‘Messengers from outer space’ sought by scientists with high-tech balloon

The project, termed Extreme Universe Space Observatory on a Super Pressure Balloon – aka EUSO-SPB2 – is searching for messengers from outer space: tiny, highly energetic particles that hit the Earth on their way from elsewhere in the universe.

The mission, which involves 280 researchers from 13 countries and 77 institutions, consists of two instruments which will be attached to a high-altitude balloon launched by Nasa. The balloon and its cargo are currently under final construction and assembly.

When completed, EUSO-SPB2 will ride wind currents about 20 miles above the Earth around the southern hemisphere, gathering data and searching for tracks left by two types of incoming particles.

EUSO-SPB2 will carry two different telescopes in order to detect two different kinds of particles that come from outer space. One type is called an ultra-high-energy cosmic ray. These are charged particles that have been accelerated to extremely high energies elsewhere in the universe and which occasionally slam into the Earth’s atmosphere. They are extremely powerful; they are the highest-energy particles we know of in the universe.

The other type of particle is a neutrino. These particles, on the other hand, rarely interact with matter at all. This aloofness makes them interesting to scientists, because they can carry information from far away in the galaxy without being distorted in the way that other particles might be affected. However, this also makes them very hard to detect in the first place.

It is believed that both these particles come from outside the Milky Way, potentially even from faraway galaxies. However, no one has yet been able to trace them back to their source in the sky. Scientists are keen to track them to their origins because this could tell us how the particles were made: whether by supermassive black holes, or by two massive neutron stars slamming into one another, or even a gigantic shock between clusters of galaxies. The particles would carry information about that event to us, millions or billions of light-years away.

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 will instead sit 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 kilometre 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.

“This is an important step towards solving the mystery of where in the universe these particles are coming from and how they could possibly be made,” said Angela Olinto, the University of Chicago Albert A Michelson distinguished service professor of astronomy and astrophysics, who heads the experiment. “These are particles that we simply cannot create ourselves on Earth; we need to use these space travellers to learn more about them.”

As nothing like this concept 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 Gs,” 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: Georgia Tech, for example, is building the Cherenkov camera, while institutions in France and Italy have 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 currently undergoing assembly at the Colorado School of Mines, before being shipped to Nasa’s facility in Palestine, Texas, for a 'hang test' designed to ensure 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.

If EUSO-SPB2 works, it will prove that space-based detection for these particles is possible. Olinto hopes it can provide proof-of-concept for follow-up missions, including one that will sit aboard a satellite orbiting Earth, picking up particle tracks.

“We are preparing for a future in which we will be able to detect lots of these particles and learn an extraordinary amount from them,” Olinto said, “but first we need to push the technology much farther.”

Additional funding for the EUSO-SPB2 project includes the Italian Space Agency, the French Space Agency, the Japan Aerospace Exploration Agency, and many others.