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 Dec. 20, it would travel for the next 23 days on a NASA balloon along the very highest reaches of the atmosphere, scanning the continent of Antarctica from its 120,000-foot vantage point for miniscule visitors from outer space known as neutrinos.
The mission, known as Payload for Ultrahigh Energy Observations, or PUEO, came back to Earth on Jan. 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 its landing site to pick up its “black box,” which contains the full set of data it took. When the data arrives back in Chicago, scientists can start combing through it 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,” said Abigail Vieregg, UChicago professor of astronomy and astrophysics and physics and David N. Schramm Director of the Kavli Institute for Cosmological Physics, who leads the PUEO mission.
NASA selected PUEO for funding as part of the inaugural class of its Astrophysics Pioneers missions. PUEO was the first of these to launch.
Searching the ice for particles
PUEO’s target, neutrinos, rain down on Earth from outer space all the time. We generally don’t notice—they only occasionally interact with our world—but scientists think there’s a small subset of these particles that is extremely energetic and has not yet been detected.
These rarer neutrinos would be far more energetic even than the particles being accelerated at the Large Hadron Collider in Europe. Finding them could give us a rare window into the violent places around the universe where such neutrinos could be created, such as black holes or in collisions between ultra-dense neutron stars.
To catch something that’s never been seen before, though, you need an extraordinary instrument.
PUEO, like its predecessor, the University of Hawaii-led ANITA, 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 NASA would launch it on a balloon that could fly it above the continent.
“Basically, we are using the entire continent of Antarctica as a detector,” said Cosmin Deaconu, research professor at UChicago who wrote the flight software for PUEO and helped build the instrument.
The team designed PUEO to be significantly more sensitive than ANITA, thanks to advancements in electronics and a new design to combine signals from multiple antennas together in real time, in order to dig more faint neutrino signals out of the noise.
Over the last five years, labs around the world, including six other collaborating U.S. institutions and institutions in Europe and Asia, built parts of PUEO and shipped them to Chicago for assembly. The final PUEO instrument was composed of concentric circles of 96 ultra-sensitive radio antennas, surrounding a “brain” that sorts through incoming signals and decides which could potentially be coming from neutrinos. It is powered by arrays of solar panels.
Last April, 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.
“You have to check a lot of things, but especially whether your electronics work in a vacuum,” said Deaconu. “The big challenge is that without air, you can’t use fans to cool down the electronics if they get too hot.”
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.
Winds for launch
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 in a continent known for its extreme weather. For balloons to launch successfully, the weather has to be not only clear but stable all the way up into near-space.
“The balloon and payload are so long that if you have strong variation in layers of the atmosphere, you could be in trouble,” explained Keith McBride, a UChicago postdoctoral researcher who led the development of major portions of the PUEO instrument.
“It’s stressful because the launch window can open any time of day or night after the polar vortex sets up in early December, and you just have to be ready,” said Deaconu.
PUEO was lucky enough to launch on its first try. Meteorologists and NASA officials determined a window for launch in the early morning of Dec. 20. A large launch vehicle called “The Boss” moved PUEO out to the launch pad—a large, flat area where the snow had been smoothed down. The go-ahead for launch came at nearly 6 a.m.
First up was the balloon. Then as it rose, it took the instrument with it, until the entire 700-foot-long ensemble was airborne. At that point, the ground crew told PUEO to extend its solar panels, which power the mission as it soars to 120,000 feet. Then, when PUEO reached its final float altitude, additional antennas were deployed below the main payload to add more sensitivity.
“There is something so exciting (and a little unnerving) seeing the last five years of your work float away,” said Rachel Scrandis, graduate student at UChicago and PUEO's Radio Frequency electronics lead. “We built PUEO to be the world's most sensitive experiment to ultrahigh energy neutrinos, but after launch, we are at the mercy of the winds to carry us over ice that will let our experiment shine.”
Sun and ice
Even with PUEO successfully aloft, it wasn’t time to relax yet.
For the next three weeks, the scientists had to take 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 the 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, heating up the instrument.
“We had to watch it very carefully for overheating so that we didn’t get too hot,” said McBride.
In missions like these, every second counts. If something on the instrument broke, the mission would lose precious data-taking time.
After over three weeks aloft, the crew made the call to bring the instrument back down to Earth.
“The meteorologists were predicting the winds would break off, and you want to get the payload to a place where it gently crash-lands, so it can be retrieved,” said Deaconu.
At the signal, 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 which could be easily reached by vehicles.
The “black box,” which contained the 50 to 60 terabytes of data taken over the duration of the mission, was retrieved and will be flown to McMurdo, Christchurch and finally Chicago. Then begins the careful process of sorting, calibrating, and analyzing the data to see whether PUEO was able to catch any of these extraordinary neutrinos—messengers from other galaxies.
The scientists said they expect the first results to be available in about a year.
“It’ll probably take us a month just to run the numbers on the computer—it’s that much data,” said McBride.
But for the scientists, a little bit of downtime is welcome.
“Now that the balloon is down, we can finally get a little sleep,” said Deaconu.
Funding: NASA.
