Spacecraft are expensive things. They can take decades to design, and test, and build. And then, apart from the modules that carry their crew back to terra firma, they are lost to the cosmos on their first and final missions.

Why not have a spaceship that can return to Earth to carry out mission after mission?

The earliest concepts of aircraft-like spacecraft were dreamt up by the German scientist Eugene Sänger in 1933. Spaceplane hopes moved to America in the decades to come. In 1959 and for most of the 1960s, Nasa and the US Air Force (USAF), with the help of Sänger’s work, launched their experimental X-15. It didn’t take-off like a rocket or an aeroplane – it was dropped from a USAF bomber, the B-52. Once dropped, the X-15 could use its rocket motors to reach an altitude the air force decided was space, above 50 miles (80.4km).