Ever since I was a kid I’ve wanted to send a probe of mine into near space. When I grew up and learned a bit more about the laws the govern the universe (at least the ones we can come up with nowadays), I realized it was not an easy task. But I didn’t want to send a full-fledged, 3-year-of-development, 1.5 ton, 3 meter tall probe into space, with the highest tech equipment on board. I just wanted a small probe, with simple, cheap sensors, a camera and little else.
A group of very clever people actually achieved this recentely: the SpaceBits project. Go there and amaze yourself.
Now, the idea of using a high altitute baloon is great, one that I also had myself back then, but not as the only means of transport. My design was based on vessel with propulsion, albeit a very weak one (to be as light as possible). The baloon rises the vessel to a certain height before bursting; the idea was to use the propulsion to go further than the baloon alone could go. The strategy was to use the propulsion a few seconds before the baloon burst to alleviate some effective weight and accelerate the set. The vessel would apply just enough power to even out it’s own wheight, or a bit more, so that the baloon would accelerate. Immediately after the baloon burst, the vessel would apply full propulsion until the power ran out. After that, it would fall again to Earth.
The fall was not caotic; the vessel could manouver during the descent, so that it could crash into a more-or-less predicted place (preferably some water, hehe). I wanted it to have a small parachute, but weight would certainly not allow it (and it’s associated firing mecanism).
I even had a sketch of the vessel made; it had a rocket-like shell, with stabilizer fins, and lateral duct-fan assemblies for propulsion. The body/fins would move to steer the vessel. I knew the fans would only work up to a certain height, but I never got to know what that limit was… if I’m thinking correctly, once the air becomes more and more rarified, the fans produce less and less impulsion; even assuming an infinte power supply, there would be a point were the impulse would equal gravity, and the vessel would move no further.
In terms of power, the idea was for it to be totally electric, with four power supplies: propulsion engines, ascent and descent control motion, main systems, and a very small backup for the main systems.
In terms of sensors, I never got to think too much about it, just some simple temperature, pressure, light, oxygen, altitude, speed, etc. It would have a camera facing forwards, GPS for positioning, and a radio telemetry and control system connected to a land-based application.
Back then, this would obviously all be done with an Amiga! 
Seriously, this was a project I would really like to build one day; electronics are much cheaper now, we have access to some pretty advanced microcontrollers and radio devices, and development (both hardware and software) has never been so much fun!
