With balloons in the target of the public, as well as public officials, questions about harmless amateur radio balloons seems to be under scrutiny.
A recent article in Scientific American suggests that some of the nefarious balloons shot down by jet fighters may have indeed been amateur radio balloons floating harmlessly overhead. Pico balloons are fairly commonplace these days. Even amateurs as famous as Tom Medlin, W5KUB, have launched quite a few in the past several years.
Pico balloons are extremely lightweight, with payloads measured in mere grams. They typically use APRS, WSPR, or both to do their reporting. The “payload” is an extremely lightweight PCB that includes solar panels for power, an extremely low powered transmitter to transmit the data, guitar strings for antennas, and a microprocessor to control the circuit. They typically only operate during sunlight, utilizing capacitors charged via the solar cells, and go “dark” at night, or if their GPS indicates they are over a nation that has restrictions on their operation (such as North Korea.)
You can read more about it, from a non-ham perspective, on the Scientific American website. To see an example of a launch, you can see one from 2022 below: