CNN reports that giant solar balloons equipped with sensitive microphones captured unexpected sounds after reaching an altitude of 70,000 feet in the "stratosphere," which is the second layer of the Earth's atmosphere, according to NASA. This layer, which contains the ozone layer, absorbs and dissipates the sun's ultraviolet rays. The stratosphere is generally a calm layer of air that is rarely affected by turbulence due to the thin and dry air found there, making it ideal for research purposes.
Daniel Bowman, the principal scientist at Sandia National Laboratory, is exploring the acoustic landscape of the stratosphere. Inspired by his studies of low-frequency sounds produced by volcanoes, he and his team built solar balloons to attach infrared recorders to capture volcanic sounds. They then decided to add microphones to these balloons and explore the platform's potential.
Bowman reports that they recorded sounds such as chemical explosions, thunder, ocean crashes, helicopters, city sounds, additional rocket launches, earthquakes, freight trains, and jet planes. However, some sounds' origins remain unclear, including mysterious infrared signals picked up from aircraft flying in the stratosphere.
Bowman and his collaborators used balloons from NASA and other aeronautical service providers but eventually built their own balloons. These balloons can be assembled with materials found in hardware and fireworks supply stores, and even on a basketball court.