US military's reusable spacecraft lifts off on mystery mission

US military's reusable spacecraft lifts off on mystery mission

Only US military knows the reason its reusable spacecraft is currently orbiting high above Earth

The US military's small, top-secret version of the space shuttle has again rocketed into orbit on a mystery mission, two years after making the first flight of its kind.

The Air Force launched the unmanned spacecraft on Tuesday hidden on top of an Atlas V rocket.

As if on cue, clouds quickly swallowed up the rocket as it disappeared out over the ocean.

It is the second flight for this X-37B spaceplane. The craft circled the planet for seven months in 2010. Another X-37B spacecraft spent more than a year in orbit.

These high-tech mystery machines measure about 8 metres in length, one quarter the size of Nasa's old space shuttles, and can land automatically on a runway.

The two previous touchdowns occurred in Southern California; this one might end on Nasa's 5km long runway, once reserved for the space agency's shuttles.

The military is not saying much if anything about this new secret mission, known as OTV-3, or Orbital Test Vehicle, flight No 3.

But one scientific observer, Jonathan McDowell of the Harvard-Smithsonian Centre for Astrophysics, speculates that the spaceplane is carrying sensors designed for spying and is likely to be serving as a testbed for future satellites. He dismisses rumours of "exotic ideas" for the X-37B such as weaponry or shadowing a Chinese satellite.

While McDowell acknowledged that he does not know what the spaceplane is carrying, he said on-board sensors could be capable of imaging or intercepting transmissions of electronic emissions from terrorist training sites in Afghanistan or other hot spots. "All the sorts of things that spy satellites generally do," he said.

The beauty of a reusable spaceplane is that it can be launched at short notice based on need, McDowell said.

What is important about this flight is that it is the first reflight. "That is pretty cool," McDowell said, "reusing your spacecraft after a runway landing. That's something that has only really been done with the shuttle."

NASA's space shuttles, now retired museum pieces, stretch 37 metres long, and have 24-metre wingspans and weights of more than 77 tonnes.

The X-37B's wingspan is a mere 4.6 metres, and it weighs five tonnes, about the same as a big Ford pickup truck, fully loaded. Once in space, it is solar-powered.

The two previous secret X-37B flights were in orbits 320km to 480km above the Earth, circling at roughly 40-degree angles to the equator, as calculated by amateur satellite trackers.

That means the craft flew over the swath between 40 degrees or so north latitude and 40 degrees or so south latitude, Russia's far north out of the spaceplane's observing realm, McDowell said.

"It might be studying Middle Eastern latitudes or it might just be being used for sensor tests over the United States," he said, speculating that this newest flight will follow suit.