[RoosterRedux]

I wonder how they accounted for the expansion of helium with altitude (a helium balloon eventually bursts because the pressure inside the balloon increases vis-a-vis the decreasing air pressure outside).

[MarchonDC09122009]

The very point I was wondering, too.
Also, is their operating altitude affected by Jet Stream winds that would make navigation difficult?
Do we have a surface-to-air missile that goes that high?

[Paladin2]

&s there negative absolute pressure?