Image copyright USAF Image caption The prior flights of the X-37B garnered worldwide attention - but few details

A famously mysterious military space plane operated by the US Air Force has launched from Florida, the third flight in a secretive test programme.

The reusable, unmanned craft is designed to operate in Earth orbit for extended periods. Its prior missions in 2010 and 2011 lasted 224 and 469 days.

The US government kept the timing of Tuesday's launch secret and has not said how long the mission will last.

That has prompted fevered speculation as to the craft's ultimate purpose.

X-37B - American military spaceplane Image copyright Other Mission: Described as a re-usable testbed for new sensors and other space technologies

Described as a re-usable testbed for new sensors and other space technologies Length: 9m Wingspan: 4.5m Height: 3m Mass: 5,000kg

9m 4.5m 3m 5,000kg Origins: Started as a Nasa project in 1999 before being handed to the military in 2006

Started as a Nasa project in 1999 before being handed to the military in 2006 Operating altitude: 180 - 800 km

180 - 800 km Cost: The budget line for the X-37B programme continues to be classified information

Tuesday's launch had been pushed back from October, delayed by two satellite launches. Patrick Air Force Base in Florida gave notice of a hazard from a launch in a window between 15:45 and 22:15 GMT (10:45 to 17:15 local time).

It is the second flight for X-37B craft launched on Tuesday, one of two in the programme, the other of which flew for the first time in 2011. Designed by aerospace giant Boeing, the craft share more than just a passing similarity to the now-retired space shuttle.

It is just a quarter the size of the shuttle, but is launched on a rocket - the Atlas V. It is coated in thermal tiles to withstand the heat of re-entry, after which it lands on its own gear autonomously.

The stated mission of the craft, according to the US Air Force, is an "experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform".

But the latest mission in particular sparked speculation that the craft was spying on the Chinese space lab Tiangong-1 - an idea that has since been largely discredited.

When it returned from its second mission in June, programme manager Lt Col Tom McIntyre said: "We knew from post-flight assessments from the first mission that OTV-1 could have stayed in orbit longer. So one of the goals of this mission was to see how much farther we could push the on-orbit duration."

But any official mission objectives seem set once again to remain secret.