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Giant solar balloons equipped with sensitive microphones were able to capture unexpected sounds after they reached an altitude of 70,000 feet from the surface, where they entered the “stratosphere”, according to CNN.
According to the US Space Agency (NASA), the “stratosphere” is the second layer of the Earth’s atmosphere, and at its lowest level is the “ozone” layer, which absorbs and dissipates the sun’s ultraviolet rays.
NASA explained that the thin, dry air in the stratosphere, where jet planes and weather balloons reach their maximum height, is a relatively calm layer of air that is rarely affected by turbulence.
Principal scientist at Sandia National Laboratory in New Mexico, Daniel Bowman, was inspired to explore the soundscape of that layer by studying low-frequency sounds produced by volcanoes. This phenomenon is known as ultrasound and cannot be heard by the human ear.
Bowman and his team had previously installed cameras on weather balloons to take pictures of the black sky above and the earth below, and then successfully built their own solar balloon.
Bowman suggested attaching infrared recorders to balloons to record volcanic sounds, but then realized, working with his advisor Jonathan Lees of the University of North Carolina, that no one had tried to put microphones in stratospheric balloons for half a century.
“We decided to move forward to explore what this new platform could do, in collaboration with Lees, who specializes in earth, ocean and environmental sciences, in addition to his experience in conducting research in seismology and volcanoes,” Bowman said.
According to Bowman, these balloons are equipped with sensors that are twice as fast as commercial aircraft.
“In our solar balloons, we’ve recorded chemical explosions, thunder, ocean crashes, helicopters, city sounds, auxiliary rocket launches, earthquakes, freight trains, and jet planes,” Bowman said in an email.
He continued, “We have recorded other sounds, but their origin is not clear.”
In a recording Bowman shared from a NASA balloon orbiting Antarctica, the ultrasound of crashing ocean waves sounds like a continuous sigh, but the explosions and other shocks are of unknown origin.
Bowman said, on Thursday, during that participation, that “inside the (stratosphere) some planes had mysterious infrared signals a few times an hour, but their source is completely unknown.”
Bowman and his collaborators conducted the research using balloons from NASA and other aeronautical service providers, but decided to build their own. They range in diameter from 6 to 7 metres.
He pointed out that the materials for making these balloons can be found in hardware and fireworks supply stores, in addition to the possibility of assembling balloons on a basketball court.
“Each balloon is made of painter’s plastic, shipping tape and charcoal dust, and costs about $50 each,” Bowman said by email.
He explained, “A two-person crew can build one balloon within a time not exceeding 3 and a half hours, and if it is launched on a sunny day after it is filled with air, it can carry a weight of one pound and reach a height of 70,000 feet.”
Coal dust is usually placed inside these balloons to darken them, and when sunlight reflects on them, the air inside them makes them float in the air and fly away.
These cheap balloons, according to experts, help researchers release many of them to collect as much data as possible. Bowman said he launched dozens of solar balloons to collect infrasound recordings from 2016 to April this year.
Accurate gauges, originally designed to monitor volcanoes, were attached to balloons to record low-frequency sounds.
The researchers tracked their balloons using the Global Positioning System (GPS) as they traveled hundreds of miles before landing in remote locations.
The longest flight so far on NASA’s helium balloon was 44 days, and it recorded 19 days’ worth of data before the batteries in the microphone ran out.
Meanwhile, solar balloon flights last about 14 hours in the summer and land after sunset.
The advantage of the higher altitude the balloons reach is the lower the noise level, the greater the detection range, and the entire planet Earth can be explored.
Despite this, balloons pose challenges for researchers, as the stratosphere is a harsh environment in which temperatures fluctuate between hot and cold, according to CNN. “The solar balloons are a bit flimsy, and we destroyed a few in the bushes trying to get them going,” Bowman said.
And he added, “We had to walk through valleys and mountains to get our loads, and one time our colleagues in Oklahoma state had to spend the entire night in a field to find a balloon before launching it again to fly all day.”
The biggest challenge now for the researchers is to identify the signals recorded during the flights. There is no doubt that it is normal or caused by air turbulence.
For her part, Sandia National Laboratories geophysicist Sarah Albert investigated an “acoustic channel” located at altitudes determined by Bowman’s research.
The acoustic duct is the channel that transmits sounds over great distances through the atmosphere.
Sarah’s recordings have captured missile launches and other unknown sounds. “This sound may be trapped in the channel and reverberate until it is completely jammed,” Bowman said.
And he added, “But whether it is close and somewhat quiet (like a turbulent spot) or far and loud (like a distant storm), this is not yet clear.”
Bowman and Sarah Albert will continue to investigate the atmospheric acoustic channel and try to determine the source of the “stratospheric rumble” and why some flights record it while others do not.
Bowman is eager to understand the sonic landscape of the stratosphere and unlock key features, such as variability across seasons and locations.
It is possible that helium-filled versions of these balloons will one day be used to explore other planets such as Venus, carrying scientific instruments above or within the planet’s clouds for a few days as a test flight for larger and more complex missions.
