Radiation in Cislunar Space (Part I); Radiation Sources and Environments

Radiation is an important factor in exploring the cosmos, and it also plays a critical role in cislunar exploration. With the increased frequency of lunar exploration and long-duration missions, the design and operation of space missions, particularly, require an understanding of radiation and radioactive effects in cislunar space. A series of articles will discuss cislunar radiation by diving into its sources, environments, effects on humans, shielding and mitigation forms, and other necessary and related aspects.

Radiation in Cislunar space spans from several sources and environments like Galactic Cosmic Rays (GCRs), Solar winds, Solar Energetic Particles (SEPs), Solar Cycles, Van Allen belts, Earth's magnetosphere, Solar storms, Geomagnetic disturbances, etc. GCRs are mostly high-energy particles, primarily protons and usually come from all directions, with support from heavier nuclei and electrons, originating from outside the solar system. Their intensity varies inversely with the 11-year period of the solar cycle, usually highest during the solar minimum when the Sun’s magnetic field is weaker and less effective at deflecting cosmic rays. But adversely, long-term exposure ripples biological tissues, raises cancer risks, and affects spacecraft electronics via single-event upsets and cumulative degradation.

Solar Radiation also contributes a great deal to cislunar radiation. Solar winds are continuous streams of charged particles (mainly protons and electrons) emitted from the Sun, with energy levels relatively low unlike other radiation sources but usually have compounding effects. Solar Energetic Particles (SEPs) are bursts of high-energy particles (protons, electrons, and heavy ions) generated during solar flares and coronal mass ejections (CMEs). Most times, SEP events are sporadic but can lead to intense short-term radiation exposure. Solar cycles are usually a series of solar activities, which usually include SEP frequency peaks during the solar maximum and frequency decline during the solar minimum.

Additionally, the Van Allen belt is another radiation source. Van Allen belts are zones of trapped radiation around Earth, containing high-energy protons and electrons. The outer belt usually overlaps with the cislunar region but becomes quite negligible near the Moon. The Earth's magnetosphere, also a major environment of cislunar radiation, extends partially into the cislunar region, providing some shielding from solar and cosmic radiation. However, spacecraft near the Moon are largely outside the protective influence of the magnetosphere.

Secondary particles, formed from certain mechanisms in the cislunar space, could also contribute to cislunar radiation. These particles are formed when primary cosmic rays or SEPs interact with materials such as spacecraft shielding, lunar regolith, or even human tissue. These interactions can produce neutrons, gamma rays, and other secondary particles that add to the radiation dose. Albedo Radiation, which is reflected radiation from the Moon’s surface, can also contribute to the environment, especially during lunar missions.

Cislunar radiation could also be induced by transient events like solar storms and geomagnetic disturbances. Solar Storms are intense solar activity that can significantly increase radiation levels due to SEP events. Prediction and monitoring are critical to protecting astronauts and spacecraft. Geomagnetic Disturbances are changes that the cislunar region can experience in radiation intensity due to geomagnetic storms that affect the Earth’s magnetosphere.

To be continued...