Space Radiation

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Space is trying to kill you. It tries to kill you in many different ways. One of those ways is to flood itself with dangerous radiation that can kill biological organisms, damage or disable electronics, and degrade some kinds of materials.

Galactic Cosmic Rays

Cosmic flux versus particle energy at the top of Earth's atmosphere.

Space is filled with energetic charged particles – primarily protons (~90%) and alpha particles (~9%) but also including other light and medium ions. These are not associated with any immediate stellar environment but instead are thought to come from outside of our solar system, originating in supernovas, neutron stars, active galactic nuclei, quasars, and gamma ray bursts.

These cosmic rays generally have much higher energies than other forms of space radiation. A typical energy common to one of these particles would be around several hundred MeV to a GeV. Some have lower energies; these are often shielded from solar systems or planets by the sun's magnetic field, the solar wind, or planetary magnetospheres. More notorious, however, are those with higher energies. Often much higher. The most energetic cosmic ray ever measured (as of 2024) had an energy of $3.2\times 10^{20}$ eV, or around 50 joules – the energy of a major league baseball pitch in a single particle.

High energy massive particles, such as these cosmic rays, will have a high gyroradius, so they will not be strongly deflected by magnetic fields. Consequently, more energetic cosmic rays can pierce a planets magnetosphere to deliver radiation dose to those in orbit. Lower energy cosmic rays can be deflected by either magnetic fields that cover a very large amount of space (such as those around planets) or magnetic fields with a very high field strength.

Cosmic rays come through at a steady sleet, delivering on the order of 1 – 2.5 mSv/day. This dose is not delivered fast enough to cause acute radiation sickness, but is roughly two orders of magnitude higher than the natural background radiation dose on Earth. This can cause issues with chronic radiation exposure. The main concern is an increased risk of cancer. However, experiments on rodents exposed to radiation from a particle beam simulating long duration exposure to cosmic radiation also suggests the possibility of reduced cognitive function after several months in deep space. The cosmic ray dose rate is lower in times of high solar activity as the increased solar wind prevents more cosmic rays from entering our solar system. A planetary magnetosphere like that of Earth can deflect enough of the lower energy cosmic rays to make a noticeable difference in the dose rate, often in the 0.2 – 1 mSv/day range in low orbits below the main radiation belts, although this depends strongly on the latitudes through which the satellite passes. Equatorial orbits offer the best protection, and polar orbits pass through the radiation belts where the cosmic rays are deflected to. A significant amount of this shielding is also afforded by the planet itself, which will block cosmic rays from close to half the sky for close orbits.

Cosmic rays passing through a computer chip can cause transient errors that can result in a glitch in operations or a corrupted bit of memory. High doses of radiation can also cause permanent damage to elctronics.

Because they can have such a high energy, cosmic rays can be difficult to shield against. A typical cosmic ray will pass through several tens of centimeters of solid or liquid matter before striking an atomic nucleus. The cosmic ray has so much energy that this shatters the nucleus, sending nuclear fragments spraying through the material and possibly (depending on the cosmic ray's energy) creating exotic particles such as pions or kaons as well as energetic electrons and positrons (and possibly the odd anti-proton or anti-neutron as well). The nuclear fragments that come out at lower energy slow down and stop inside the material before colliding with another nucleus, producing a very high ionization density near the end of their track that can cause significant radiation damage. Higher energy fragments, along with the pions and kaons, are likely to continue the radiation cascade by slamming into more nuclei every few tens of centimeters or so and making more showers of nuclear particles until the energy of the primary cosmic ray is distributed among so many secondary particles that there is not enough energy left to shatter additional nuclei. Meanwhile, the high energy electrons and positrons make extensive electron-gamma showers.

On Earth, we have the benefit of ten tons of air over every square meter of ground to help intercept and stop this space radiation. This stops essentially all of the particles from the radiation showers, except for the pions that decay in flight into muons, which do not strongly interact with nuclei and so they can reach the ground. However, cosmic rays initially interact with the atmosphere at altitudes of several tens of kilometers. The great distance that the particles have to travel to reach the ground means that even most of the muons decay before reaching us, and the electrons the muons decay into are quickly stopped (the pion and muon decays also produce neutrinos, which are not stopped. By anything. Even the ground. They just go right through the Earth without interacting, and consequently are of little interest when considering the effects of radiation).

In space, it is expensive to carry this much shielding. Even worse, a moderate amount of shielding might make things worse, by allowing the impacting cosmic rays to produce more secondary particles. The best shielding uses hydrogen-rich materials with only light elements to limit the secondary radiation. One of the preferred materials is polyethylene, composed of two hydrogens for each carbon atom and naught else. On a planetary or sub-planetary body lacking an atmosphere, native ice or regolith could be used as shielding by piling it over and around any facilities.

Solar Radiation

Proton energy spectra at 1 AU, showing the increase in solar energetic particles during solar particle events.

Solar Energetic Particles

The sun is an erratic source of high energy particles, ranging from keV to GeV energies. These solar energetic particles or SEPs, as they are called, are often produced in solar flare or coronal mass ejection events (see below). Such an event that produces SEPs is called a solar particle event. SEPs are primarily protons, with some alpha particles and a small amount of light and medium ions. As protons below about 30 to 50 MeV energy can't penetrate even thin spacecraft hulls, we are mostly concerned about those SEPs in the 100 MeV to GeV range. When the sun is quiescent, SEPs in this energy range are negligible compared to cosmic rays. However, in a solar particle event the flux of SEPs can jump by six orders of magnitude, posing a significant radiation hazard to anyone in space and not protected by a planetary magnetosphere. The Earth's magnetosphere does a good job stopping SEPs from reaching close orbits at low latitudes, but funnels the deflected particles to the poles where they produce auroras.

Solar Wind

The solar wind is an outflowing plasma streaming from the Sun's outer layer called the corona. These are low energy particles, generally ranging from sub-keV to several keV, and quite incapable of penetrating spacecraft hulls or space suits. This solar wind is of little concern from a radiological perspective.

Stellar Corpse Environments

Supernovas

If you are near a supernova, space radiation is probably one of the smaller of your concerns. However, Type II supernovas are notable in being one of the only phenomena known that can produce dangerous levels of neutrino radiation. Neutrinos are normally so penetrating that they go through everything without significant interactions. However, the core collapse of Type II supernovas makes neutrinos in such prodigious quantities that enough of them can interact and cause radiation sickness and death within approximately the distance of the inner solar system.

Artificial Radiation Sources

The main focus of this article is on natural sources of radiation. But if you expect to operate in space you will also need to consider common artificial radiation sources. Many spacecraft and other space infrastructure are expected to be powered by fission or fusion reactors, or to use fission or fusion propulsion. All of these will produce copious amounts of nuclear radiation in the form of energetic neutrons, gamma rays, and the emissions of radioactive isotopes produced through fission or neutron capture. Without an atmosphere to attenuate the radiation produced, high power radiation sources can have an effect over a much larger distance than a similar unshielded source on Earth. This will produce a hostile radiation environment that will require large exclusion zones or shielding.

In addition, space conflict scenarios are likely to use particle beam weapons, x-ray or gamma-ray lasers, and nuclear explosives. All of these produce radiation as a primary effect or side effect of their operation.

Nuclear reactors and explosions in the vicinity of a planet with a magnetic field can make artificial radiation belts that persist for days to years (depending on the altitude), and can severely damage electronics operating within or passing through the belt.

Credit

Author: Luke Campbell