The project started ~a decade ago, and back then a drone that could stay airborne 24x7 wasn't really doable - at least not with several kg of radio equipment and several hundred watts of power for networking gear.

Even today that's challenging - today's 24x7 drones glide all night because that's more efficient than carrying the weight of a battery that can last all night.

Your point about needing to glide all night is interesting. Is this true for Zephyr as well?

I'm only guessing, but perhaps because balloons have fewer moving parts? They're the lowest-tech aircraft. Minimise aerospace engineering, focus on networks for now.

Mostly payload and Airborne time. Idk what is the current status of Zephyr (google search suggests 26 days?) but Loon balloons were consistently reaching 300 days of airborne time.

Zephyr's latest flight had 64 days airborne:

https://newatlas.com/aircraft/zephyr-8-mission-ends-abruptly...

Unfortunately, it ended with a crash.

Thanks for the link. Pretty impressive mission despite the unfortunate ending! Edit: Did it actually crash? The article says it was “brought down” but that could have still been a glide and regular landing if it lost power.

Baloons are simpler, sturdier and much cheaper.

Zepher's payload is 5kg and only stays up for a month and costs $$$$.

Balloons are a bag that can carry tonnes and last forever in theory.

The obvious choice is balloons.

You can tether balloons for local.

In practice what works best is a hard question.

What will the military fund etc.

How do you tether a balloon at 30,000ft?

Perhaps a happy medium would be a solar airship with a rotor for maneuvering during the day.