Astronauts on deep space missions will be exposed to galactic cosmic rays (GCR) that are composed of a collection of ions (atoms with their electron stripped away) traveling at speeds close to that of light. The types of ions involved and the speeds at which they travel are quite variable. Some, such as iron ions, are rather massive and these exist side by side with much smaller ions such as high energy (very fast moving) protons (hydrogen nuclei). As these ions encounter atoms, say, inside a space ship or an astronaut’s body, they can pull off electrons from these atoms in a process known as ionization. Ionization events can ultimately produce chemical reactions that can damage vital structures within the cells of an astronaut such as DNA. This ionization damage can break DNA strands or in other ways eliminate important genetic information. Such changes can lead to mutation, cancer, or can even kill the cell.
A key factor in nature and severity of the effects produced is the spacing of the ionizations along the path or track that the ion takes through a cell. This is dependent on the mass and charge (the number of positively charged protons making up the ion) of the particle as well as the speed at which the ion is traveling. This spacing we refer to as linear energy transfer (LET) or the amount of energy that is used to produce ionizations along a specific length of the ion’s track. Ionizations are spaced far apart with low LET radiation but become increasingly closer together as LET increases.
A key concern to NASA is how dangerous is radiation to space flight crews. Most of the epidemiological data we have is from low LET radiations such as the X-rays used for radiation therapy or from the exposures received by the survivors the atomic bomb attacks in 1945. High LET data is harder to come by and derives largely from experiments using cells or animal model systems. These experiments use ground-based particle accelerators to accelerate ions to energies found in space and simulate potential space radiation exposures. Nearly all of this data come from experiments using a single ion at a solitary energy. As mentioned above, the space radiation environment is far more complex with many ions traveling with large ranges of energies all of which could impact an astronaut’s health, particularly since the probability of a cell in an astronaut’s body being hit sequentially by a small number of different ion species over a Mars mission lasting 600 to 900 days is quite high. In order to provide more realistic simulations of the space radiation environment, NASA is developing the GCR Simulator which has the potential to irradiate samples with multiple ion species within the shortest period of time allowed by ion switching, in order to mimic the effects of coincident (simultaneous) exposure. For the GCR simulator to best duplicate the natural space radiation environment, it is important to optimize dose delivery in ways that allow the results to be directly scalable to the low doses and dose rates that are encountered in deep space.
It will be difficult to match the low doses and dose rates directly. From a practical sense higher doses will need to be used in order to obtain statistically meaningful results. Likewise, higher dose rates are also required as exposing samples over long periods of time is just not feasible. Classically, the dose problem has been resolved by exposing samples to a series of higher doses then extrapolating the results back to the doses of interest. Similarly, it has long been known that as the dose rate declines the yield of biological endpoints is also diminished. This diminishment, however, only occurs to a point beyond which no additional decline in the dose response is detectable. The dose rate at which this point occurs has been termed the limiting low dose rate (LLDR). We plan to determine the LLDR for chromosome aberration induction by first measuring the dose response at high dose rate then using a mathematical model estimate the LLDR. The derived value for the LLDR will be tested in subsequent experiments. If we have achieved the LLDR the dose response will be strictly linear making extrapolations back to the conditions in the space radiation environment relatively straightforward. Future experiment will determine how best to fractionate the dose (deliver the dose in small increments with periods of time between each exposure) in a way that will achieve the same result as a continuous low dose rate exposure.
This is the first year for our project. As scheduling beam time at the NASA Space Radiation Laboratory (NSRL) is a somewhat long and involved process, we applied for time at the earliest opportunity after funding was secured. At present, our experiments have yet to run but our first run at NSRL is scheduled for April 11, 2019 using 250 MeV/n Helium ions. This is to be followed by a second experiment using 1 GeV protons on April 15. With this in mind, we have not yet been able to produce any results so far but hope to have our initial high dose rate experiments completed in the next few months.
Abstracts for Journals and Proceedings
Loucas BD, Cornforth MN. "Protracted exposure to NASA's GCR-Simulator: Cytogenetic validation and beam time optimization." 2019 NASA Human Research Program Investigators’ Workshop, Galveston, TX, January 22-25, 2019.
Abstract Book. 2019 NASA Human Research Program Investigators’ Workshop, Galveston, TX, January 22-25, 2019. , Jan-2019