A 29 year-old physicist named Victor Hess discovered cosmic rays, and his research won him the Nobel Prize in Physics for his work on cosmic radiation in 1936. He immigrated to the United States after he received threats from the Nazis (his wife was Jewish), where he became the professor at Fordham University. He kept contributing to the field until his death in 1964. By the end of World War II, researchers had more or less determined the true composition of cosmic rays. It wasn’t pretty. 90% are protons, the nuclei of hydrogen atoms. About 9% are alpha particles, hefty globs containing two protons and two neutrons, the nucleus of a helium atom. Rarer cosmic rays are the nuclei of yet heavier elements, making up about 1% of all cosmic rays. Their overall composition mirrors the makeup of the universe! Apparently, a fairly accurate sample of the cosmos steadily arrives at Earth equally from all directions, before our magnetic field starts channeling it.
If a cosmic ray has an energy of 100MeV, it must be moving at exactly 43% of the speed of light, which is impressively fast. Yet many cosmic rays have energies of 10GeV, and this corresponds to 99.6% of the speed of light. It’s common sense that a faster moving object can do more damage than a slow one. The highest energy cosmic rays have the amazing power of 1,020eV. Each delivers the same energy as getting hit by a well-flung fastball, but packed into the tiny impact zone of a single proton! As Hess discovered, cosmic rays are more intense the higher up we go. So those who live at high altitudes get twice the cosmic radiation exposure of those below. Career pilots and flight crew get twice as much radiation as the rest of us. It’s frequent flyer radiation, translated into a 1% higher cancer rate than the general population.
The heaviest cosmic ray intensity streams in from deep space in spurts. Much of this is usually deflected by a boundary at the edge of the Solar System called the heliopause, a shock wave where the outrushing solar wind goes from supersonic to subsonic. But the Sun’s power, which provides this barrier varies with its 11-year solar cycle. Since 2000, this termination shock zone had become weaker, and its protective barrier becomes much more porous. Hence, deep space cosmic rays are currently streaming to Earth with a greater intensity than normal. When the 27 Apollo astronauts sped outward from Earth between 1968 and 1972, they experienced something no human had ever been subjected to, before or since. They ventured beyond the magnetosphere, Earth’s protective magnetic field. Each astronaut saw flashes of light in their field of vision every 2.9 minutes. NASA physicians had an immediate theory that was later verified: powerful cosmic rays were zooming through the liquid in the astronauts’ eyeballs.
Cosmic rays start to seriously strike air molecules at a height of about 35 miles (56 km). They break atoms apart with the contents raining down towards the surface at nearly the speed of light. Among the detritus flying off are muons. With a mass of 208 electrons, they have a short life span, with a half-life of just two-millionths of a second, but can cause harm if they strike the wrong bit of genetic material in a cell nucleus. A Nature study published in 2016 showed a correlation between cancer rates and cosmic ray activity and some 240 muons flash through your body every second.
Credit: Bob Berman in the Astronomy November 2016