The Sun can kill. Until Earth developed its ozone layer hundreds of millions of years ago, life couldn’t venture out onto dry land for fear of exposure to the Sun’s deadly ultraviolet radiation. Even now, the 1% of its UV radiation that reaches the surface can cause cancer and even death.
Astronauts outside of Earth’s protective ozone layer and magnetic shield are exposed to far more radiation than on the planet’s surface. Exposure to radiation from the Sun and elsewhere in the cosmos is one of the main hurdles that must be cleared in long-duration space travel or missions to the lunar and Martian surfaces.
Unfortunately, there’s no harmonized approach to understanding the complexity of the hazard and protecting astronauts from it.
Astronauts haven’t gone further into space than the ISS for decades. But if Artemis lives up to its promise, they’re about to leave Earth and its protective environment behind. Artemis will land astronauts on the Moon, which could be an intermediate step to an eventual landing on Mars. What hazards does radiation pose, and how can astronauts be protected?
A new research editorial in the Journal of Medical Physics examines the issue. It is titled “System of radiological protection: Towards a consistent framework on Earth and in space.” The lead author is Werner Rühm from the Federal Office for Radiation Protection, München (Neuherberg), Germany. The same issue of the Journal of Medical Physics contains several other articles about radiation exposure. Together, they’re part of a research effort by the International Commission on Radiological Protection (ICRP) to update and harmonize radiation exposure guidelines.
The term ‘radiation’ is descriptive enough that most of us recognize the potential threat. However, when it comes to variable space environments and human physiology, the word holds a lot more detail. The authors use the term ‘mixed radiation field’ to describe the radiation environment astronauts must endure.
“The mixed-radiation field outside and within a space vehicle is of particular complexity involving not only low-linear energy transfer (LET) radiation such as gamma radiation, electrons, and positrons but also high-LET radiation such as neutrons and heavy ions,” the authors write. The components of the field contain a wide span of particles with different energy levels. “The quantitative and even qualitative risks of exposure to the combined impact of a complex radiation environment, microgravity, and other stressors remain unclear,” they explain.
One problem in preparing for exposure to these mixed radiation fields is the different approaches taken by different countries and space agencies.
According to lead author Rühm, this disharmony is caused by “the complex and dynamic radiation environments and an incomplete understanding of their biological consequences. Because of this, space agencies follow somewhat different concepts to quantify radiation doses and their resulting health effects.”
This paper and its companions are part of an effort to unify our understanding of radiation and its hazards and to harmonize the various approaches to dealing with them. The goal is to develop a “consistent radiological protection framework.” To do that, the authors explain that several questions need answers:
- Which radiation-induced health effects should be considered?
- What dose quantities are the best for the radiological protection of astronauts?
- Which metrics should be used to quantify radiation-related health risks?
- How do we address sex and age differences in radiation risk?
- What kind of protection criteria should be applied?
- How do we decide on the tolerability of radiation-induced risks, given that astronauts are exposed to many other occupation-related risks?
- How do we deal with the fact that increased health risks due to radiation exposure may persist after an astronaut’s career ends?
- How do we communicate radiation risk and make a comparison with other health hazards in a meaningful way?
- How do we harmonize national radiological protection guidelines, given that different subpopulations might have different levels of risk tolerance?
This list of questions vividly illustrates the complexity of the radiation exposure problem. Answering them will help harmonize the approach to radiation on space missions.
Rühm and his colleagues want to support space agencies as they harmonize and coordinate their guidelines for astronauts’ exposure to radiation. The goal is to develop an approach consistent with the thorough guidelines followed here on Earth.
The difference between how males and females respond to radiation illustrates one of the problems in developing radiation exposure guidelines. In past decades, much medical research was based on males and the results were applied to females as well. According to Rühm, the same thing has happened with radiation.
“It is worth mentioning that on Earth, the System developed by ICRP does not include any systematic differentiation between recommendations on limits for males and females,” the authors write. This is in spite of the fact that it is “well known that there are individual differences in radiation sensitivity between males and females.” The difference is largely because reproductive tissue is more susceptible to radiation than other tissue, and women have more of it.
NASA has developed a different approach to radiation exposure because of this. “This standard is based on a REID (Risk of Exposure-Induced Death) of 3% calculated for cancer mortality in the most vulnerable group of astronauts––35-year-old females,” the authors write. Scientists understand that females are more vulnerable to radiation than males and that younger females are more sensitive than older females. It’s worth noting that astronauts are unlikely to be under the age of 35.
The difference between the sexes isn’t the only thing that needs to be addressed when it comes to astronauts’ exposure to radiation. Different sub-populations might have different risk factors; there are lifestyle-related risks, different mission architectures hold different risks, and many other factors come into play. Harmonizing an approach with all of these different factors is a daunting task.
Difficult or not—and there’s nothing easy about space travel—a harmonized and coordinated approach to understanding the radiation risk is the logical next step. Artemis itself is a collaboration between different nations and agencies, and it’s only fair to the astronauts themselves that they have the same protections and considerations when it comes to radiation exposure.
Rühm and his colleagues hope that their work will help lead to a harmonized approach to assessing the radiation hazards faced by astronauts in mixed radiation fields. We owe it to the people willing to put their lives on the line and serve as astronauts.
“Adventurous people have always tried to widen their horizon, this is part of our very nature as humans,” Rühm says. “Our work contributes to and supports one of the most exciting and challenging human endeavors ever undertaken.”