Skylab, the American space station, was a curious world in which what goes up, does not necessarily come down. It was a unique experiment in conditions never found on Earth.
When everything is weightless, lifting heavy objects is easy - but so is losing tiny ones. You have to devise new ways of sleeping, drinking, shaking salt on your food and going to the lavatory, because nothing falls. How does a spider spin its web? Which way up will a goldfish swim?
For the human body, life without gravity involves loss of body fluid, dissolving bones, crazily deformed blood cells and inexplicable bouts of space nausea. But, as astronauts never tire of demonstrating, it can be enormous fun - like playing with spheres of water, frictionless gyroscopes, and floating objects in general; not to mention oneself - weightlessness turns everybody into a champion gymnast.
Horizon explores all these aspects, with footage from Skylab and reminiscences from ex-inhabitants - the astronauts who call this world zero gravity - zero G for short.