New Zealand is, hopefully, just a few days away from becoming a space-nation. The private company Rocket Lab (what a great name – I like names that describe what a business actually does) aims to put up its Atea-1 rocket from Great Mercury Island sometime around November 30th. The payload will reach an altitude of 120 km before returning to earth and being recovered.
But how high do you have to put something to get it into space? The earth’s atmosphere does not have a well-defined edge – atmospheric pressure decays roughly exponentially as altitude increases. At about 6 km altitude, the pressure is about half what it is on the earth’s surface; and it approximately halves for every further 6 km you go up. So it doesn’t take long for there to be little left. The lowest layer of the atmosphere is the troposphere (to about 10 km in altitude, but rather variable); here it gets colder as you go upwards. Passenger jets typically cruise at around 10 km in altitude.
Above the troposphere is the stratosphere; this goes to about 50 km in altitude and curiously temperature increases again as you go upwards. However, with so little air pressure, temperature starts to become somewhat less meaningful. With pressure so low at 50 km, the top of the stratosphere is not accessible to conventional aircraft (jet or propellor), but has been reached by rocket plane.
Beyond the stratosphere is the mesosphere (temperature decreases with height again), and beyond that is the thermosphere (temperature, for what it’s worth, does a reverse again and now increases with height.) There’s not much air left at all, and at 100 km the Karman line marks an arbitrary boundary as the ‘edge of space’. Karman calculated that at about that altitude a place would have to travel so fast to gain enough lift from its wings it would be in orbit. The Karman line is where the Federation Aeronautique Internationale sets the boundary for ‘space’, and it is where the Atea-1 rocket will have to reach before it is recognized as reaching space.