The layer of sky above the cruising height of commercial aircraft and below satellite orbits was, until recently, regarded by most people to be empty space.That changed after a gigantic Chinese balloon drifted across North America 20km above the ground, followed by three more mysterious objects with lower altitude flight paths, which were all shot down.

The “forgotten space” between about 15km and 40km above the Earth’s surface — sometimes called near space — is well suited to longer-lasting surveillance balloons, said professor Alan Woodward, a physicist and security expert at the University of Surrey.

“The Chinese are not the only ones doing this,” said Woodward. “The Americans realised long ago that high-altitude balloons were a ‘sweet spot’ for intelligence gathering.” The US operated a balloon surveillance programme called Project Genetrix over the Soviet Union in the 1950s.