Risk of Radiation Exposure to Astronauts
Anuti Joshi shares her ethics & society case study, which she completed as part of our Young Scientist Program.
Astronauts are inevitably exposed to radiation whether they are trapped solar protons or Galactic Cosmic Radiation (GCR). Heavy ions produce distinct types of biological damage to cells and tissues compared to X-rays or gamma-rays. An increase in the Solar Proton Event (SPE) or GCR can cause a greater chance of DNA mutation and therefore an even higher probability of contracting cancer. Thus the National Aeronautics and Space Administration (NASA) has been recommended a set of regulations by the National Council on Radiation Protection and Measurements (NCRP) that needs to be followed to safeguard the physical and mental well being of an astronaut during a spaceflight. They seek to formulate guidance on radiation protection and measurements, which represents the consensus of leading scientific thinking.
The NCRP sets different radiation dose limits according to the location of the activity. For low earth orbit (LEO) missions an astronaut may not receive more than 0.5 Sv of radiation dose per year. The dose absorbed depends on the shielding provided by the spacesuits and spacecraft. For example, a three-month mission in a 2 g/cm2 shielded spacecraft in an almost constant atmospheric density orbit with a varying altitude would result in a depth-dose equivalent of roughly 0.1 Sv. The Mars Radiation Environment Experiment (MARIE), launched in 2001, estimate that shielded humans would receive annually 500 to 1000 mSv of radiation.
By the early 1980s several major changes in epidemiology data had occurred leading to the need for a new approach to define dose limits for astronauts. NASA requested NCRP to re-evaluate dose limits to be used for the LEO missions. In 1989, the NCRP Report No. 98 recommended age and gender dependent career dose limits using as a common risk limit of a 3% increase in cancer mortality. On one hand it is extensively held that the social and scientific benefits of space flight continue to provide justification for the 3% risk level for astronauts participating in exploration mission but on the other hand improvement of space safety places pressure on radiation protection to also improve.
Short term dose limits are imposed to prevent non-cancer health effects. Dose limits for cataracts, skin and heart disease are imposed to reduce risk of degenerative tissue diseases that could occur post mission. Career limits for the heart are intended to limit the Risk of Exposure Induced Death (REID) for heart disease to be below a few percent.
Spacesuits are designed in accordance with ALARA (As Low As Reasonably Achievable) to provide shielding from harmful radiations. Each spacesuit has its own design based on the duration and environmental conditions of the intended mission. The current spacesuits used on the International Space Station (ISS) are specifically designed to work in microgravity. These suits, however, are not compatible in an environment with substantial gravity as the lower torso of the suit is stiff. For longer duration flights designed for planetary exploration, the configuration of the suits will be made different. They will be made more flexible, allowing the astronaut to move around to thoroughly examine the surface of planets e.g. on Mars. Some spacesuits will incorporate shape memory to better fit an astronaut’s body shape and sensors in the gloves to make it easier for a cosmonaut to feel objects.
Besides shielding and spacesuits, chemoprevention agents are the employed to reduce chronic radiation bio-effects. They reduce mutant yield by acting on the DNA to enhance repair or prevent lesions from developing that could possibly affect other cell activities.
In the future improved and verified models for calculating organ dose equivalents are required. Re examination of our understanding of the radiation in different environments will help to model tools to address issues such as variability of atmospheric shielding on Mars which occurs due to seasonal changes in atmospheric pressure. A vigorous radiation health program must be implemented. Constant improvements in radiation dosimetry and an extensive international research into the biological effects of radiation exposure will ensure the well being of the next generation of astronauts.