A thought that the orbital design limit of the shuttle (about 690 miles) was due to the Van Allen radiation belts. The highest orbit for the shuttle, carrying full OMS fuel (and resulting reduced payload capacity) supposedly is just inside the lower boundary of the inner Van Allen belt.
I have a book somewhere called "The Space Shuttle Operator's Manual" (blue cover, here it is on Amazon -
http://www.amazon.com/Space-Shuttle-Operators-Manual-Revised/dp/0345341813 ) which gets into a lot of this stuff. Great read and worth picking up if you get the chance - I got my copy back in 1981 or 1982 I think.