A single particle from space once carried the punch of a thrown baseball
In 1991 detectors caught one subatomic particle carrying about 50 joules, roughly the energy of a baseball flying at 90 kilometres an hour. That so-called OMG particle belongs to the cosmic rays, a ceaseless drizzle from space, and right now about one of its muons is crossing a region the size of your head every second.
The discovery began with a puzzle about air. After Henri Becquerel found radioactivity in 1896, physicists assumed the faint electrical charge in the atmosphere came from radioactive rocks and radon. Theodor Wulf built a sealed electrometer in 1909 and found the Eiffel Tower's summit more strongly ionised than its base, though few accepted his paper. Domenico Pacini then showed in 1911 that readings fell underwater, hinting at a source beyond the Earth.
Victor Hess settled it. In 1912 he rode a free balloon to 5,300 metres carrying three improved Wulf instruments and found ionisation had doubled compared with ground level. He even flew during a near-total eclipse to rule out the Sun, and still saw the rise. His verdict was that highly penetrating radiation was pouring in from above. Werner Kolhörster confirmed it at 9 kilometres, and Hess won the 1936 Nobel Prize in Physics.
The name is a slight misnomer. Early researchers thought the rays were mostly electromagnetic, but they turned out to be particles with mass. About 99 percent of those arriving from space are bare atomic nuclei, and roughly nine in ten of those are simple protons, with helium nuclei making up most of the rest and a sliver of heavier elements. Striking the upper air, they shatter atoms into cascades of pions, muons and neutrinos. Most incoming particles are turned aside by Earth's magnetic field or the Sun's heliosphere.
Their origins are gradually coming into focus. Fermi Space Telescope data from 2013 suggest exploding stars supply a large share, and a flare of neutrinos and gamma rays from the blazar TXS 0506+056 in 2018 implicated active galactic nuclei too. The most energetic cosmic rays exceed the design energy of the Large Hadron Collider more than ten million times, and they also damage electronics and living tissue wherever an atmosphere offers no shield.
Source: Cosmic ray
