Ground sensors like those used on the moon and Mars can measure seismic waves. Balloon-based pressure sensors can measure infrasound waves, a form of low-frequency waves in the atmosphere created ...

It’s pretty easy to take a balloon, fill it up with helium, and send it up in to the upper atmosphere. It’s much harder to keep track of it and recover it when it falls back to Earth.

Last summer, Politico reported that the US was itself developing plans to deploy its own high-altitude balloons to, among other things, track the deployment of hypersonic missiles in China and Russia.