At a glance
Cosmic rays are charged particles, mostly atomic nuclei, that arrive at Earth from space at nearly the speed of light. Their energies span more than ten orders of magnitude, up to values no human-made accelerator approaches. Neutrinos, neutral and almost massless, are produced in many of the same sources and travel to us in straight lines. Together they form the particle side of multimessenger astronomy.
| Quantity | Value | Context |
|---|---|---|
| Intensity above 1 GeV | ~10⁴ particles m⁻² s⁻¹ sr⁻¹ | Mostly protons below ~10 TeV |
| “Knee” | a few PeV (10¹⁵ eV) | Spectral index steepens from ~2.7 to ~3 |
| “Ankle” | a few EeV (10¹⁸ eV) | Spectrum flattens to index ~2.5 |
| Flux suppression | above a few tens of EeV | Fewer than 1 particle per km² per year above 30 EeV |
| Amaterasu particle | ≈ 2.4 × 10²⁰ eV | Telescope Array, 27 May 2021 |
| KM3-230213A | ≈ 220 PeV (neutrino, median estimate) | KM3NeT/ARCA, 13 February 2023 |
| IceCube | ≈ 1 km³ of ice; 5,160 optical modules | South Pole, completed December 2010 |
Sources and dates: spectral features and intensities from the Particle Data Group review of cosmic rays (2024 edition); Amaterasu from the University of Utah (November 2023); KM3-230213A from the KM3NeT Collaboration (Nature, 12 February 2025); IceCube from the IceCube Collaboration. 1 PeV = 10¹⁵ eV; 1 EeV = 10¹⁸ eV.
Discovery: radiation from above
At the start of the twentieth century, physicists noticed that electroscopes slowly lost their charge even when shielded, as if the air were always slightly ionised. The natural assumption was radioactivity in the ground, which should weaken with height. In 1912 the Austrian physicist Victor Hess carried instruments aloft in balloon flights and found the opposite: above a certain altitude, ionisation increased the higher he went. The radiation came from space. Hess shared the 1936 Nobel Prize in Physics “for his discovery of cosmic radiation”.
The name “rays” stuck, although we now know that the primary particles are not electromagnetic radiation. Early cosmic-ray studies were also the birthplace of particle physics: the positron, the muon and the pion were all discovered in cosmic-ray experiments before accelerators existed.
Sources: [1]
Composition and the energy spectrum
By number and by energy, protons dominate the cosmic-ray flux below about 10 TeV, followed by helium and heavier nuclei. Above 1 GeV the intensity is around 10⁴ particles per square metre per second per steradian. The flux falls steeply with energy, close to a power law with index about 2.7: each tenfold increase in energy brings roughly five hundred times fewer particles.
The spectrum is not a perfect power law, and its breaks are clues to where cosmic rays come from. At a few PeV is the knee, where the index steepens to about 3; a second knee follows at around 100 PeV, with an index near 3.3. At a few EeV the ankle marks a flattening to about 2.5. Above a few tens of EeV the flux is strongly suppressed: fewer than one particle per square kilometre per year arrives above 30 EeV. A common interpretation is that Galactic accelerators, such as supernova remnant shocks, run out of power around the knee, and that an extragalactic population takes over near the ankle.
The suppression at the highest energies had been predicted by Greisen, Zatsepin and Kuzmin (the GZK effect): protons above a threshold collide with photons of the cosmic microwave background and lose energy, which limits how far they can travel. Whether the observed cut-off is mainly due to this effect or to sources reaching their maximum energy is still debated, and the answer depends on the composition at the highest energies.
Not every nucleus that reaches Earth left its source in that form. Lithium, beryllium and boron are rare in stars but relatively common among cosmic rays, because they are produced when heavier nuclei such as carbon and oxygen break apart in collisions with interstellar gas. The ratio of these “secondary” nuclei to their “primary” parents, for example boron to carbon, tells physicists how much matter cosmic rays have crossed, and therefore how long they stay trapped in the Galaxy’s magnetic fields. Such ratios are among the precision measurements made in space and are collected in public databases (see Data for researchers).
Cosmic-ray energy scale lab
Pick an energy and a particle, or start from the presets for an LHC proton beam, the knee, the ankle and the Amaterasu particle. Compare the gyroradius in the Milky Way’s magnetic field with the thickness of the Galactic disc, and see at which energy the Galaxy can no longer hold on to a proton, and how much higher that limit is for an iron nucleus.
For a proton at the knee in a 3 μG field the gyroradius is about one parsec, so such particles wander for a long time inside the Galaxy. At the ankle it is close to 2,000 parsecs, larger than the disc’s thickness, which is one reason why the highest-energy cosmic rays are thought to be extragalactic. A heavy nucleus carries 26 times the charge of a proton, so at the same energy its path curves 26 times more tightly; this is why composition matters when cosmic rays are traced back to their sources. As the lab’s note explains, the real Galactic field is turbulent, so these radii are indicative.
Air showers and the detectors that catch them
Only the lowest-energy cosmic rays can be caught directly, above the atmosphere. Higher-energy particles are so rare that they must be observed through the extensive air showers they trigger. The primary particle collides with a nucleus high in the atmosphere and produces a burst of secondary particles, mainly pions. Neutral pions decay into gamma rays that start electromagnetic cascades of electrons, positrons and photons; charged pions decay into muons and neutrinos. By the time it reaches the ground, a shower can be enormous: the Amaterasu shower described below triggered 23 detectors spread over 48 km².
The Pierre Auger Observatory on the Pampa Amarilla near Malargüe, Argentina, is the largest cosmic-ray detector. More than 1,600 water-Cherenkov stations, usually 1,500 m apart, cover about 3,000 km²; 27 fluorescence telescopes at four sites on the array’s edge watch, on dark nights, for the faint ultraviolet light emitted as the shower excites atmospheric nitrogen. Combining both techniques gives a calibrated energy scale. In the northern hemisphere, the Telescope Array in Utah has 507 surface detector stations over 700 km². On 27 May 2021 it recorded the Amaterasu particle, about 2.4 × 10²⁰ eV, roughly 40 joules in one subatomic particle, the second most energetic cosmic ray recorded after the “Oh-My-God” particle seen by Utah’s Fly’s Eye detector in 1991. Puzzlingly, its arrival direction points back to the Local Void, a region with few galaxies.
At lower energies, direct measurements from space are possible. The Alpha Magnetic Spectrometer (AMS-02) has been operating on the International Space Station since May 2011 and has recorded more than 250 billion cosmic-ray events. It measures the charge, momentum and sign of each particle, separating matter from antimatter.
Neutrino astronomy
Because cosmic rays are charged, magnetic fields bend their paths and scramble their arrival directions. Where cosmic rays are accelerated and collide with gas or light, however, they also make neutral pions and charged pions, and the charged pions decay into neutrinos. Neutrinos have no charge, are barely deflected and pass through dust and gas, so they point back to their sources. The price is that they interact so rarely that detectors must be enormous.
Neutrino astronomy beyond the Sun began on 23 February 1987, when the Kamiokande detector in Japan captured twelve neutrinos from supernova SN 1987A in the Large Magellanic Cloud, out of about 10¹⁶ that passed through it. Masatoshi Koshiba shared the 2002 Nobel Prize in Physics for this work.
The IceCube Neutrino Observatory at the South Pole turns a cubic kilometre of Antarctic ice into a detector: 5,160 digital optical modules on 86 strings, between 1,450 and 2,450 m deep, record the faint light from charged particles produced when a neutrino interacts. The last string was installed on 18 December 2010. Detectors like this see two main event shapes. A muon neutrino that interacts can produce a muon that travels a long way through the ice, leaving a track whose direction can be reconstructed well; other interactions deposit their energy in a compact, nearly spherical cascade, which measures energy better but direction less precisely. On 22 September 2017 IceCube detected a neutrino of about 290 TeV coinciding with a flaring gamma-ray blazar, TXS 0506+056, the first evidence for a specific high-energy neutrino source. In 2022 it reported an excess of about 79 neutrinos from the nearby active galaxy NGC 1068, at 4.2σ, using data from 2011 to 2020. In 2023, with machine-learning methods applied to ten years of data, it identified neutrino emission from the plane of the Milky Way at 4.5σ.
Current research (2024–2026)
The most energetic neutrino yet. On 12 February 2025 the KM3NeT Collaboration reported in Nature an extremely high-energy muon that crossed its ARCA detector in the Mediterranean on 13 February 2023, when only 21 detection lines were operating. The muon’s energy was estimated at 120 PeV (90 per cent interval 35–380 PeV), and the median energy of a neutrino producing such a muon is about 220 PeV, far above any neutrino seen before. The event, KM3-230213A, may come from a cosmic accelerator or from cosmic rays interacting with background photons; one event cannot yet decide.
Upgrading the detectors. During the 2025/26 austral summer, five new strings carrying new photosensor designs were deployed as a dense infill at the centre of IceCube, the IceCube Upgrade, which will improve calibration and low-energy measurements. On the ISS, AMS-02 is being prepared for a new tracking layer, Layer-0, expected to triple the detector’s acceptance; CERN reported in 2026 that integration of the flight hardware was under way, with transport on a SpaceX Dragon and installation by spacewalks to follow.
Cosmic rays, space weather and astronauts
For spacecraft and astronauts, cosmic rays are a hazard as well as a research topic. NASA distinguishes three sources: galactic cosmic rays, heavy nuclei stripped of their electrons and moving at nearly the speed of light; solar particle events, bursts of protons and electrons from the Sun; and particles trapped in Earth’s Van Allen belts. Galactic cosmic rays are the hardest to stop, as they can pass practically unimpeded through a typical spacecraft. Their flux at Earth varies with the solar cycle, being lower when the Sun is active; the Sun’s side of the story is in our sun and space weather dossier.
NASA gives effective doses for astronauts in the range of 50 to 2,000 mSv, where 1 mSv is roughly the dose of ten chest X-rays. Beyond low Earth orbit, the agency lists increased lifetime cancer risk, effects on the central nervous system and degenerative diseases among the concerns. This is why radiation shielding, storm shelters and dosimetry are central to planning long missions to the Moon and Mars, discussed on our human spaceflight page.
Sources: [14]
Data for researchers
The Pierre Auger Observatory open data portal provides about 10 per cent of the observatory’s cosmic-ray events, in raw-like and reconstructed form, together with Python notebooks and a catalogue of the highest-energy events. IceCube publishes neutrino event samples on its data releases page, including a track sample covering 2008 to 2022. The Cosmic-Ray Database (CRDB) collects published fluxes and ratios from AMS, PAMELA, Voyager, Auger and other experiments in one searchable place.
Known vs open questions
Established results
- Cosmic rays come from space, are mostly protons and nuclei, and follow a steep spectrum with a knee, an ankle and a suppression at the highest energies.
- Particles above 10²⁰ eV exist, although only a handful have ever been recorded.
- High-energy astrophysical neutrinos exist; the active galaxy NGC 1068 and the Galactic plane are identified neutrino sources.
- Galactic cosmic rays are a major, hard-to-shield radiation risk for crewed missions beyond low Earth orbit.
Open questions
- Which objects accelerate the highest-energy cosmic rays, and how?
- Is the suppression above ~50 EeV mainly the GZK effect or the limit of the sources?
- Where did the Amaterasu particle and KM3-230213A come from?
- How does the composition change across the knee and the ankle?
Related on this site
Sources and further reading
- Nobel Prize Outreach — Victor F. Hess: Facts — The Nobel Prize in Physics 1936
- Particle Data Group — Cosmic Rays (review) — Review of Particle Physics, Physical Review D 110, 030001 (2024)
- LIP Auger group — The Pierre Auger Observatory — Laboratório de Instrumentação e Física Experimental de Partículas (Auger member institute)
- University of Utah — Telescope Array detects second highest-energy cosmic ray ever — Department of Physics and Astronomy (November 2023)
- Telescope Array Collaboration — An extremely energetic cosmic ray observed by a surface detector array — Science 382, 903 (2023); arXiv:2311.14231
- CERN EP Newsletter — AMS-02: A New Chapter in Orbit to Probe the Nature of Cosmic Rays — CERN Experimental Physics Department (2026)
- Nobel Prize Outreach — The Nobel Prize in Physics 2002: press release — Royal Swedish Academy of Sciences (8 October 2002)
- IceCube Neutrino Observatory — IceCube — IceCube Collaboration / University of Wisconsin–Madison
- IceCube Collaboration et al. — Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A — Science 361, eaat1378 (2018); arXiv:1807.08816
- IceCube Collaboration — Evidence for neutrino emission from the nearby active galaxy NGC 1068 — Science 378, 538 (2022); arXiv:2211.09972
- IceCube Collaboration — Observation of high-energy neutrinos from the Galactic plane — Science 380, 1338 (2023); arXiv:2307.04427
- KM3NeT Collaboration — Observation of an ultra-high-energy cosmic neutrino with KM3NeT — Nature 638, 376–382 (12 February 2025)
- IceCube Collaboration — IceCube Upgrade status and perspectives — arXiv:2609.24387 (2026)
- NASA — Why Space Radiation Matters — NASA Space Radiation Laboratory