I wonder how many of these are in the sky: https://hackaday.com/2017/10/30/quick-and-easy-solar-hot-air-balloon/ https://hackaday.com/2017/10/30/quick-and-easy

I wonder how many of these are in the sky: https://hackaday.com/2017/10/30/quick-and-easy-solar-hot-air-balloon/ https://hackaday.com/2017/10/30/quick-and-easy-solar-hot-air... Since they are filled with plain air they could stay up a long time. They probably have no radar cross section, though they'd probably have a thermal signature.