Radiation Exposure During Space Travel: Using Radioisotopes for a Comparative Study of Human Feces and Urine as Integrated Shield Components

Earth’s atmosphere and magnetosphere deflect and absorb the majority of harmful radiation traversing space; however, once outside Low Earth Orbit (LEO), payloads are exposed to Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE). While humans possess capabilities that hardware does not, we are uniquely vulnerable to radiation. Detrimental effects range from nausea and dizziness brought by intense, short-term doses to increased cancer risk and impaired cognitive function associated with chronic exposure. This paper aims to explore the use of human waste (feces and urine) as radiation shields in a comparative study of urine vs. water and hydrated vs. dehydrated feces. GCRs contain particles with energy orders of higher magnitude than SPEs, which makes them impractical to shield against. Conversely, SPEs occur with higher frequency and at a lower level that is practically attenuated. To this end, an SPE surrogate was validated and exposure (Counts) on the leeward side of the respective shields was measured. Counts per Minute (CPM) were obtained by applying a multiplicative factor to Counts. As expected, CPM behind a urine shield did not differ from CPM behind a water shield (t-test, p < 0.05). Similarly, no difference in leeward CPM was found between hydrated and dehydrated feces shields (t-test, p < 0.05). The lack of differences between water and urine may be a result of urine being primarily composed of water. While fecal matter is made primarily of water as well, the solid content likely masks the loss of water’s attenuation properties. Acronyms CPM Counts per Minute GCR Galactic Cosmic Ray GMW Geiger-Mueller tube window Radiation Exposure During Space Travel: Using Radioisotopes for a Comparative Study of Human Feces and Urine as Integrated Shield Components By Noah Falck 38 Symposium | Student Journal of Science & Math HIMAC Heavy Ion Medical Accelerator in Chiba LEO Low Earth Orbit OLTARIS Online Tool for Assessment of Radiation in Space RI Radioisotope SPE Solar Particle Event Introduction As humans continue to travel farther from Earth to explore space, we encounter a Pandora’s box of biological hazards. Chief among dangers to Homo sapiens is the damage caused by radiation (Horneck et al., 2006), which can be classified into two main categories: acute and chronic symptoms. Acute symptoms include nausea and vomiting and are typically the result of exposure to a high dose during a short timespan. Chronic symptoms vary from increased cancer risk to cataract development (Langford, n.d.) and are associated with prolonged exposure to radiation. With space travel, there are two 1,000 lb gorillas in the room: mass and money. This paper will involve the former. The mass limitation requires that waste is minimized and that many materials serve multiple purposes, such as the use of drinking water as part of a Solar Particle Event (SPE) shelter (Simon, Clowdsley, & Walker, n.d.). While humans possess capabilities hard payloads do not, they bring their own set of limitations. One of the commonly accepted characteristics of life is the production of waste; during a long-term mission such as the journey to Mars and back, biological waste will be an influential factor. Approximately 128 g of feces wet mass and 1.4 L (59 g dry solids) of urine are produced by each human daily (Rose, Parker, Jefferson, & Cartmell, 2015). With a crew of six and an estimated Earth-Mars transfer of 202 days (Horneck et al., 2006), such approximations predict 155 kg of feces wet mass and 1,697 L of urine (72 kg dry solids) produced upon Martian arrival. Consequently, biological waste will be a factor that cannot be ignored. Given that waste and radiation are two immovable constants, this paper aims to explore the idea of using human waste as a radiation shield. While urine is primarily recycled for potable liquid and oxygen production (Jr., Carter,& Higbie, n.d.), feces have not been similarly utilized. For expeditions outside Low Earth Orbit (LEO), storm shelters will likely integrate compacted foodstuffs, equipment and biowaste (Simon et al., n.d.). Therefore, this investigation will explore the comparative qualities of urine and feces as elements of such a shield. Urine is primarily composed of water, which will likely result in no significant difference in attenuation compared to pure water. Human feces are composed of approximately 75% water and 25% solid matter (Britannica, 2002), which suggests a possibly significant negative effect on its attenuation properties after desiccation. Because the high energies required to directly simulate SPE particles would be impractical to achieve in the Space Environment Laboratory of California Polytechnic State University, San Luis Obispo, a surrogate must take the place of a particle accelerator. While the β decay (two protons and two neutrons) of radioisotopes (RIs) occurs at orders of magnitude below that of SPEs, they are composed of similar particles: protons (Simon et al., n.d.); therefore, they may produce similar exposure effects on the leeward side of a shield. In order to determine if RIs may be used as SPE surrogates, they will be tested against shielding scenarios predicted by NASA’s Online Tool for the Assessment of Radiation in Space (OLTARIS). The RIs that can replicate the trends to a statistically significant level will be used for the aforementioned experiments. Volume 4, Issue 1 39