Cosmic Rays

cosmic rays
An artist's concept of the heliosphere, a magnetic bubble that partially protects the solar system from cosmic rays. Walt Feimer/NASA GSFC's Conceptual Image Lab

The term "cosmic ray " refers to high-speed particles that travel the universe. They're everywhere. Chances are very good that cosmic rays have passed through your body at some time or another, particularly if you live at high altitude or have flown in an airplane. Earth is well-protected against all but the most energetic of these rays, so they don't really pose a danger to us in our everyday lives.

Cosmic rays provide fascinating clues to objects and events elsewhere in the universe, such as the deaths of massive stars (called supernova explosions) and activity on the Sun, so astronomers study them using high-altitude balloons and space-based instruments. That research is providing exciting new insight into the origins and evolution of stars and galaxies in the universe. 

What Are Cosmic Rays?

Cosmic rays are extremely high-energy charged particles (usually protons) that move at nearly the speed of light. Some come from the Sun (in the form of solar energetic particles), while others are ejected from supernova explosions and other energetic events in interstellar (and intergalactic) space.  When cosmic rays collide with Earth's atmosphere, they produce showers of what are called "secondary particles".

History of Cosmic Ray Studies

The existence of cosmic rays has been known for more than a century.

They were first found by physicist Victor Hess. He launched high-accuracy electrometers aboard weather balloons in 1912 to measure the ionization rate of atoms (that is, how quickly and how often atoms are energized) in upper layers of Earth's atmosphere. What he discovered was that the ionization rate was much greater the higher you rise in the atmosphere — a discovery for which he later won the Nobel Prize.

This flew in the face of conventional wisdom. His first instinct on how to explain this was that some solar phenomenon was creating this effect. However, after repeating his experiments during a near solar eclipse he obtained the same results, effectively ruling out any solar origin for, Therefore, he concluded that there must be some intrinsic electric field in the atmosphere creating the observed ionization, though he could not deduce what the source of the field would be.

It was more than a decade later before physicist Robert Millikan was able to prove that the electric field in the atmosphere observed by Hess was instead a flux of photons and electrons. He called this phenomenon "cosmic rays" and they streamed through our atmosphere. He also determined that these particles weren't from Earth or the near-Earth environment, but rather came from deep space. The next challenge was to figure out what processes or objects could have been creating them. 

Ongoing Studies of Cosmic Ray Properties

Since that time, scientists have continued to use high-flying balloons to get above the atmosphere and sample more of these high-speed particles. The region above Antartica at the south pole is a favored launching spot, and a number of missions have collected more information about cosmic rays.

There, the National Science Balloon Facility is home to several instrument-laden flights each year. The "cosmic ray counters" they carry measure the energy of cosmic rays, as well as their directions and intensities. 

The International Space Station also contains instruments that study the properties of cosmic rays, including the Cosmic Ray Energetics and Mass (CREAM) experiment. Installed in 2017, it has a three-year mission to collect as much data as possible on these fast-moving particles. CREAM actually began as a balloon experiment, and it flew seven times between 2004 and 2016.

Figuring out the Sources of Cosmic Rays

Because cosmic rays are composed of charged particles their paths can be altered by any magnetic field that it comes into contact with. Naturally, objects like stars and planets have magnetic fields, but interstellar magnetic fields also exist.

This makes predicting where (and how strong) magnetic fields are extremely difficult. And since these magnetic fields persist throughout all space, they appear in every direction. Therefore it is not surprising that from our vantage point here on Earth it appears that cosmic rays do not appear to arrive from any one point in space.

Determining the source of cosmic rays proved difficult for many years. However, there are some assumptions that can be assumed. First of all, the nature of cosmic rays as extremely high-energy charged particles implied that they are produced by rather powerful activities. So events like supernovae or regions around black holes seemed to be likely candidates. The Sun emits something similar to cosmic rays in the form of highly energetic particles.

In 1949 physicist Enrico Fermi suggested that cosmic rays were simply particles accelerated by magnetic fields in interstellar gas clouds. And, since you need a rather large field to create the highest-energy cosmic rays, scientists began looking at supernova remnants (and other large objects in space) as the likely source. 

In June 2008 NASA launched a gamma-ray telescope known as Fermi — named for Enrico Fermi. While Fermi is a gamma-ray telescope, one of its main science goals was to determine the origins of cosmic rays. Coupled with other studies of cosmic rays by balloons and space-based instruments, astronomers now look to supernova remnants, and such exotic objects as supermassive black holes as sources for the most highly energetic cosmic rays detected here on Earth.

Edited and updated by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "Cosmic Rays." ThoughtCo, Jan. 5, 2018, thoughtco.com/history-and-sources-of-cosmic-rays-3073300. Millis, John P., Ph.D. (2018, January 5). Cosmic Rays. Retrieved from https://www.thoughtco.com/history-and-sources-of-cosmic-rays-3073300 Millis, John P., Ph.D. "Cosmic Rays." ThoughtCo. https://www.thoughtco.com/history-and-sources-of-cosmic-rays-3073300 (accessed January 18, 2018).