How high must we climb before we can claim to be "in space"? To answer "above the atmosphere" will not suffice, for although the Earth's atmosphere grows progressively thin with altitude, traces of it extend beyond 800km and slowly merge with the swarms of particles trapped within our planet's magnetic field or billowing outwards from the Sun in the solar wind.

On such a definition, the International Space Station, orbiting at about 400km at present, would not be worthy of its name. The ISS, like all satellites in low Earth orbit, feels the minute drag of the atmosphere and must boost its orbit from time to time if it is not to spiral down to re-entry and destruction – in fact another small boost is due on Wednesday.

The Fédération Aéronautique Internationale (FAI), formed 107 years ago and widely recognised as the governing body for aeronautics, astronautics and related activities, puts the beginning of space at 100km. This is now sometimes dubbed the Kármán line after the person who calculated that aerodynamic lift was impossible at higher levels without attaining orbital velocity. Travellers who cross this line can claim to have reached space, whether or not they have reached orbit, and have earned their astronaut's or cosmonaut's wings. On the other hand, and despite being a founder-member of the FAI, the US has taken a contrary view in awarding astronaut's wings to anyone reaching 50 miles or 80km.