In a friendly talk with ChatGPT he told me that NASA uses NuttX, so I requested: please show me that link that proves it and he did it:

https://ntrs.nasa.gov/api/citations/20160011507/downloads/20160011507.pdf

So NASA used Pixhawk that runs NuttX on research balloon in 2016 and the article is really good for people willing to create some balloon project.

It is interesting they used RFD900 transceiver and also suggested to use APRS (Automatic Position Reporting System) for telemetry data to be captured by other people around the world and send automatically to the Internet.