On 2016-05-28 15:17, Henry Spencer wrote:
On Sat, 28 May 2016, Rand Simberg wrote:
It makes some sense for LEO, where you really care about the
difference between altitudes of (say) 200km and 500km...
When you go much beyond LEO, it changes from being part helpful and
part nuisance to being all nuisance, alas.
Seems helpful for any low planetary orbit, including major moons.
True. There's also a helpful numerical accident: despite the great
differences in their atmospheres, the "reasonable" low-orbit altitudes
happen to be essentially identical for Earth, Venus, and Mars!
Venus's surface atmosphere is far denser than Earth's, but the upper
atmosphere is very cold and hence its density drops off very quickly.
Mars's surface atmosphere is very thin, but low gravity makes it fall
off quite slowly with altitude. By chance the
reasonable-orbital-altitude numbers come out nearly equal.
Alas, when you add Titan to the list, that bollixes up your altitude
intuition severely. :-) Huygens was at terminal velocity under its
main parachute 150km up!