Yep, cost effectiveness of project is amazing & really nice application for WSPR. WSPRnet for checking readings seems to be still online https://www.wsprnet.org/drupal/wsprnet/map .

If you keep the payload under 4 pounds I believe it's theoretically unregulated in the US except section 101.7 - don't create a hazard. I would still try to be approximately in compliance with part 101 though. Not a laywer.

Four pounds of hardware ingested by a jet engine is going to do a shitload of damage.

Exactly. Given that most birds weigh less than 4 pounds and bird strikes can take an engine out completely.

Plus, if @jacquesm says it, it must be true. ;-)

I agree, but apparently that's the rule.

These pico balloons weigh grams.

Yeah, I think ours was less than 10 grams.

As rules go, IIRC at least here in the Czech Republic anything below 100 g is unregulated, you just need to have a licensed amateur radio operator, so that the probe can broadcast in the 20 meter WSPR band under his callsign.

Parent to mine mentioned 4 pounds.