C H A P T E R
N ° 3
The Outer Space Environment: Radiation vs Space Radiation
What is radiation?
Radiation is something found both on Earth and in outer space. If you, therefore, ask multiple people to define or explain what radiation is you will receive different answers. Some may refer to cancer treatments or nuclear facilities, whilst others to things like Galactic Cosmic Radiation (GCR) and Solar Energetic Particles (SEPs). But what is radiation and is there a difference between radiation found on Earth compared to that found in space?
Radiation can be defined as a form of energy that is emitted in the form of rays, electromagnetic waves, and particles. Sometimes, radiation can be seen like visible light or even felt as infrared radiation. Other forms such as x-rays and gamma rays are not visible and can, thus, only be observed using special equipment.
The different explanations and definitions that people have when you ask them about radiation depends on their experiences and knowledge on the subject. If you ask someone in the healthcare system, you are more likely to hear positive things, such as how helpful it is to combat cancer. However, if you ask people within the space domain, you will most likely experience that they have different opinions depending on their work. Some may have a very negative view on radiation as it can have negative effects on the human body and mechanical systems. Others may tell you how amazing it is as it enables observations of the universe and different phenomena.
Image Credit: NASA: The electromagnetic spectrum (EM).
Where does radiation come from?
There are two types of radiation: radiation found on Earth and radiation found in outer space.
Radiation found on Earth can be created by humans or naturally. Radiation created by people can be found in things like microwaves, phones, radios, light bulbs, and diagnostic medical applications such as x-rays.
In contrast, naturally occurring radiation can be emitted from the Sun, radioactive elements in the Earth’s crust, radiation trapped in the Earth’s magnetic field, stars, and other astrophysical objects like quasar or galactic centers. Yet, the biggest source of radiation is from the Sun which emits all wavelengths in the electromagnetic spectrum (EM). Most of it is in the form of visible, infrared, and ultraviolet (UV) radiation. However, occasionally, solar activities cause space weather events which are loaded with x-rays, gamma rays and streams of protons and electrons. These can have serious consequences to astronauts, their equipment’s, spacecrafts etc.
Radiation found in outer space - also known as space radiation or cosmic radiation - is defined as high-energy, charged particles (i.e., ionized particles) produced in outer space. It is comprised of atoms where the electrons have been stripped away as the atom accelerated in interstellar space to speeds approaching the speed of light. At the end, it is only the nucleus of the atom which remains.
When referring to space radiation what we really refer to are three kinds of radiation:
1. Particles trapped in the Earth’s magnetic field.
2. Solar Energetic Particles (SEPs) which are particles bursting into space from the Sun during Solar Particle Events (SPEs).
3. Galactic Cosmic Radiation (GCR) / Galactic Cosmic Rays (GCRs) which are particles originating from outside our solar system.
All of the above represent ionizing radiation.
The biggest difference between radiation found on Earth and that found in outer space is the energy level of the particles. Thus, whereas radiation can be found on Earth, space radiation can not.
Image Credit: GeeksforGeeks: Illustration of Rutherford’s atomic model.
Non-ionizing and ionizing radiation
Radiation can either be non-ionizing and, thus, composed of low energetic particles, or ionizing and, thus, composed of highly energetic particles. The energy level of non-ionizing radiation is low and does, therefore, not have enough energy to remove electrons from atoms or molecules that it crosses. Examples of non-ionizing radiation include radio frequencies, microwaves, infrared, visible light, and ultraviolet (UV) light.
In contrast, ionizing radiation consists of particles that have enough energy to completely remove an electron from its orbit, consequently creating a more positively charged atom. Examples of ionizing radiation are alpha particles/alpha rays/alpha radiation which are positively charged nuclear particles identical with the nucleus of a helium atom that consists of 2 protons and 2 neutrons and is ejected at high speed in certain radioactive transformations. Other examples are gamma rays, x-rays, Galactic Cosmic Radiation, and Solar Energetic Particles.
It is no secret that ionized radiation is much more dangerous than non-ionized radiation in regards to space and terrestrial infrastructures. However, one should not underestimate the potential damage caused by non-ionized radiation. The most significant difference is, however, the shielding capabilities provided by today’s scientists, engineers, and health professionals. Whilst it is easier to protect against non-ionized radiation such as ultraviolet (UV) radiation, implementation of radiation shielding measures against ionized radiation is another story. The complexity of mitigation measures against ionized radiation is a longer and much more complex discussion. Yet, for today’s post SR Hoplon will simply provide a short introduction:
Ionized radiation has the ability to move through substances and alter them as it passes through. Through the penetration process, the radiation ionizes the atoms (i.e., the atom loses electrons) in the surrounding material that it interacts with. Depending on the density level of the material, ionized radiation - most often than not - penetrates through the material, leaving significant damage behind. If the material is too dense for the ionized radiation to penetrate through, the ionized radiation will hit the material with full force and shatter into several other particles - also known as a particle shower - producing secondary particles. Secondary particles have a lower energy level than that of the primary particle. However, they can cover a larger area, consequently causing the impact zone to be bigger. Due to this, some would argue that secondary particles propelled into motion by a primary ionized particle can cause more or the equal amount of damage of that of a primary particle.
Ionized particles found in space are categorised into three main groups: Galactic Comic Rays (GCRs), Solar Energetic Particles (SEPs), and radiation particles within the Van Allen Belts (radiation belts composed of ionized radiation surrounding Earth).
Galactic Cosmic Radiation (GCR) vs Solar Energetic Particles (SEPs)
Galactic Cosmic Radiation (GCR) originates from events like supernova outside the solar system but primarily from the Milky Way Galaxy. It is a type of radiation that consists of high-energy protons and heavy ions from elements that have had all their electrons stripped away as they travelled through the galaxy at nearly the speed of light. When passing through other atoms, they can cause them to ionize and can pass through astronauts and most spacecrafts.
Solar Energetic Particles (SEPs) – also known as solar cosmic rays – are high-energy charged particles originating from the Sun. They consist of protons, electrons, and heavy ions with energies ranging from a few tens of Kiloelectron volt (KeV) to many Gigaelectron Volt (GeV). They travel at the fraction of the speed of light (relativistic). Solar Particle Events (SPEs) consisting of Solar Energetic Particles (SEPs) are highly hazardous to electrons and humans in space. They can, additionally, be hazardous to aircrafts, pilot, and crew in particular polar routes, and to technology systems.
Whereas Galactic Cosmic Radiation is dominating the space environment in normal circumstances, it can get ‘replaced’ by Solar Energetic Particles during severe and extreme space weather events. This is due to the energy level of the particles emitted from the Sun. Thus, when the Sun releases plasma filled with ionized radiation into the outer space environment and it is for example Earth-directed, it increases the radiation intensity level in that environment, consequently intensifying the radiation exposure to objects in the space environment surrounding Earth. This does not necessarily mean that there is a higher amount of ionized particles in the environment but that the already present ionized radiation increases in intensity level, consequently increasing the risk of effects on nearby electronics, astronauts, planets and moons.
Image Credit: Georgina Torbet: Cartoon of solar activity (i.e. Solar Particle Event (SPE) emitting Solar Energetic Particles (SEPs).
Space Radiation and Earth
Earth has a global dipole magnetic field which means that the planet is fully surrounded by a magnetic field. This field acts as a shield against the usual influx of space radiation and, additionally, enables a dense atmosphere which provides further protection. However, during an Earth-directed severe or extreme space weather event the magnetic field can experience challenges.
Image Credit: Peter Reid; NASA: Schematic illustration of Earth’s global dipole magnetic field showcasing the magnetic field lines, magnetic poles, and geographic magnetic poles.
Furthermore, whilst Earth is mostly protected, the same cannot be said about its surrounding space environment. The magnetic field provides some level of shielding to its surrounding environment, yet not enough for spacecrafts and astronauts to be protected the same way as they would, had they been on Earth.
Learning and understanding solar and plasma physics and the outer space environment is crucial in order to understand the relation between the Sun and Earth and its surrounding space environment in order to enable the creation and implementation of mitigation measures. It is only when we understand the who, where, how, and what that we can act and do something about it. Furthermore, if the world wishes to enable human exploration missions to other planets and moons or to enable space tourism for all of humanity, we have to establish a deep and thorough understanding of the phenomena originating from the Sun and find or create highly protective shielding properties and emergency plans. Resilient mitigation measures should be established and implemented for all parts of space and terrestrial infrastructures, especially when discussing exploration and habitation beyond Earth.
Sources
NASA (2017): “Why Space Radiation Matters”. https://www.nasa.gov/missions/analog-field-testing/why-space-radiation-matters/.
Yihua Zheng; Rebekah M. Evans (2014): “Solar Energetic Particles (SEPs)”. SW REDI Boot Camp. https://ccmc.gsfc.nasa.gov/RoR_WWW/SWREDI/2014/SEP_YZheng_20140602.pdf.
Kathryn Whitman et al. (2023): “Review of Solar Energetic Particle Prediction Models”. Advances in Space Research. Volume 72, Issue 12. Pages 5161-5242. ISSN 0273-1177. DOI: https://doi.org/10.1016/j.asr.2022.08.006.
Space Center Houston: “What are the Van Allen radiation belts?”. https://spacecenter.org/what-are-the-van-allen-radiation-belts/. Last edit, May 19 2020.
Richard Wigmans (2017): “The Physics of Shower Development”. Oxford Academic. Calorimetry: Energy Measurement in Particle Physics 2nd edit. DOI: https://doi.org/10.1093/oso/9780198786351.003.0002.
