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| Space plane: Mysterious US military plane returns to Earth | Space plane: Mysterious US military plane returns to Earth |
| (about 3 hours later) | |
| An unmanned US plane on a top-secret, two-year mission to space has returned to Earth and landed in California. | An unmanned US plane on a top-secret, two-year mission to space has returned to Earth and landed in California. |
| The aircraft, resembling a miniature space shuttle and known as the Orbital Test Vehicle or X-37B, spent 674 days in orbit around the planet. | The aircraft, resembling a miniature space shuttle and known as the Orbital Test Vehicle or X-37B, spent 674 days in orbit around the planet. |
| It was the unmanned plane's third space flight, but its mission has been shrouded in mystery. | |
| A theory that it was taking a look at China's space lab has been downplayed by experts. | |
| Air Force officials have only told US media the aircraft performs "risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies". | |
| The X-37B programme, started in 1999 and is currently run by the Air Force's Rapid Capabilities Office. | |
| The first plane flew in April 2010 and returned after eight months. The second launched in March 2011 and remained in space for 15 months. | |
| The current aircraft - built by Boeing - uses solar panels for power in orbit, measures over 29ft (9m) long, has a wingspan of nearly 15ft and a weight of 11,000lbs (4,989 kg). | |
| It looks like a mini space shuttle and can glide back down through the atmosphere to land on a runway, just like Nasa's re-usable manned spaceplane used to do before its retirement. | |
| A fourth X-37B mission is said to be planned for launch from Cape Canaveral, Florida, in 2015. | |
| Europe is to test its own automated space plane technology in the coming weeks. | |
| The IXV vehicle will be launched into space atop a rocket from French Guiana and then make its way back through the atmosphere to splash down in the ocean. | |
| BBC News science correspondent Jonathan Amos | BBC News science correspondent Jonathan Amos |
| The reality is that no-one really knows what this vehicle does. The only credible explanation I have seen is that it is testing technologies that could find their way on to future satellite missions. | |
| If you consider how expensive a satellite mission is - several hundred million dollars - you'd like to be sure that any innovations are going to work straight out of the box. | If you consider how expensive a satellite mission is - several hundred million dollars - you'd like to be sure that any innovations are going to work straight out of the box. |
| By flying early prototypes on the X-37B, you can test these technologies so that when you put them on future satellite missions, you can be sure they will deliver. | By flying early prototypes on the X-37B, you can test these technologies so that when you put them on future satellite missions, you can be sure they will deliver. |
| Nasa recently agreed to give over work space formerly used to service the shuttles at the Kennedy Space Center to the X-37B programme, which tells us this is a long-term project for the Air Force. Whatever it is they are doing up there, they deem it to be high value. | Nasa recently agreed to give over work space formerly used to service the shuttles at the Kennedy Space Center to the X-37B programme, which tells us this is a long-term project for the Air Force. Whatever it is they are doing up there, they deem it to be high value. |