Image from Steve Bowers
The exosphere of Nova Terra, a long-terraformed world with an Earth-like atmosphere
[1] The outermost layer of a planetary atmosphere. On Earth or an Earth-like Garden World or terraformed planet, the exosphere extends from about 650 to 1,300 km. The lower boundary of the exosphere is called the critical level of escape, where atmospheric pressure is very low (the gas atoms are very widely spaced) and the temperature is very low.

[2] The edge of the technosphere, where there has been only minor penetration and colonisation by SI:1 and higher powers; the astrographical and toposophical border or membrane between Terragen and non-Terragen space (whether idea-space, virchspace, or physical space).
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Development Notes
Text by M. Alan Kazlev
Initially published on 24 October 2001.