How high above us does sky become space and how does this boundary manifest itself?
Outer space begins where airspace ends, though exactly where that is has never been determined. A commonsense definition might be: anywhere above the Earth where aircraft cannot fly because they cannot derive support in the atmosphere from the reactions of the air. This is the classic definition of airspace.
Tony Aust, London W6
“Sky” means the atmosphere, which tails off gradually into the magnetosphere of charged particles that orbit the Earth. Beyond this is the heliosphere of charged particles that orbit the Sun, extending well beyond the most distant planets. There are no sharp edges.
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In practice, aircraft can fly on wings supported by air up to about 19 miles (30km — the Helios unmanned propeller-driven aircraft reached 96,863ft in 2001, the air- breathing rocket plane X-43A 95,000ft in 2004). The hybrid X-15 manned rocket-plane reached 108km in 1963. Above this, near-Earth satellites orbit through “space”. The International Space Station is currently in space at an altitude of 360km, but, due to residual air drag even at this height, it is falling at 500 metres per week towards its minimum altitude of 278km, when it would definitively fall to ground unless boosted upwards.
The British Government has an exact definition of the boundary between sky and space of 100km. Above this altitude the science is astronomy, funded by the Particle Physics and Astronomy Research Council, and below it the atmospheric sciences, funded by the Natural Environment Research Council.
Paul Murdin, Royal Astronomical Society, London W1
I was born in 1947 in Widnes, which was then in Lancashire. Since then the boundaries have been changed and Widnes is now in Cheshire. When I am asked to state my place of birth on official forms, do I put “Widnes, Lancashire”, or “Widnes, Cheshire”?
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Further to your last answers (September 14), in Wales we suffer a similar problem. I was born in Carmarthenshire, which then changed to Dyfed (taking in all of west Wales: Carmarthenshire, Cardiganshire and Pembrokeshire). Some ten years ago the previous regime was reinstated and I now once more live in Carmarthenshire — but whenever I have dealings with computerised systems, including government agencies, I apparently still live in Dyfed. No wonder our postmen don’ t know whether they’re coming or going.
Only politicians and film-makers adapt history to taste, so your correspondent Judy Adams was born in Lancashire, surely. Excepting official forms, I sidestep any confusion by putting down West Wales, which also has the added attraction of giving the reader a quick visual image of where, roughly, Llandeilo is.
Huw Beynon, Llandeilo, Carmarthenshire/Dyfed/West Wales
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