The Three Sources of Space Radiation

Lesson 2 of Space Radiation Foundations: trapped particles, solar particle events, and galactic cosmic rays, and how they differ in origin, energy, and predictability

Is the radiation a spacecraft encounters one continuous hazard, or several unrelated ones layered on top of each other? Lesson 1 established what counts as radiation in the first place; this lesson answers the question underneath it for anything operating in space — because “space radiation” is not one thing with one energy scale and one schedule. It is three physically distinct populations, each with its own origin, its own energy range, and, most consequentially for anyone planning around it, its own timescale of change.

!The three sources of space radiation: trapped particles in the Van Allen belts, solar particle events from the Sun, and galactic cosmic rays from outside the solar system

The Direct Answer

Space radiation comes from three sources: particles trapped in Earth’s own magnetic field (the Van Allen belts), bursts of protons thrown out by the Sun during flares and coronal mass ejections (solar particle events, or SPEs), and high-energy nuclei arriving from outside the solar system (galactic cosmic rays, or GCRs). A NASA public-education article lays out this same three-way split directly, naming galactic cosmic rays, the Van Allen belts, and solar particle events as the three main sources spacecraft and astronauts encounter. The three do not just differ in where they come from — they differ in how predictable they are, which matters as much as how energetic they are.

The Concept

Trapped Particles: The Van Allen Belts

The Van Allen belts are populated by protons and electrons trapped by Earth’s magnetosphere — radiation that originates from Earth’s own magnetic field rather than from any external source, per the same NASA classification cited above. NASA’s Van Allen Probes mission found that the belts’ basic structure has been remarkably stable: since their discovery in the late 1950s, the overall shape of the belts has remained largely unchanged, and the mission measured outer-belt electrons at energies exceeding 1 million electron-volts (>1 MeV). That structural stability is exactly what makes the belts one of the best-characterized radiation environments a mission will encounter — Reed and Pellish’s NASA technical report on the state of space-radiation models points to long-standing empirical engineering models (AE8/AP8 and their newer successors AE9/AP9) that have modeled trapped-particle populations for decades. For the specific inner- and outer-belt energy ranges, see the Van Allen belts glossary entry.

That said, “well-modeled” and “predictable” are not the same claim, and the same Van Allen Probes findings are explicit about where the predictability ends: the belts’ overall geometry is stable, but the storm-driven intensity of their particle flux is a different matter entirely. NASA describes that flux behavior, in the aftermath of geomagnetic storms, as currently unpredictable. A mission can count on the belts being in roughly the same place; it cannot yet count on knowing exactly how intense they will be at a given moment.

Solar Particle Events: Bursts From the Sun

Solar particle events are proton bursts released by solar flares and coronal mass ejections — episodic events tied directly to activity on the Sun, rather than a permanent feature of near-Earth space like the belts. NOAA’s Space Weather Prediction Center (SWPC) tracks these events as “solar radiation storms,” using a specific operational threshold: a storm is declared when the flux of protons at energies of 10 MeV or higher crosses a defined level at geosynchronous orbit. A NASA Johnson Space Center technical report on solar particle events, part of the THREE (The Health Risks of Extraterrestrial Environments) series, documents the proton energy range involved: SPE protons span roughly 10 MeV up to several GeV. The National Academies of Sciences’ 2018 consensus report on radiation risk independently arrives at the same order of magnitude for these events.

Unlike the belts’ stable geometry, SPEs are genuinely hard to predict in advance. NOAA SWPC’s forecasting for solar radiation storms is explicitly probabilistic and short-range — a 3-day forecast window, not a long-range schedule — which is a direct reflection of how quickly these events develop once the Sun produces one.

Galactic Cosmic Rays: The Interstellar Background

Galactic cosmic rays are the only one of the three sources that originates outside the solar system entirely. NASA traces their origin to supernova remnants, describing GCRs as nuclei stripped of their surrounding electrons — everything from hydrogen through uranium on the periodic table — traveling at speeds close to the speed of light. The National Academies’ 2018 report gives the energy range in per-nucleon terms: from around 1 MeV per nucleon up to more than 10,000 MeV per nucleon, the widest and highest-reaching energy span of the three sources, and consistent with why GCRs are typically the hardest of the three to shield against.

Where GCRs land on the predictability spectrum is the detail most easily missed. The same National Academies report frames all three sources together on a single predictability scale, and its own words are the clearest statement of the pattern: “The first two sources are relatively constant or change on time scales of days to years (e.g., the 11-year solar cycle). The third is highly variable, on the scale of minutes to a few days, in response to events on the Sun.” Trapped particles and galactic cosmic rays are the two sources that change slowly — GCR flux itself is modulated by the same 11-year solar cycle referenced in that quote. Solar particle events are the outlier: the one source that can go from absent to hazardous within minutes.

Editorial note — glossary discrepancy flagged, not yet corrected. The live glossary entry for solar particle events currently states an SPE proton energy ceiling of “several hundred MeV.” This lesson instead follows the NASA JSC THREE report and the National Academies’ 2018 report (both cited above), which independently place the ceiling at several GeV — roughly an order of magnitude higher. Both are publisher-grade, dual-sourced figures, so the lesson uses them rather than the glossary’s current text. A follow-up correction issue for the glossary entry itself is tracked separately; the glossary entry should not be treated as authoritative on this specific number until that correction lands.

Worked Example: Comparing the Three Sources

Takeaway

Energy alone does not sort these three sources into a risk ranking — GCRs reach the highest per-nucleon energies, but they are also among the slowest-changing sources, modulated only by the 11-year solar cycle rather than by sudden events on the Sun. The dimension that actually separates them operationally is timescale: two sources a mission can plan around on a days-to-years horizon, and one — solar particle events — that can turn hazardous within minutes and is only forecastable a few days out.

Key Facts

• Space radiation comes from three physically distinct sources — trapped particles, solar particle events, and galactic cosmic rays — not one uniform hazard (NASA, “Positive, Negative, or Neutral? NASA Explains Space Radiation,” 2024).

• The Van Allen belts’ overall structure has remained largely unchanged since their discovery in the 1950s, and outer-belt electrons exceed 1 MeV, but storm-driven flux intensity is currently unpredictable (NASA Van Allen Probes, 2016).

• Trapped-particle populations are modeled with long-standing empirical engineering models, AE8/AP8 and their successors AE9/AP9 (Reed & Pellish, NASA NTRS, 2016).

• Solar particle event protons range from about 10 MeV to several GeV; NOAA SWPC defines a solar radiation storm using a ≥10 MeV proton flux threshold and forecasts only 3 days ahead, probabilistically (NASA JSC THREE report, Hu, 2017; NOAA SWPC, “Solar Radiation Storm”).

• Galactic cosmic rays originate from supernova remnants outside the solar system, span hydrogen through uranium at near-light speed, and range from about 1 MeV/nucleon to more than 10,000 MeV/nucleon (NASA, 2024; National Academies of Sciences, 2018).

• Of the three sources, only solar particle events are highly variable on a minutes-to-days timescale; trapped particles and galactic cosmic rays both change more slowly, over days to years, including the 11-year solar cycle (National Academies of Sciences, 2018).

FAQ

Which of the three sources is hardest to shield against?

Galactic cosmic rays. They reach the highest per-nucleon energies of the three sources — up to more than 10,000 MeV/nucleon — and include heavy, fully ionized nuclei traveling near the speed of light, which is what makes them the most difficult of the three populations to shield against.

Are the Van Allen belts predictable?

Partially. Their overall geometry has been stable since the 1950s and is well-modeled by decades-old empirical models. But the intensity of their particle flux during and after geomagnetic storms is, per NASA’s own Van Allen Probes findings, currently unpredictable — structure and intensity are two different questions with two different answers.

Why are solar particle events described as “sporadic” instead of cyclical, like galactic cosmic rays?

Because they are tied to individual events on the Sun rather than a smooth cycle. NOAA SWPC can only forecast solar radiation storms about 3 days out, and probabilistically at that, which reflects how quickly a flare or coronal mass ejection can turn into a hazard — on the order of minutes to a few days, per the National Academies’ 2018 report.

Do the energy ranges of the three sources overlap?

At the proton end, yes. Solar particle event protons start around 10 MeV, which overlaps the upper end of inner-belt proton energies (up to hundreds of MeV). Galactic cosmic rays are on a different, per-nucleon energy scale and extend far beyond both, past 10,000 MeV/nucleon.

What’s Next

With the three sources identified and compared, Lesson 3 goes deeper on the first of them — the Van Allen belts — covering their inner/outer structure and the altitude ranges a mission actually has to plan around.