmanned Space Plane

    This picture provided by the US Air Force shows personnel inspecting the X-37B, the Air Force's first unmanned re-entry spacecraft, after landing on December 3, 2010 at Vandenberg Air Force Base in California. The X-37B, named Orbital Test Vehicle 1 (OTV-1), conducted on-orbit experiments for more than 220 days during its maiden voyage, and fired its orbital maneuver engine in low-earth orbit to perform an autonomous reentry before landing, according to the Air Force. (Getty)

  • X-37B Launch

    A United Launch Alliance Atlas V rocket with the U.S. Air Force's second Orbital Test Vehicle (OTV-2) launches from Space Launch Complex 41 at Cape Canaveral Air Force Station at 5:46 p.m. Eastern time on March 5, 2011. (Boeing).

  • X-37B Unmanned Space Plane

    The second Boeing X-37B Orbital Test Vehicle, built for the U.S. Air Force, is shown here during encapsulation within the United Launch Alliance Atlas V 5-meter fairing at Astrotech in Titusville, Fla., on Feb. 8, 2011. (Boeing)

  • X-37B Unmanned Space Plane

    The X-37B is shown here after landing at 1:16 a.m. Pacific time on December 3, 2010, concluding its more than 220-day experimental test mission. It was launched from Cape Canaveral Air Force Station, Fla., on April 22, 2010. (Boeing)