"Maarten" no_one@nowhere wrote

So my question is, why this preference for 400 km

For a long-duration space station, 400 km is kind of the
"Goldilocks" altitude between atmospheric drag and radiation
exposure from protons in the inner Van Allen belt.


and why isn't it an orbit of 250km

Too much air.

or 600km or even more ?

Too many protons. Also, present transport vehicles are limited
in their ability to carry masses of the desired size to such
altitudes.

http://www.estec.esa.nl/wmwww/wma/Ba...d/rad_env.html
http://www.windows.ucar.edu/spaceweather/sat_drag.html

http://www.heavens-above.com/ssorbitdecay.asp shows rather
graphically, so to speak, why 250 km is too low.