NASA scientists in Antarctica are set to launch a balloon-borne instrument to collect information on cosmic rays, high-energy particles from beyond the solar system that enter Earth’s atmosphere every day. The instrument, called the Super Trans-Iron Galactic Element Recorder (SuperTIGER), is designed to study rare heavy nuclei, which hold clues about where and how cosmic rays attain speeds up to nearly the speed of light. It will be launched on December 10. In 2013, the SuperTIGER had broken flight duration records while flying over Antarctica. “The previous flight of SuperTIGER lasted 55 days, setting a record for the longest flight of any heavy-lift scientific balloon,” said Robert Binns, the principal investigator at Washington University in the US, which leads the mission. “The time aloft translated into a long exposure, which is important because the particles we’re after make up only a tiny fraction of cosmic rays,” said Binns. The most common cosmic ray particles are protons or hydrogen nuclei, making up roughly 90 per cent, followed by helium nuclei (eight per cent) and electrons (one per cent). The remainder contains the nuclei of other elements, with dwindling numbers of heavy nuclei as their mass rises. With SuperTIGER, researchers are looking for the rarest of the rare – so-called ultra-heavy cosmic ray nuclei beyond iron, from cobalt to barium.
“Heavy elements, like the gold in your jewelry, are produced through special processes in stars, and SuperTIGER aims to help us understand how and where this happens,” said John Mitchell at NASA’s Goddard Space Flight Center in the US. “We’re all stardust, but figuring out where and how this stardust is made helps us better understand our galaxy and our place in it,” said Mitchell. When a cosmic ray strikes the nucleus of a molecule of atmospheric gas, both explode in a shower of subatomic shrapnel that triggers a cascade of particle collisions. Some of these secondary particles reach detectors on the ground, providing information scientists can use to infer the properties of the original cosmic ray.
However, they also produce an interfering background that is greatly reduced by flying instruments on scientific balloons, which reach altitudes of nearly 40,000 metres and float above 99.5 per cent of the atmosphere. The most massive stars forge elements up to iron in their cores and then explode as supernovas, dispersing the material into space. The explosions also create conditions that result in a brief, intense flood of subatomic particles called neutrons. Many of these neutrons can “stick” to iron nuclei. Some of them subsequently decay into protons, producing new elements heavier than iron.
Supernova blast waves provide the boost that turns these particles into high-energy cosmic rays. As a shock wave expands into space, it entraps and accelerates particles until they reach energies so extreme they can no longer be contained. Over the past two decades, evidence accumulated from detectors on NASA’s Advanced Composition Explorer satellite and SuperTIGER’s predecessor, the balloon-borne TIGER instrument, has allowed scientists to work out a general picture of cosmic ray sources. Roughly 20 per cent of cosmic rays were thought to arise from massive stars and supernova debris, while 80 per cent came from interstellar dust and gas with chemical quantities similar to what is found in the solar system.