Peggy Whitson, the former head of the NASA Astronaut Office, believes NASA’s radiation standards are too restrictive and discriminate against female astronauts.
Whitson expressed her views at a workshop on Ethics Principles and Guidelines for Health Standards for Long-Duration and Exploration-Class Spaceflights, conducted by the National Academy of Science’s Institute of Medicine. The story was reported by Space.com.
NASA limits astronauts’ lifetime radiation exposure to keep the probability of radiation-induced cancer death below 3%. The limit for NASA astronauts is higher than the limit for terrestrial radiation workers. Women are more susceptible to certain forms of cancer, however, so the limit for female astronauts is lower than the limit for male astronauts. A female astronaut can fly only 45-50% as many missions as a male astronauts, Whitson said.
Many of NASA’s astronauts are already limited by their lifetime radiation exposure. According to Col. Robert Behnken, who replaced Whitson as head of the Astronaut Office, only three of NASA’s 50 astronauts were eligible for the recent one-year mission to ISS, because of radiation limits. A mission to Mars is probably impossible within current radiation limits.
Questions for NASA
Three questions come to mind:
First, is NASA calculating the radiation risk correctly? At present, no one has good data on the biological effects of long-term low-level radiation exposure. Instead, the risks are inferred from data on the effects of short-term, high-level radiation exposure (collected from events such as Hiroshima and Nagasaki). This data is applied to low-level radiation using an extrapolation model known Linear No Threshold (LNT).
The LNT model assumes that a given dose of radiation will have the same effect regardless of the dose rate. It is a highly conservative model, which assumes the body has no way to recover from the effects of gradual, low-level radiation exposure over time. It ignores the usual principles of toxicology and much of what we know about the biological repair mechanisms. There is no scientific evidence to prove the LNT model is correct, but the government has adopted it out of an abundance of caution (the “precautionary principle”).
The LNT model has been criticized many times in the past, but environmentalists and anti-nuclear activists have resisted any attempted change. In space, however, the precautionary principle may cost NASA the chance to go to Mars.
Second, does the 3% fatality limit makes sense? The Shuttle had about a 2% fatal accident rate on each flight, as does Soyuz. Future launch systems may do better, but right now, an astronaut who flies multiple missions has a just of death that is significantly greater than 3%.
Third, is the male-female dichotomy really the best way to classify radiation risks? There’s no doubt that cancer risks differ between men and women, but gender is only one of many variables that affect cancer risks. With the development of modern genomics, we’re reaching the point where it’s possible to determine risks on a personal level based on individual genetic markers. This is part of an emerging field known as personalized medicine, which treats people as individuals rather than broad statistical groups. Inspiration Mars is looking at personalized medicine to help select crew members for its proposed Mars flyby mission. Is NASA doing the same?
These are questions which NASA needs to answer as it plans for future exploration activities.