Being a passenger on a sailing ship doesn't mean you're a 'sailor' — so is Katy Perry an 'astronaut'?

Contrary to what might seem obvious, 'space' is not defined by the height at which you become weightless. Near-weightlessness can be achieved on one of those fairground experiences which drop you vertically: being 'weightless' is not the same thing as 'being an astronaut'

singer Katy Perry after her flight in Blue Origin's New Shepard NS-31 spaceship. The 40-year-old US pop star was part of a six-woman crew on Jeff Bezos's New Shepard NS-31 mission, organised by his partner, journalist Lauren Sanchez. Picture: Blue Origin/YouTube/PA Wire

The question of 'when is an astronaut an astronaut?' has received much attention with the most recent flight by the space company Blue Origin that included a number of high-profile individuals. 

While the question appears to be a simple one, the answer turns out not to be so.