EUSO-SPB2 cannot directly detect these particles, but it can look for telltale signs in the atmosphere as neutrinos and cosmic rays collide with the molecules of the ground and atmosphere. Both its instruments look for the traces from these collisions. One detects the UV light produced by cosmic rays hitting atmospheric molecules and producing a particle shower. The other looks for a special kind of light called Cherenkov radiation that is produced after a neutrino hits a molecule in the Earth; the collision sends a tau particle streaking away, which then decays and produces a shower of billions of secondary particles that create the telltale Cherenkov light.
Most previous experiments to find these particles have sat on the ground looking up at the atmosphere. EUSO-SPB2 instead sits just above the atmosphere looking down. This gives the instruments a much wider potential view of the traces of these collisions.
“The more atmosphere you can observe, the better, since ultra-high energy cosmic rays are extremely rare,” said UChicago physicist Rebecca Diesing, who is helping to build one of the instruments that will ride aboard the balloon. “A square kilometer patch of Earth will be hit by one of these particles only about once per century.”
EUSO-SPB2 will also launch while several gravitational wave detectors are running. These observatories are designed to catch the ripples in space-time that happen when black holes or neutron stars collide. If the gravitational wave detectors pick up such a collision, EUSO-SPB2 can swing around to try to look for neutrinos in the aftermath.
Putting the pieces together
Even to get this far has been a journey, the scientists said. Because nothing like this has been done before, dozens of scientists and engineers have worked to ensure the instruments and the balloon will work together.
“For example, we had to choose materials that were light enough to fit within the weight limit for what the balloon can carry, but also strong enough to withstand the shock of launch—when the parachute deploys, the gondola experiences up to eight to 10 G’s,” explained Johannes Eser, a UChicago scientist who has worked on EUSO-SPB2 since its inception.
Different pieces of EUSO-SPB2 have been built at institutions around the world; for example, Georgia Tech is building the Cherenkov camera and institutions in France and Italy built the UV fluorescence camera. Meanwhile, the University of Chicago is building several parts, including a device to measure cloud cover around the balloon and the innovative gondola that carries the instruments, as well as running simulations and overseeing the full project.
The entire cargo is being assembled at the Colorado School of Mines this summer and fall, then shipped to NASA’s facility in Palestine, TX for a “hang test”—to make sure the entire device holds together and works well when hung from the balloon. Finally, it will make its way to New Zealand for launch, planned for spring 2023.