Wikipedia wrote:The Raven Aerostar balloons Google is using typically have a maximum life of about 55 days, although Google claims that its twerked design can stay aloft for more than 100 days.
A maximum life of 55 days would probably indicate an average life of somewhere around 45 to 50 days, but I'll be generous to Google and use their number and grant an average life of 100 days.
Now, parachutes designed for human beings seem to have a malfunction rate of somewhere from 1 in 1000 to 1 in 5000. Of course, human parachutists usually have a secondary chute, making the fatality rate much closer to 1 in a million, but I assume Project Loon will not be using backup chutes. So, I'll assume the parachutes fail once in a thousand times on average.
It then becomes a factor of how many balloons are aloft.
Wikipedia wrote: Google hopes to eventually have thousands of balloons flying in the stratosphere at an altitude of 20 km (12 mi).
1000 balloons in the air, each lasting 100 days, means one needs to come down every 2.4 hours on average, and if one in 1000 of these will not have a successful parachute, every 2400 hours = 100 days one will fall uncontrolled through controlled airspace.
10,000 balloons means this happens every 10 days on average.
Sounds like fun...
I hear velociraptor tastes like chicken.