After years of work building an exquisitely sensitive instrument, University of Chicago scientists stood and watched as it flew up and out of sight into the fiercely blue Antarctic sky.

Launched on December 20, 2025, it traveled for the next 23 days on a NASA balloon through the very highest reaches of the atmosphere, scanning the continent of Antarctica from its 120,000-foot vantage point for minuscule visitors from outer space called neutrinos.

The instrument, known as Payload for Ultrahigh Energy Observations, or PUEO, came back to Earth on January 12, when it landed a few hundred miles from the South Pole.

Although PUEO transmitted some of its readings during the trip, a retrieval crew traveled to the landing site to pick up its payload, which contains the full set of data it recorded. Now scientists can start combing through that data for PUEO’s target: a set of energetic particles that have not yet been detected, but which could tell us about the most extreme events in the universe.

“We might discover the highest energy particles in the universe with PUEO, but even if we don’t find any, we’ll still learn about how the most energetic places in our universe work,” says Abigail Vieregg, the David N. Schramm Director of the Kavli Institute for Cosmological Physics and professor of physics and astronomy and astrophysics, who leads the PUEO mission.

Neutrinos rain down on Earth from outer space all the time and only occasionally interact with normal matter. But PUEO sought rarer neutrinos—ones thought to be far more energetic even than the particles accelerated at the Large Hadron Collider in Europe. Finding them could offer a new window onto the violent places around the universe where such neutrinos are likely created, such as black holes or in collisions between ultradense neutron stars.

To catch something that’s never been seen before, you need an extraordinary instrument.

PUEO, like its predecessor, the University of Hawai‘i–led Antarctic Impulsive Transient Antenna, relies on the fact that ice conducts radio waves very easily. If a high-energy neutrino hit an atom in the enormous sheet of ice that covers Antarctica, the collision would produce radio waves that travel through the ice and the air above. So all the team had to do was design a new instrument that could catch these extremely faint signals, and then NASA would launch it on a balloon that could fly above the continent.

“Basically, we are using the entire continent of Antarctica as a detector,” says Cosmin Deaconu, a research assistant professor at UChicago who wrote the flight software for PUEO and helped build the instrument.

Over the past five years, labs around the world, including six other collaborating US institutions as well as institutions in Europe and Asia, built parts of PUEO and shipped them to Chicago for assembly. The final instrument was composed of concentric circles of 96 ultrasensitive radio antennas surrounding a “brain” that sorts through incoming signals and decides which could potentially be coming from neutrinos. It was powered by arrays of solar panels.

In April 2025 the instrument traveled to Palestine, Texas, for testing at a NASA facility to make sure it could function in the challenging conditions of near space. After more tweaks and tests, the group took the instrument apart again so it could be shipped by road to California; by boat to Christchurch, New Zealand, the closest major port to McMurdo Station in Antarctica; and then, finally, by air to NASA’s balloon station in Antarctica.

There a team of sleep-deprived scientists worked swiftly to reassemble the payload for the last time before the launch window opened in mid-December.

So far PUEO had passed all of its tests. But perhaps the biggest lay ahead.

Launching anything aboard a gigantic balloon is risky. It’s even riskier when the instrument is so complex and everything depends on the conditions being just right—a tricky prospect on a continent known for its extreme weather. For balloons to launch successfully, the weather has to be clear and stable all the way up into near space.

PUEO was lucky enough to launch on its first try. Meteorologists and NASA officials determined a window for launch in the early morning of December 20. A large launch vehicle called “The Boss” moved PUEO out to the launchpad—a large flat area where the snow had been smoothed down. The go-ahead came just before 6 a.m.

First up was the balloon. As it rose, the balloon took the instrument with it until the entire 700-foot-long ensemble was airborne. “There is something so exciting (and a little unnerving) seeing the past five years of your work float away,” says Rachel Scrandis, SM’24, a physics graduate student and PUEO’s radio frequency electronics lead.

Even with PUEO successfully aloft, it wasn’t time to relax yet. For the next three weeks the scientists took shifts to constantly monitor the readouts as the device flew in the thin air at the edge of space.

PUEO was built to function as independently as possible, but the crew still had to make adjustments during its flight. For example, the payload turned out to be rotating more slowly than they had expected, which meant the sun beat down on one side for a long time. “We had to watch it very carefully for overheating,” says Keith McBride, a UChicago postdoctoral researcher who led the development of major portions of the instrument.

After more than three weeks aloft, the crew made the call to bring the instrument back down to Earth. NASA cut the line between PUEO and its balloon and deployed a parachute. PUEO drifted gently to Earth about 200 miles north of the South Pole, on a stretch of ice that could be easily reached by vehicles.

The payload, which contained the 50 to 60 terabytes of data taken over the duration of the mission, was retrieved and flown to McMurdo, to Christchurch, and finally to Chicago. Now begins the careful process of sorting, calibrating, and analyzing the data to see whether PUEO was able to catch signs of these extraordinary neutrinos—messengers from other galaxies.