The United States’ hush-hush robot spaceplane, the mysterious X-37B, is due to touch down at the US military space base in California this week, following a 22-month clandestine mission in orbit above undisclosed nations.

The US Air Force has so far launched two of the clandestine spaceships, which look a little like mini-space shuttles,on three missions over the last few years. The first X-37B has blasted off twice, while the second has done one trip so far.

Spokespersons at Vandenberg Air Force Base - the US military spaceport on the West Coast - said in a statement that the current X-37B mission, also known as Orbital Test Vehicle 3 or OTV-3, would be landing this week. However, the exact time and date will depend on weather and technical factors.

The X-37Bs are around nine metres long and are referred to as experimental vehicles, though again, the exact use for these unmanned reusable space planes is unknown - the more so as the payload is likely to be different for every flight. The US military has sought to suggest that the idea of the little spaceplanes is to get new space technologies - for instance new surveillance instruments for spy satellites - into space quickly to try them out, without the need to build a dedicated one-shot satellite every time. The X-37B also offers the option of bringing such trial equipment back down again for adjustments, unlike a normal spacecraft.

Some observers, however, have pointed out that this sort of thing wouldn't require the X-37Bs to have their Shuttle-style delta wings. Indeed, there might not be any need for runway landing at all if all that was desired was recoverability - parachutes as used on various manned capsules could do the job.

It would appear that if the US military so desired, the X-37Bs with their wings and resulting "cross range" capability could perhaps carry out certain unusual covert missions originally planned for - but never carried out by - the space shuttles. This potential capability might be the reason why the X-37B was dubbed a "space warplane" by Iranian state media channel Press TV.

The first OTV mission stayed in space for eight months and that was prolonged to 15 months after a second launch in March 2011. This third mission will be the longest time the craft has spent in space before coming in to land. The X-37Bs, unlike the shuttles, have a solar array and thus are not limited in endurance by electrical power. They also carry a large tank of manoeuvring fuel, perhaps allowing a lot of sneaky orbit changing to escape observation from below.

Last week, the Air Force and NASA signed a contract to relocate the X-37B from California to the Kennedy Space Centre in Florida, but this time, the ship will land at Vandenberg as usual. ®