Researchers examined the space radiation environment and the risk it poses to long-duration astronauts remains limited. There is a disparity between research results and observed empirical effects seen in human astronaut crews, likely due to the numerous factors that limit terrestrial simulation of the complex space environment and extrapolation of human clinical consequences from varied animal models.

Given the intended future of human spaceflight, with efforts now to rapidly expand capabilities for human missions to the moon and Mars, there is a pressing need to improve upon the understanding of the space radiation risk, predict likely clinical outcomes of interplanetary radiation exposure, and develop appropriate and effective mitigation strategies for future missions. 

Aerospace Community

To achieve this goal, the space radiation and aerospace community must recognize the historical limitations of radiation research and how such limitations could be addressed in future research endeavors.

They have sought to highlight the numerous factors that limit understanding of the risk of space radiation for human crews and to identify ways in which these limitations could be addressed for improved understanding and appropriate risk posture regarding future human spaceflight.

While space radiation research has expanded rapidly in recent years, large uncertainties remain in predicting and extrapolating biological responses to radiation exposure in humans. The study of human health risks of spaceflight typically involves analogs that closely represent the space environment.

As future missions explore outside of low-Earth orbit (LEO) and away from the protection of the Earth’s magnetic shielding, the nature of the radiation exposures that astronauts encounter will include higher radiation exposures than any experienced in historical human spaceflight.

 The space radiation environment

Biological stressors related to space radiation are due to the effects of energy transfer from a charged particle to the human body. The combination of a particle’s charge, mass, and energy determine how quickly it loses energy when interacting with matter.

In biological tissue, the absorbed dose that a particular target organ receives from heavy-charged particle radiation depends not only on the energy spectrum of the particles but also on the depth and density of the tissue mass that lies between the skin surface and the target organ.

Limitations of terrestrial analogs

Mechanisms of biological impact

There are numerous limitations of current terrestrial analogs used for studying and predicting space radiation effects on biological tissues. The mechanisms that cause biological damage from space radiation are uniquely different from those associated with terrestrial radiation sources that are frequently used as surrogates in space radiobiology studies. 

Cumulative dose delivery and tissue distribution

Models of the space environment outside of LEO have predicted that astronaut crews may receive a total body dose of approximately 1–2 mSv/day in interplanetary space and approximately 0.5–1 mSv/day on the Martian surface. These doses would increase with any SPE encountered over the course of the mission.

Translation of space radiobiology research to human health outcomes

Biological damage from radiation exposure is generally classified as deterministic, dose threshold-based effects related to significant cell damage or death, where increased exposure is associated with increased risk through no threshold dose is necessary for biological impact.

As humans seek to explore space outside of the proximity and protection of LEO, they have the responsibility to address the space radiation risk to the extent of terrestrial capabilities to provide the best information and protection possible for our future explorers.