Summary:

• Cosmic rays are subatomic particles accelerated by the Sun during solar flares or by other stars when they explode in a supernova.
• Proxy data from isotope concentrations in ice cores and tree rings suggest evidence of extreme cosmic ray events in the past, far larger than measurements of recent events.
• Cosmic rays can ionise particles in electronic devices. This can cause malfunction and device failure, which could be catastrophic for safety-critical systems like those in nuclear power stations, military uses, and hospitals.
• During an extreme cosmic ray event, it is plausible that disruption to electronic devices and satellite services would be widespread. The impact of this on modern society is difficult to estimate but could be large.
• While cosmic rays can cause human health impacts like increased risk of cancer, the health burden is small on Earth. The health burden could be significantly greater for a space-faring civilisation.
• Relative to other cause areas recommended by major charity evaluators, this cause area does not seem to be extremely pressing from either a global health point of view or from an existential risk point of view, although there are some risks of both.

What are cosmic rays?

Cosmic rays are subatomic particles generated by the Sun or other stars that bombard the Earth and spacecraft. Cosmic rays are categorised based on their origin as either solar or galactic. Solar cosmic rays, also known as solar energetic particles (SEP), are accelerated from the Sun in both a constant stream from the solar wind and during large releases of energy from tightly bound magnetic fields during solar flares. Galactic cosmic rays (GCR) are particles that originate from other stars during huge explosions that either destroy the star or create a black hole. Cosmic rays are mostly protons (~90%) and alpha particles (~9%); electrons and heavier nuclei make up the remaining flux.

The flux of galactic cosmic rays is approximately constant^[1] and is often considered as the background rate. Solar energetic particles have a background component generated from the solar wind and rapid and transient increases in flux during large solar flares. The Sun follows a regular 11-year cycle of pole-reversal between periods of low magnetic activity, few sunspots, and few low-energy flares known as solar minima and periods of high magnetic activity, many sunspots, and many high-energy solar flares. At the time of publication (2022), the Sun is increasing its magnetic activity towards solar maximum around 2025.

Cosmic rays from both solar and galactic origin travel through space at close to the speed of light, with GCRs tending to have significantly higher energy. Cosmic rays are deflected, fully or partially depending on particle energy and incident angle, by the Earth’s magnetic field, thus protecting us from much of the damage that cosmic rays could cause. Like a bar magnet, the Earth’s magnetic field is approximately a dipole field and therefore offers greater magnetic protection around the equator than at the poles. This can be seen by the fact that the aurorae (the northern and southern lights), produced when cosmic rays ionise atmospheric gases, are usually visible only at the most poleward latitudes.

When cosmic rays hit the Earth’s atmosphere, they can induce an atmospheric shower where a series of collisions, ionisations, and decays, leads to a shower of secondary particles at ground level. These secondary particles are mostly neutrons. At the Earth’s surface, a constant but small flux of background neutrons caused by GCR is observed. Occasionally, SEPs are energetic enough to cause an increase in the neutron flux at ground level during events known as ground level enhancements (GLE).

How likely are extreme cosmic ray events?

At ground level, the background cosmic ray flux is around 1 neutron per square centimetre per second. This has a very low ionising effect on human health and a very low risk of effects on electronic devices.

However, solar energetic particle events have the potential to increase this cosmic ray flux by a large factor during GLEs. Since neutron monitor records began in around the 1940s, 73 GLEs have been recorded, just less than one per year on average. The distribution of peak fluxes during GLEs seems to be heavy-tailed, meaning that although the average GLE is relatively modest (~10% increase above background neutron flux), the most extreme GLEs are many times more extreme. The largest GLE since records began was recorded at a neutron monitor in Leeds, UK, in February 1956 and peaked around 5,000% (50 times) the background flux, for around 30 minutes, before slowly decaying.

Using a statistical approach known as extreme value theory, which fits carefully chosen statistical models to the largest events from a dataset, attempts have been made to estimate the probability of even more extreme events than the 1956 GLE. Mason 2015 estimated that the 10,000-year return level peak flux (that is, the peak flux expected to occur once over 10,000 years is around 50 times that of the 1956 event, i.e. around 250 times the background flux.

Proxy data from isotope levels measured in ice cores and tree rings have suggested that GLEs with significantly higher fluxes have occurred previously. Events in 7,176 BC, 5,259 BC, and 774 AD have been estimated as being tens to hundreds of times more intense than the 1956 GLE. However, there is a lot of uncertainty on these estimates due to the low temporal resolution of ice core and tree ring measurements.

Another rare originator of a rapid increase in cosmic rays are gamma-ray bursts. Gamma-ray bursts are end-of-life stellar explosions that emit radiation along a single axis, rather than symmetrically in all directions, and have the potential to send huge fluxes of cosmic rays towards Earth. They have been hypothesised to cause mass extinction events. In The Precipice (pg. 96), Toby Ord estimates that the probability of catastrophe caused by gamma-ray burst catastrophe in the next century is 1 in 2.5 million. This is several orders of magnitude lower probability than other natural extinction risks estimated in the book.

What impact could cosmic rays have?

Cosmic rays can have a negative effect on electronic devices in space, at aircraft altitudes, and at the Earth’s surface. They can also have a negative impact on human health.

High energy particles can have two effects on electronic devices: single event effects and total ionising dose effects. Single event effects occur when individual cosmic rays ionise atoms in an electronic device, which can cause a multitude of localised problems, including rapid energy discharge in power devices and bit-flips in memory devices. The resultant effect of these local disruptions depend strongly on the type of device and where in the device the particle is incident. The resultant effects can range from nothing at all to temporary or permanent malfunction.

Total ionising dose effects are progressive degeneration of electronic devices due to the accumulation of ionising radiation. The main reason for device degradation caused by total ionising dose effects is because of charge buildup in the silicon chip. This leads to gradual performance deterioration of the circuits and potentially to system failure. Total ionising dose effects tend to be minor at the Earth’s surface as the cosmic ray flux is usually so low.

Disruption of terrestrial electronics

At ground level, the Earth’s magnetic field gives significant protection by deflecting most cosmic rays away. However, the most energetic cosmic rays do reach Earth’s atmosphere causing a shower of secondary particles at ground level. While the particle flux is much lower at ground level, the amount of electrical infrastructure is many orders of magnitude larger than in space. Therefore, the total impact of a huge cosmic ray event would likely be larger on terrestrial devices than spacecraft.

Temporary or permanent malfunction or failure are some of the possible outcomes of cosmic ray impacts in ground-level electronics. This is particularly worrying for safety critical electronic devices, such as those used by hospitals, nuclear power stations, and weapons of mass destruction.

Rather alarming estimates of the failure rates of common electronic components have been made by testing their functionality in neutron bombardment experiments. There now exist neutron bombardment experimental facilities that have an energy spectrum that matches the GLE atmospheric spectrum, thus giving a close representation of the effects of a GLE. In such experiments, Dyer et al. 2020 estimated failure rates of around 1-2% per component for power MOSFETs and IGBTs (two common electronic components in many electronic devices) during the February 1956 GLE. This rises to over 50% during a 10,000-year GLE. Since electronic devices often have several such components, each required for the device to function, you can imagine the electronic disruption such an event would cause if these numbers are even close to being true!

There may be a possibility of cosmic rays accidentally triggering nuclear weapons or accidentally creating a signal that leads to the start of a nuclear war. This scenario seems unlikely, but I don’t know of any attempt to ensure that this probability is acceptably small. My best guess is that the probability is small compared to other possible causes of nuclear warfare. However, this is a key open question as it may be the most likely existential risk related to cosmic rays, given how high the component failure rates are during extreme GLEs.

This would not be the first time that phenomena of a solar origin had caused accidental detonation of military equipment. In 1972, a large geomagnetic storm induced the detonation of around 4,000 US military deep sea mines near Vietnam. These mines were designed to be triggered by variations in the Earth’s magnetic field caused by passing enemy ships, but the large disturbance of the Earth magnetic field caused by a large coronal mass ejection was sufficient to detonate them. No one is thought to have been injured in this instance. In his wartime memoirs, the US Navy Sailor, Chief Petty Officer Michael Gonzales, states that “During the first few weeks of August, a series of extremely strong solar flares caused a fluctuation of the magnetic fields, in and around, South East Asia. The resulting chain of events caused the premature detonation of over 4,000 magnetically sensitive DSTs (Destructor mines).”

In the same solar energetic particle event, the flux of cosmic rays during the 1972 space weather event was enough to trigger a warning as a possible breach of the nuclear weapons test ban in place at the time. High energy particle flux was measured by the Vela neutron counter on a satellite orbiting at the time in real time at Air Force Global Weather Central (AFGWC).

A possible ground-level cosmic ray event that has garnered a lot of attention is the 2003 elections in Brussels' municipality Schaerbeek, in Belgium, where an anomalous number of votes triggered an investigation that suggested a single event effect induced by cosmic rays was responsible for giving a candidate 4,096 extra votes. The main argument in favour of this explanation being that the erroneous votes was a power of 2. A sceptical view has since been given on this interpretation of the event.

Disruption of spacecraft and aircraft electronics

Electronic devices at spacecraft altitudes have the least protection by the Earth’s magnetic field and are vulnerable to the full impact of cosmic rays.

As an example, during the famous Halloween geomagnetic storms of 2003, some of the largest space weather events in the modern era, a large number of satellite anomalies were reported. NASA stated that 59% of their space science missions were affected, with a full and permanent loss of contact with the ADEOS II satellite. The loss of satellites can have an impact on scientific missions, but also on the services that satellites provide in the modern world, such as global positioning (GPS) and telecommunications.

A major impediment to estimating the impact that cosmic rays have on the space industry is that there is often no way to attribute causation of electronic issues to cosmic rays. Causal attribution can only be made once all other possible causes are deemed impossible. A 2017 study tried and ultimately failed to estimate the risk that space weather poses to the space sector by eliciting feedback from industry practitioners about experiences with space weather impacts. In an earlier study, which attempted to quantify the number of spacecraft anomalies caused by space weather, Iucci et al. 2005 compiled a database of about 5,700 anomalies registered by 220 satellites between 1971 and 1994. They show that satellite anomalies in high-altitude (>15,000 km) near-polar (inclination >55°) orbits and to a much smaller extent to anomalies in geostationary orbits were correlated with intense fluxes of solar cosmic rays.

Beyond cosmic rays, other space weather impacts on satellites are possible, including surface charging, radio signal blackouts over the poles, and increased atmospheric drag. The latter caused the loss of 40 SpaceX satellites in 2022. These effects are beyond the scope of this article, but would contribute to the overall impact of space weather on today’s society and a future space-faring civilisation.

Aircraft have partial protection from cosmic rays by the Earth’s magnetic field. The magnetic protection is reduced to about one third at normal subsonic cruising altitudes and to one tenth at supersonic altitudes leading to background radiation levels that are 300 to 1,000 times higher than at sea level. Space weather forecasts are monitored in the aviation industry and occasionally airlines are rerouted or delayed. The aviation industry also uses accelerated testing and some radiation-hardened devices for safety-critical uses. However there have still been several events where disruption to avionics has led to malfunction.

One such event occurred in 2008 on Qantas flight 72 between Singapore and Perth, Australia, when cosmic rays caused a malfunction of the autopilot system. This induced the plane into a rapid nose dive, sending everything that wasn't tied down, including passengers and crew, rapidly into the ceiling. At least 110 of the 303 passengers and nine of the 12 crew members were injured; 12 of the occupants were seriously injured and another 39 received hospital medical treatment.

Thankfully, these events are few and far between. Given the relative safety of air travel over the past 50 years, even during periods of increased cosmic rays, the risk of future catastrophe in this area seems low.

Human health

Cosmic rays can have a negative effect on human health. The main mechanism is through damage to DNA which can lead to cell mutation and eventually cancer. Other effects include damage to the cardiovascular system, damaging the heart, hardening and narrowing arteries, and damaging cells in linings of the blood vessels, leading to cardiovascular disease. Furthermore, radiation damage to neurons can lead to cognitive impairments and memory problems.

The doses of ionising radiation received at ground level with comparisons to various space flights is given in the figure below. Human health impacts are made significantly worse if the radiation is received over a short period of time. As a reference, 50 mSv is the US occupational dose limit per annum, 1,000 mSv is the maximum allowed radiation exposure for NASA astronauts over their career, and 4,000 - 5,000 mSv is the dose required to kill a human with a 50% chance within 30 days, if the dose is received over a very short duration.

It is worth noting that these dosages are based on normal space weather conditions. The dosages received during the most extreme solar energetic particle events would be hundreds of times higher. At ground level this is not particularly worrying, but the effects on an unshielded human in space during this event could be serious.

Attempts have been made to quantify the health burden of cosmic rays at ground level. Correlations have been found between solar energetic particle events and morbidities. However, the health burden is small compared to other priority public health issues and mitigation options are limited as there is no warning or prediction possible for GLEs.

It is clear that humanity is likely to develop into a space-faring civilisation, if we are able to survive the coming decades. This would radically increase the population of humans who are not protected from cosmic rays by the Earth’s magnetic field. So this may become a more significant health issue for the long-term.

What can be done to mitigate the effects of cosmic rays?

Cosmic rays are a known problem in the space industry and there are various countermeasures already in place including shielding of devices and increasing the component specification so that they are hardened to the effects of space radiation. Accelerated testing of electronic equipment in high energy particle bombardment experiments to test for failure probability is also commonplace. However, as the space industry booms, the incentive to cut costs by using commercial off-the-shelf (COTS) electronic devices in spacecraft that do not have these mitigation measures in place increases. This is compounded by the fact that space-qualified devices are not available for some of the high-performance requirements in the modern-day space industry. The market may not provide an adequate incentive for new satellites to be built with space-quality electronic components if we continue to have a quiet period of space weather activity. In which case, the space sector, and the myriad terrestrial uses of space technology, could set itself up for disaster if there is a huge cosmic ray event in the medium-term future.

Shielding of vulnerable electronics (or humans) can significantly reduce the probability of damage. Any material that contains water will have an attenuating (reduction in flux) and moderating (reduction in energy) effect on neutrons. Concrete, one of the most abundant building materials, has a significant water content and thus provides shielding of GLE neutrons. Based on experimental testing and computational radiation modelling, we know that every 1m of concrete provides around a 10x attenuation, but varying significantly based on the density and reinforcement of the concrete. This is good news for the resilience of electronic devices used in heavily reinforced buildings like nuclear power stations. Additionally, heavy metals like lead and iron provide around an order of magnitude more shielding than concrete.

Error-correcting code in memory devices can improve resilience to single event effects caused by cosmic rays. Some of the error mitigation approaches include parity check, cyclic-redundancy check coding, Hamming code, Reed-Solomon coding, convolutional encoding, and overlying protocol. These methods help to stop the localised single event effects propagating to macro-scale failures in the whole electronic device. Error-correcting code is commonplace nowadays, although it is unclear to what extent they reduce the probability of failure during extreme cosmic ray fluxes.

Grantmaking to reduce risks from cosmic rays

While there have been grants given and philanthropic research completed in other areas of space weather, I am aware of no philanthropic attempts to reduce the risk from cosmic rays. Relative to other cause areas recommended by major charity evaluators, this cause area does not seem to be extremely pressing from either a global health point of view or from an existential risk point of view, although there are some risks of both.

Uncertainty on the estimates of the risks, both on the probability of extreme events and the effect of such an event on critical electronic infrastructure. In my experience as a space weather researcher, very little research is conducted with the explicit aim of reducing space weather risk. Funding well-chosen research in this area would improve the outlook. There could also be improvements in how information trickles down from researchers to operators of safety-critical infrastructure, who are likely unfamiliar with the potential effects of cosmic rays.

Finally, there may exist policy changes that could be lobbied for that would significantly improve society’s resilience to cosmic rays. For example, for safety-critical electronic devices in hospitals, nuclear power stations, or for military use to be stored or built under sufficient shielding to reduce the risks to an acceptable level. Space policy could also be lobbied to ensure that future satellites are built with components that are more resilient to cosmic rays so that simultaneous global disruption to satellite services is not a possibility.

Selected open questions

• What is the joint probability of solar cosmic ray events occurring simultaneously with a large geomagnetic storm? And what would be the joint effects of these phenomena?
• Since the best estimates of extreme cosmic ray fluxes rest upon them, how trustworthy are the estimates of ground level enhancement fluxes from proxy data sources (such as isotope concentrations in ice cores and tree rings)?
• How resilient are the electronic components of weapons of mass destruction to cosmic rays?
• As society becomes evermore reliant on electronic devices and satellite services, what is the plausible worst-case scenario impact of a cosmic ray event?
• How should the possibility of extreme cosmic ray events affect humanity’s approach to space travel and space colonisation?
• Are there any policy changes that could improve society’s resilience to extreme cosmic ray events?

1. ^{^}

   While the flux of GCRs is mostly constant, it decreases following large coronal mass ejections. Coronal mass ejections are large ejections of solar plasma into space. When they travel Earthward, their magnetic field adds to the deflection of GCRs, thus decreasing the flux that reaches Earth. This is known as a Forbush decrease.