The National Aeronautics and Space Administration (NASA) works to realize the benefits of space exploration, and these benefits accrue, for the most part, to society through technological and scientific advances, as well as national and international pride and collaboration (IOM, 2014). Astronauts are in a unique class of employees as they work for an agency whose mission is exploration, which implies both a high level of risk and uncertainty as astronauts explore space as well as NASA’s duty to care for their safety during a mission and throughout an astronaut’s lifetime. NASA has long recognized that crewed space missions carry a range of unique hazards and challenges, including health-related risks.
As NASA prepares for space exploration missions that extend to greater distances into our solar system and for longer durations, including missions to near-Earth objects, the Moon, and Mars, as well as prolonged stays on the International Space Station (ISS), these challenges are further amplified. To ensure the safety and success of these missions, health and performance risks associated with spaceflight must be adequately characterized, controlled, and mitigated through novel tools and technologies (NASA, 2021a). NASA uses an evidence-based approach to assess the likelihood and consequence of a risk (Romero and Francisco, 2020), in which risks are assigned a rating for their potential to affect in-mission crew health and performance and for their potential to affect long-term health outcomes and quality of life.
NASA recognizes a range of physiological and behavioral impacts to the health of its astronauts during and after spaceflight. One of the most challenging risks to assess and communicate about is ionizing radiation (simply referred to as “radiation” throughout this report), because of
incomplete knowledge of the complicated radiation environment in space, limited data on the mechanisms of how space radiation causes cellular damage, and additional uncertainties associated with modeling radiation-induced health risks. Constant exposure to radiation places astronauts at an increased lifetime risk of cancer and non-cancer health problems, including central nervous system damage, cataracts, cardiovascular damage, potential heritable effects, impaired wound healing, and infertility (Blakely et al., 2010; Chylack et al., 2009, 2012; Cucinotta and Durante, 2006; Cucinotta et al., 2001; NRC, 2006). The great degree of uncertainty around whether an astronaut will develop any of these health problems, especially cancer, following exposure to space radiation contributes to the challenges of communicating these risks effectively to astronauts, NASA personnel, policy makers, and the public.
To protect astronauts from unacceptable risks due to space radiation exposure, NASA has set space permissible exposure limits (SPELs) (NASA, 2014). The current permissible exposure limit is set so that astronauts shall not exceed 3 percent risk of exposure-induced death (REID) from cancer, adjusted for age and sex. REID estimates the probability that an individual will die from cancer associated with radiation exposure (UNSCEAR, 2000). As currently implemented by NASA, the SPEL is set from the calculated upper 95 percent confidence limit of the REID distribution, and is dependent on the age and sex of the astronaut. Based on detailed individual uncertainty calculations, an individual astronaut’s permissible exposure limit can vary. For instance, under NASA’s current approach, a 30-year-old female would be limited to ~180 millisieverts (mSv) of radiation exposure in space, whereas a 60-year-old male would be limited to ~700 mSv. For reference, a 180-day mission to the ISS would expose an astronaut to 50–120 mSv.1
NASA’s exposure limit is based on recommendations from National Council on Radiation Protection and Measurements (NCRP) reports (NCRP, 2000, 2014), and is intended to apply only to radiation exposure incurred during missions in low Earth orbit (LEO).
Several reports and papers have cautioned NASA about the significant increases in radiation exposure expected on exploratory and long-duration missions and the increased cancer risks that will likely result. In Safe Passage: Astronaut Care for Exploration Missions, the authoring committee said:
Deep space is a unique environment with special hazards for humans…. In addition, technological problems, such as radiation protection, re-
main unsolved, making long duration space travel probably unacceptably dangerous. (IOM, 2001, p. 191)
Cucinotta and Durante concluded:
Radiation-induced cancer is one of the main health risks for manned exploration of the Solar system…. The issue of radiation risk during space exploration is unlikely to be solved by a simple countermeasure, such as shielding or radioprotective drugs. The risk will be understood and controlled only with further basic research in cancer induction by charged particles. (Cucinotta and Durante, 2006, p. 434)
NCRP reported, “The issue of radiation protection limits for exploratory missions is more complex given the likelihood that radiation exposures will be increased in magnitude and biological effectiveness” (NCRP, 2014, pp. 1–2). NASA scientists have also noted that staying below the current radiation SPEL can be difficult for astronauts with previous spaceflight experience and for young female astronauts selected for lunar surface missions (Simonsen and Slaba, 2020). In addition, the authors concluded that all astronauts on a Mars mission will exceed the SPEL for space radiation (Cucinotta et al., 2013; Simonsen and Slaba, 2020).
To assess cancer risks from space radiation, NASA uses the NASA space cancer risk model (currently NASA Space Cancer Research [NSCR] 2012) (Cucinotta et al., 2013). This model incorporates the current state of knowledge about the physics of space radiation, its effect on the induction of carcinogenesis, and the translation of terrestrial epidemiological data along with other factors such as mission duration and where the Sun is in the solar cycle. Because REID is calculated probabilistically, uncertainties are defined for every model component. The REID output of the model significantly affects many facets of a mission, such as engineering (e.g., shielding and propulsion systems), design of habitable spaces in spacecraft and planetary habitats, and flight eligibility of astronauts. NASA also uses REID when communicating with astronauts about radiation-induced health risks.
In the near future, as NASA prepares for a crewed space mission to the Moon and, eventually, Mars, the agency is proposing changes to its health standard for space radiation exposure limits and its approach to managing and communicating the cancer and non-cancer risks associated with space radiation exposure. There are a number of reasons why NASA feels these changes are necessary. These are described in the next section.
WHY NASA IS CONSIDERING REVISIONS TO THE CURRENT RADIATION EXPOSURE STANDARD AND ASSOCIATED RISK MANAGEMENT
According to presentations by NASA at the committee’s public session in January 2021, there are several reasons why an update to the radiation exposure standard is being considered. The initial reason was because the current standard is for LEO missions exclusively. Now that the Artemis lunar mission, additional longer-duration lunar missions, and missions to Mars are in planning and development, NASA needs to define a radiation exposure standard that considers both missions in LEO and missions into deeper space. This is supported by section 22.214.171.124 of NASA-STD-30001, volume 1 (NASA, 2014, p. 23), which says:
Exploration Class Mission radiation exposure limits shall be defined by NASA based on NASA-requested recommendations from the National Academy of Sciences, the Institute of Medicine, and the National Council on Radiation Protection [and Measurements] (NCRP).
The other reasons presented to the committee for revising the standard and associated risk management are the following:
- NASA formed a Bioethics Advisory Panel in 2019 charged to perform a bioethical review and counsel NASA on a range of issues including the long-standing concern that NASA’s current radiation exposure results in an unequal work environment that limits female astronauts to shorter space careers because of scientific data indicating that, compared to men, females have an increased risk of cancer from exposure to ionizing radiation.
- In 2020, NASA convened an advisory panel of clinicians with expertise in cancer and other radiation health effects to individually advise NASA on both radiation risk characterization and how the standard can be aligned and viewed in context with other risks. Panelists recommended increasing the exposure limit, revising how both REID and the risk of exposure-induced cancer (REIC) are communicated, encouraging conversation with astronauts on risk limits and risk communication, and expanding the model
- input data to include more epidemiological data from occupational exposure studies.2
- Space missions are increasingly carried out as collaborations among international space agencies and commercial enterprises. NASA relies on other governments, their space agencies, and commercial rockets to provide LEO transportation for its astronauts to NASA-supported facilities in space since the retirement of the Space Shuttle Program (IOM, 2014; NASA, 2021b). This increase in collaboration has led to challenges associated with coordinating regulations and policies, including those that address health risks to astronauts. Because each agency sets its own radiation exposure limits, NASA thought it prudent to reconsider its space radiation health standard in that context.
- The U.S. government is developing a space-based branch of the military, and U.S. commercial enterprises are planning for crewed spaceflight ventures, which may be regulated by the Federal Aviation Administration. While NASA will not be responsible for these endeavors, NASA recognizes that the technical and health standards it establishes and follows within its agency are frequently used by other agencies as the benchmarks to follow.
- NASA recognizes that a key component of managing risk is appropriate risk communication with astronauts and other stakeholders. While considering revisions to the radiation exposure standard, NASA is also evaluating how best to communicate that risk to its astronauts.
In 2020, NASA asked the National Academies of Sciences, Engineering, and Medicine (the National Academies) to convene a committee of experts to review and assess NASA’s processes for long-term risk assessment and management for currently anticipated crewed missions with respect to cancer (see Box 1-2 for the committee’s complete Statement of Task). This report provides the committee’s recommendations to NASA for assessing, managing, and effectively communicating about the space radiation–induced cancer risk for astronauts. The study committee was not asked to develop a new radiation exposure health standard or to perform a detailed evaluation of the NASA stochastic cancer model that informs the standard.
To accomplish the task, the National Academies empaneled a committee of 18 members with expertise in the areas of radiation and cancer biology;
biostatistics and mathematical modeling; risk communication, management, and uncertainty; medical genetics; clinical medicine; ethics; occupational health and safety; radiation dosimetry and physics; epidemiology; and two former astronauts with clinical medicine expertise (see Appendix B for the biographical sketches of the committee members and staff).
From December 2020 through May 2021, the committee held five full committee meetings, including two public information-gathering sessions with NASA, as well as a joint meeting with the International Commission on Radiological Protection, and many working group meetings (all meetings were held virtually) and email communications. At the first public session in January 2021, NASA provided more specifics on its objectives for the study; factors considered and background on the proposed radiation standard; health and medical risk characterization; sex difference considerations; cancer incidence within the NASA Astronaut Corps; and a crew perspective (see Appendix A for the public session agendas). At the second public session in February 2021, NASA presented an updated set of options for updating the standard for the committee’s consideration and answered clarifying questions about the options.
Over the course of the study, NASA submitted three white papers to the committee, each building on, providing additional context for, and sometimes revising NASA’s proposed approach for updating the space radiation health standard.3 In the first white paper (shared with the committee in advance of the January 2021 public meeting), NASA provided background information and additional context on the scope of the task. NASA also posed specific questions to the committee related to a set of options NASA proposed for updating the standard. NASA laid out what the agency was asking (and not asking) the committee to consider (see Box 1-3). Then, in response to the committee’s questions at the January meeting, NASA provided a second white paper in February 2021 that answered specific questions from the committee and provided updates to NASA’s preferred strategy for updating and communicating the standard. In March 2021, NASA provided a final white paper in response to additional questions posed by the committee at the February public session meeting.
There were several important updates to NASA’s proposed approach as communicated to the committee throughout the study process. In the January white paper and public meeting, NASA proposed to use a 75 percent confidence level to set the career total dose limit, but solicited the committee’s input on other options (using a 95 percent confidence level or using the mean). In the February white paper, NASA indicated that it instead preferred to use the mean REID to set the limit and to use a confidence
interval to communicate the wide distribution of the risk produced by the model. The agency also proposed the use of dose-based thresholds universal for sex and age and using a 35-year-old female astronaut as the basis for setting the universal standard. In the March white paper, NASA provided the final proposed language for the updated standard, which utilized the mean REID to set the career total dose limit (universal for sex and age). NASA also indicated that the agency would use the 95 percent confidence interval to communicate the broad distribution of the risk, along with the
mean risk. The March white paper also included additional information on median and mean calculations, and examples of how NASA communicates risks to astronauts post-mission.
There was also a change to NASA’s proposed three-band risk communication tool (see Figures S-2 and 4-1) for the standard. In January, the risk band figure was black and white. In the February update, NASA redesigned this figure to have a “traffic light” color-coding scheme to communicate “high,” “medium,” and “low” risk. The table was also reorganized and additional information on the rationale behind the bands and NASA management protocols was included.
In the materials that NASA provided to the committee and during the public information-gathering sessions, the updated standard and the risk communication strategies were often presented together. This sowed some confusion and highlighted for the committee the importance of establishing a clear separation between the space radiation standard and the strategies for communicating radiation-induced cancer risks to astronauts and other stakeholders. One way that the committee has emphasized the separation between their evaluation of the proposed radiation standard and how best to communicate cancer risk caused by radiation is to cover each topic in a separate chapter. Throughout the report the committee acknowledges the interconnectedness of the steps in the risk management process, while emphasizing the importance of risk communication as distinct from the standard itself.
NASA recognizes that there are bioethical considerations that must be addressed adequately as it executes its plan for a mission to Mars. NASA has asked the committee to confirm whether these considerations have been addressed adequately in its proposed standard and in the assessment, management, and communication of radiation-induced cancer risks. NASA has acknowledged that under current conditions, a mission to Mars would expose all astronauts to space radiation that exceeds the SPEL, despite taking measures to keep radiation exposure as low as reasonably achievable (the ALARA principle). It would therefore be necessary for NASA to use a waiver process that evaluates and explains why such a mission is critical and why astronauts would be allowed to fly. The committee has considered NASA’s likely need for both mission and individual waivers for certain long-duration missions and has addressed that need in the context of the ethical framework previously recommended to NASA (IOM, 2014).
A glossary of terms used in this report is presented in Box 1-4.
This report is organized into four chapters. Chapter 1 provides general background and context on the issues to be addressed and also describes
the Statement of Task and the committee’s approach. Chapter 2 provides an overview of ionizing radiation in space, its impact on cancer risk, and cancer risk models used to assess and project the risk of cancer from exposure to ionizing radiation. Chapter 3 considers NASA’s proposed changes to its radiation exposure standard, and provides the committee’s recommendations on the implementation and application of the updated standard and on risk assessment for cancer risks associated with radiation exposure during crewed space missions. Chapter 4 considers the aims and methods of communicating to astronauts the cancer risk from ionizing space radiation, and offers conclusions and recommendations on communicating cancer risks associated with radiation exposure during crewed space missions, as well as considerations of waivers for missions that exceed the radiation exposure standard. Finally, Appendix A contains the methods used by the committee to develop this report, information on materials provided by NASA to the committee, and the committee’s public session meeting agendas. Appendix B presents short biographical sketches of the committee members and staff.
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