Ultrahigh-energy neutrinos could help scientists unravel some of the biggest mysteries in astrophysics—and the best place to find them may be the South Pole.

IT WAS A crisp December morning in 2016 at the icy airfield near McMurdo Station in Antarctica, and Peter Gorham was watching a massive balloon fill with helium. Attached to the balloon was a gondola the size of a semitruck cab that was designed to turn the entire frozen continent into the world’s largest radio dish. The experiment was known as Anita—short for the Antarctic Impulsive Transient Antenna—and its hulking frame was a checkerboard of square white antennas and black solar panels. When the balloon was full, it carried Anita 20 miles into the atmosphere, where it spent the next month riding the polar vortex in circles over Antarctica.

“It was surreal,” says Gorham. “The balloon looks like a giant jellyfish, and it just kind of hovers there for a moment when it’s released. It rustles in this majestic way as it rises up, and it’s just an unforgettable sound.”

For the past decade, Gorham and a small team of scientists had regularly traveled to Antarctica to send Anita on missions to detect signs of cosmic neutrinos. These rare subatomic particles could provide a window onto some of the most violent processes in the universe—but there’s a catch. Neutrinos are nearly massless and rarely interact with other matter, which makes them extraordinarily difficult to study. Anita was scouting for the faint burst of radio waves created when they pass through the Antarctic ice, one of only two proven ways to detect the elusive particles. Some of the cosmic neutrinos that reach Earth have been traveling through space at the speed of light for billions of years, but their provenance remains a mystery. Gorham and his colleagues hoped Anita could lead them to the source of these particles like a hound tracking its prey—but first it had to find some.

Gorham has been chasing cosmic neutrinos for most of his life. After working in the 1980s on a failed seafloor neutrino detector called the Deep Underwater Muon and Neutrino Detector Project, he and a colleague ran an experiment in a particle accelerator in 2000 that proved it was possible to detect the radio waves created by neutrinos when they interact with ice. Like water, ice would reveal neutrinos when they passed through, and it would avoid the leaks faced by submarine detectors. So in 2003, he pulled together a crack team of astroparticle sleuths to create the Anita project, which would search for signs of neutrinos using a balloon flying 20 miles above Antarctica.

“It was such a preposterous idea,” Gorham says. “People couldn’t tell if it was a joke or not.” (Still, Gorham and his team were in good company; Victor Hess, the physicist who discovered cosmic rays in 1912, did so from a hot air balloon.)

Gorham says the Anita team had to pitch the idea to NASA several times, but eventually the agency signed on to the project. So in 2006, the team made their way down to Antarctica for Anita’s first flight. Once the balloon was launched, it would use its array of 32 antennas to search for short bursts of radio emissions caused by ultrahigh-energy neutrinos slamming into the nuclei of hydrogen and oxygen atoms in the ice. These neutrinos that Anita was searching for would be classified among the most energetic particles in the universe, which physicists believe are produced by the same processes that generate jets of charged particles called ultrahigh-energy cosmic rays. Physicists still don’t know how these particles are produced. If Anita detected even a single ultrahigh-energy cosmic neutrino, it would be a major step toward resolving that mystery.

As with any scientific work in Antarctica, each Anita mission came with a number of challenges, ranging from technology issues to poor weather and crew illnesses. During Anita’s third flight in 2014, Gorham almost died after being stricken by an unknown virus three days after arriving in Antarctica. (He had to be evacuated to New Zealand.) But despite a decade of hardships endured by Gorham and his colleagues, by 2016 Anita had yet to detect a single cosmic neutrino. As Gorham watched Anita rise into the blue that day in 2016, he was optimistic that the experiment might finally find its mark. But no luck. Now that the Anita team is wrapping up the data analysis from that flight, Gorham says it didn’t pick up any evidence of ultrahigh-energy neutrinos.

But taking to the skies isn’t the only way to go neutrino hunting in Antarctica. Just a few miles from where Anita came crashing back to Earth near the South Pole, a network of detectors buried deep in the ice has kept an around-the-clock lookout for the elusive particle for the last decade. Known as IceCube, the experiment became the first to ever detect cosmic neutrinos in 2013. Only a year after Anita’s last mission, it was the first to trace one of the particles to its origin: a galaxy over 4 billion light years away.

Now, physicists are working on next-generation versions of Anita and IceCube that will not only find more cosmic neutrinos, but trace them to their sources on the other side of the universe. Together, they will usher in the era of neutrino astronomy, an entirely new way to study the most extreme phenomena in the cosmos.

Ghostbusters

Neutrinos are everywhere. At any given time, there are trillions of these nearly massless particles passing through your body at the speed of light. Given their abundance, you’d expect that detecting them would be about as challenging as catching fish in a barrel. But neutrinos are the ghosts of the subatomic world. They pass through solid material like sunlight streaming through a window. On occasion, they do interact with other matter, but these instances are incredibly rare. Consider this: Even though any given human encounters quadrillions of neutrinos per day, over the course of their lifetime only a single neutrino will interact with one of the billions of atoms in their body. The challenge for physicists is to figure out how to detect these rare interactions so the neutrinos can be studied.

Neutrinos are produced whenever the nuclei of radioactive elements break down. This means that the Earth’s atmosphere, nuclear reactors, and even bananas are all neutrino factories. But not all neutrinos are created equal—some have vastly more energy than others. At the low-energy end of the spectrum are neutrinos produced by our sun, which constitute the vast majority of natural neutrinos streaming through the Earth. At the other end are ultrahigh-energy cosmic neutrinos like the kind created around the supermassive blackholes in the turbulent hearts of so-called active galactic nuclei. These have around a billion times more energy than solar neutrinos.

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