Mysterious US space plane arrives back after long, unexplained mission

'Each successive mission advances our nation's space capabilities,' says Air Force, without explaining how

The US's mysterious space plane has come back to Earth – and still nobody will explain what it is for.

The craft, known as X-37B, has been in the sky since 2017.

When it dropped down at Nasa's Kennedy Space Center over the weekend, it brought an end to the 780-day mission that is the longest ever carried out by the reusable vehicle.

The Air Force said the mission was a success and the craft had achieved its objectives, though not what those objectives are.

The X-37B has been on five such flights in the past.

The next is planned next year, when it will launch into space from Cape Canaveral.

"Each successive mission advances our nation's space capabilities," said Air Force secretary Barbara Barrett. But she, like everyone else involved wit the project, has not said what those capabilities are or what happens in those missions.

The X-37B looks like a space shuttle but it a quarter of the size, and is only 29 feet long.

It is launched on a rocket but then flies around above the Earth on its own, conducting work including experiments from the Air Force Research Laboratory.