[jacquesm]

At the speed your average jet is travelling and the fact that approximately half the traffic is at night I'd hope for a radar transponder on the balloon (or at least some kind of reflector) because I highly doubt you'd be able to respond in time when going on visual cues alone, you'd need at least a few kilometers warning.

[plantain]

They have ADS-B transponders. You see them sometimes on flightradar24 floating around at FL650.

[plantain]

http://theaviationist.com/2014/03/01/google-project-loon/

[jacquesm]

> The helium balloons are all “floating” around 1,000 feet to the southeast of New Zealand

I can't make soup of that.