OP answered that question earlier - 120K ft. The balloon has to float 'on' or 'in' the gas of the atmosphere - once the atmosphere runs out there is no longer any lift. The ballon would then need some sort of peroxide jet pack on the side of it to get higher. Like oil in a half full salad dressing bottle it only rises to the top of the water not to the top of the bottle.
Two further questions though:
How much further would the setup need to get to become weightless ie be largely immune to gravitational pull and with what kind of lateral speed? (Geostationary orbit)
Is it possible to install a transmission system to send images back to your computer?
How long would the batteries last at that temperature?
You can see where I am going with this - building your own observation satellite...
Last edited by Arjay Bee; 04-02-2010 at 05:08 PM.