NASA’s super pressure balloon at 40

The groundtrack of NASA's Super Pressure Balloon is pictured here. The green track represents the first mid-latitude circumnavigation and the red represents the current track. The balloon is flying over the Pacific Ocean nearing the west coast of South America.
The groundtrack of NASA’s Super Pressure Balloon is pictured here. The green track represents the first mid-latitude circumnavigation and the red represents the current track. The balloon is flying over the Pacific Ocean nearing the west coast of South America.

Forty days at float as of Saturday, June 25, and NASA’s super pressure balloon (SPB) is presently flying above the south Pacific as it continues on its long-duration technology test and science flight.

For the past two weeks, the balloon has etched out a whimsical groundtrack over the Pacific, slipping out of the more southerly stratospheric cyclone pattern and then floating nearly to the equator before turning west, south, and now east toward South America.

“The balloon has flown longer than its predecessor, flying through the rigors of the heating and cooling experienced in the day/night cycle,” said Debbie Fairbrother, NASA’s Balloon Program Office chief. “We continue to monitor the balloon, conducting daily analyses of the balloon’s health and trajectory.”

The balloon is designed to float at a constant pressure altitude of 7 millibars. However, the actual altitude at float is also dependent on the environmental conditions where colder weather below could lead to lower float altitudes. To date, the balloon has flown over a number of cold storms, which has resulted in the balloon losing its differential pressure and experiencing some altitude variance at night.

“The heating and cooling cycle puts a lot of stress on the balloon, but it was engineered to withstand the extra wear,” said Fairbrother. The previous record flying through the day/night cycle was 32 days, 5 hours, set in 2015 with an SPB flight that launched from Wanaka, New Zealand. The overall record for any SPB flight is 54 days, set by a 7-million-cubic-foot SPB flight over Antarctica in 2009.

Aside from the technology test of the balloon itself, the SPB is carrying the Compton Spectrometer and Imager (COSI) gamma ray telescope. The COSI team reports that the science instrument continues to collect good data.

Balloon flight operators at NASA’s Columbia Scientific Balloon Facility in Palestine, Texas, are monitoring the mission around-the-clock. Anyone can track the progress of the flight from this website: 

High-Flying Recording Studio Captures Deep Bass Infrasound

Carolina Infrasound Instrument
The Carolina Infrasound instrument is shown here prior to shipping to New Zealand for incorporation onto the super pressure balloon gondola. (Credit: Danny Bowman)

While the Compton Spectrometer and Imager (COSI) gazes into space, another experiment is listening for whispers from the Earth itself. Housed in two humble white boxes tucked behind COSI’s solar panel, a triad of microphones are recording infrasound – sound too deep for humans to hear. Infrasound sources include volcanoes, earthquakes, ocean waves, the aurora, explosions, rocket launches…and many more.

The power of infrasound lies in its ability to travel vast distances. Higher frequency sounds dissipate rapidly: this is why bass notes carry much further than treble notes, and why thunder from a nearby lightning strike is a sharp crack, as opposed to the muffled boom of a faraway storm. The exceptionally deep sounds in the infrasound range can travel around the world multiple times.

Networks of ground-based infrasound detectors are located around the world. The sensitivity of these microphones can be their own worst enemy, however. The slightest gust of wind can overwhelm faint acoustic waves from the other side of the planet. Also, the temperature structure of the troposphere (the lowest layer of the atmosphere) tends to channel sound waves away from the Earth’s surface.

Infrasound sensors on balloons may offer significant advantages. Since they move at the same speed as the wind, they never feel its effects. Modeling suggests they can record sounds at much greater distances compared to those on the ground, and perhaps detect signals that never reach the surface at all. Despite this, no infrasound microphones have been deployed above 8 kilometers from the early 1960s to 2014, a gap of over half a century. Thus, the true diversity of atmospheric sounds remains unknown.

The infrasound payload on board the super pressure balloon was developed by the University of North Carolina at Chapel Hill. The exceptionally light (<3 kg) instrument package has three primary objectives: 1) characterize the sound field of the stratosphere 2) record an “event” (earthquake, bolide, etc) and 3) quantify the impact of local acoustic or electromagnetic noise from COSI and the balloon itself.

If successful, this experiment will collect the most infrasound data ever recorded during a single flight; indeed it will increase the time coverage of available acoustic data in the stratosphere by an order of magnitude. The detection range and sensitivity of free flying acoustic stations will be well characterized, and the magnitude of humanity’s contribution to the global infrasound wave field will be quantified.

Unlike COSI, however, the data are not telemetered. The recorder must be recovered, or the microphones will take their secrets to the bottom of the sea. Thus, the UNC team watches nervously as the superpressure balloon drifts across the open ocean, and crosses their fingers that nothing goes wrong.

Listen to previous infrasound recordings here: 

Danny Bowman, University of North Carolina at Chapel Hill