Cosmic rays are high-speed particles created by supernova explosions and other space interactions, carrying elements that can reveal how stars, matter, and the universe evolve. The Alpha Magnetic Spectrometer (AMS-02) on the International Space Station has collected trillions of cosmic ray events, enabling scientists to classify 20 elements into distinct groups of primary and secondary cosmic rays with unprecedented precision. The study was published in Physical Review Letters.
The Alpha Magnetic Spectrometer (AMS-02) is visible at center left, as installed on the International Space Station. Image Credit: NASA
Millions of years ago and millions of light-years away, a star underwent a catastrophic explosion.
During this event, the star expelled vast amounts of material, including carbon, nitrogen, and oxygen, the fundamental elements of life, as well as other elements up to iron on the Periodic Table. These elements were scattered throughout space, leaving behind only the star's burned-out core.
At the time, no one on Earth knew the star was exploding. To start, humans had not yet evolved, and the cosmic rays (streams of high-energy particles released during the event) are only now reaching Earth. Although they have traveled for millions of years from their source, these particles provide valuable insights into how the universe works
The understanding of these cosmic rays is growing due to recent findings from the Alpha Magnetic Spectrometer (AMS-02), an experiment on the International Space Station funded by the Department of Energy's Office of Science.
Particles from the Stars
Cosmic rays are not the same as X-rays or UV light rays. They are collections of particles that are falling from space. Phosphorus, chlorine, potassium, argon, and calcium are just a few of the elements that may be present in these cosmic rays.
Some of these elements are produced by supernovae, or exploding stars. Others are created when heavier element nuclei, originally from supernovae, collide with cosmic gases.
Although cosmic rays constantly strike Earth's atmosphere, relatively little is known about them. One of the biggest questions is why these particles continue to travel at such extraordinary speeds. While a star's explosion gives them an initial burst of energy, they should eventually slow down. The fact that many are still traveling at nearly the speed of light suggests that something has continued to accelerate them.
What These Rays May Reveal
To determine whether the former USSR's leadership was testing nuclear weapons, the US investigated cosmic rays decades ago. It became irrelevant, however, when scientists discovered that cosmic rays were coming down from the sky rather than up from the Earth.
NASA and the Department of Defense found other reasons to research cosmic rays, even if they were not useful for espionage. Future astronauts will be exposed to more cosmic rays as space exploration grows, whether on the Moon or aboard a space station. Additionally, satellites used for GPS, communications, and other functions are being bombarded by cosmic rays. These organizations seek to comprehend how these rays affect both people and equipment.
Dark matter, one of science's most important mysteries, may also be resolved by cosmic rays.
Dark matter exclusively interacts through gravity, in contrast to conventional matter. Its impacts on stars, galaxies, and the astrophysical records of the early universe are the sole reasons astronomers are aware of its existence.
Scientists have failed to discover it, directly or indirectly, despite estimates that it accounts for around 25% of the mass-energy of the cosmos. In order to comprehend the fundamental components of the cosmos, the DOE's Office of Science is interested in finding dark matter.
Dark matter particles may collide and annihilate one another, according to certain dark matter theories. Positrons, the antimatter equivalent of electrons, would be produced if dark matter behaves in this way. Cosmic ray statistics should reflect this overabundance of positrons.
Earth’s atmosphere provides protection from the potentially harmful effects of cosmic rays. The amount of water vapor in the atmosphere is equivalent to a 10-meter-deep layer of water. However, this protective barrier prevents accurate measurement of cosmic rays from the ground, making space-based observations essential for their study.
Introducing the Alpha Magnetic Spectrometer
The AMS-02 is the ideal instrument for measuring cosmic rays in space, not simply a device for doing so. The only instrument that has collected this data for a considerable period is the AMS-02. It has spent more than 13 years on the International Space Station, where it has been subjected to the severe conditions of space.
The AMS-02 is essentially a particle detector. On Earth, particle accelerators, such as CERN's Large Hadron Collider, are frequently equipped with particle detectors. Others are located in isolated areas to collect particles free from human intervention. However, the AMS-02 does not require a particle source or a site far from people, given its position in space. It is also far smaller than most particle detectors, measuring about the size of a large coffee table.
Since its installation in 2011, the AMS has recorded more than 230 trillion cosmic-ray events. Most of these events involve the universe's most abundant elements, such as hydrogen and helium, and have been detected billions of times. Interactions involving lithium are far less common, numbering around 100,000. Among those trillions of events, only a few hundred involve heavier elements such as iron, nickel, and zinc.
Scientists determine the momentum and parent element of the particles based on the data gathered. They compute a quality known as "rigidity" using this data. This characteristic tells how cosmic rays may be created, accelerated, and dispersed over space. The knowledge of nuclear physics on Earth can likewise be enhanced by information about these cosmic nuclei.
Insights Into the Cosmos
New discoveries are being sparked by these trillions of data points.
The AMS-02's observations of sodium, aluminum, neon, magnesium, and sulfur were detailed in earlier analyses. There were primary and secondary rays in these cosmic rays. The beams that emerge from outer space largely unharmed are called primary rays. Over time, secondary ray particles have interacted with interstellar gases. Because the ions are mixed with other particles such as electrons and muons, these rays are more disorganized.
It is interesting to note that whether an element was "even" or "odd" determined whether the rays were primary or secondary. It makes sense that odd elements, like sodium and aluminum, have an odd number of protons, whereas even elements, like neon, magnesium, and sulfur, have an even number.
Even and odd elements are created differently in stars, even though they are adjacent on the Periodic Table. Odd nuclei are also less stable. The link between whether the ions were even or odd and whether they interacted with anything as they traveled through space may provide insight into the formation and behavior of these fundamental components of the cosmos.
The most recent findings from the AMS-02 expand that information to include interactions involving phosphorus, chlorine, potassium, argon, and calcium, which are much less common or have more limited reliable data. Some of these elements had been detected in previous cosmic ray investigations, but their measurement errors were quite significant.
This study demonstrated that some elements were mostly found in primary cosmic rays, whilst others were primarily found in secondary ones. Two classes of primary cosmic rays were found in the data: one is composed of helium, carbon, oxygen, and iron, while the other is composed of neon, magnesium, silicon, and sulfur.
Researchers also discovered two classes of secondary rays: one composed of boron, lithium, and beryllium, and another composed of fluorine, phosphorus, and potassium.
This study demonstrates that the 20 elements the AMS-02 team has identified, all of which fall between helium and iron on the Periodic Table, can be divided into four classes when paired with the earlier findings.
Above all, this data contradicts existing theories of cosmic rays. Because the AMS-02 data is so accurate, it suggests that there may be a significant phenomenon that cannot be explained by scientific theories. Determining the reason for this disparity may provide more insight into how the universe functions.
Even after more than a decade of operation, scientists continue to uncover unexpected insights in the data collected by AMS-02. Although supernovae last only a few days, the cosmic rays they produce provide information that researchers will continue to study for years to come.
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Journal Reference:
Aceituno, A., et al. (2026) Properties of Heavy Cosmic Nuclei Phosphorus, Chlorine, Argon, Potassium, and Calcium: Results from the Alpha Magnetic Spectrometer. Physical Review Letters. DOI: 10.1103/d2vf-fw3v. https://journals.aps.org/prl/abstract/10.1103/d2vf-fw3v.