Space Radiation: The Number One Risk to Astronaut Health beyond Low Earth Orbit

Abstract

Projecting a vision for space radiobiological research necessitates understanding the nature of the space radiation environment and how radiation risks influence mission planning, timelines and operational decisions. Exposure to space radiation increases the risks of astronauts developing cancer, experiencing central nervous system (CNS) decrements, exhibiting degenerative tissue effects or developing acute radiation syndrome. One or more of these deleterious health effects could develop during future multi-year space exploration missions beyond low Earth orbit (LEO). Shielding is an effective countermeasure against solar particle events (SPEs), but is ineffective in protecting crew members from the biological impacts of fast moving, highly-charged galactic cosmic radiation (GCR) nuclei. Astronauts traveling on a protracted voyage to Mars may be exposed to SPE radiation events, overlaid on a more predictable flux of GCR. Therefore, ground-based research studies employing model organisms seeking to accurately mimic the biological effects of the space radiation environment must concatenate exposures to both proton and heavy ion sources. New techniques in genomics, proteomics, metabolomics and other "omics" areas should also be intelligently employed and correlated with phenotypic observations. This approach will more precisely elucidate the effects of space radiation on human physiology and aid in developing personalized radiological countermeasures for astronauts.

Figure 1

The interplanetary space environment showing the toxic combination of galactic cosmic radiation (GCR) and (largely) proton radiation due to solar particle events (SPEs). Figure courtesy of NASA/JPL-Caltech.

Figure 2

Relative abundance of GCR nuclei from hydrogen (Z = 1) to iron (Z = 26) [1].

Figure 3

Energetic SPEs affecting low Earth orbit (LEO) space missions since 1991 are plotted as a function of the solar cycle. Shown here are events (red circles) that have been measured since 1991 to 2013 and include Solar Cycle 22 (partially), 23 and 24 (partially). Energetic solar events contain a higher fluence of >100 MeV protons that can penetrate typical spacecraft shielding and significantly impact the health of astronauts.

Figure 4

Select health effects due to space radiation exposures.

Figure 5

Examples of two complex aberrations involving three or more chromosomes observed post-mission in astronauts. Chromosomes were hybridized with painting probes for chromosome 1 (red), chromosome 2 (green) and chromosome 4 (yellow). All other chromosomes were counterstained with DAPI (blue). Adapted from Cucinotta et al. [29] and republished with permission from Radiation Research.

Figure 6

Acute radiation outcomes: (a) Blood cell counts (lymphocytes) following exposure to SPE-like gamma and proton radiation in a mouse model. Graph adapted from Romero-Weaver et al. [69]. (b) WBC counts in a mini-pig model following exposure to SPE-like electron and proton radiation. The WBC counts did not return to normal levels at the 30-day time point, with proton radiation exposure having the more detrimental effect. Graph adapted from Ann Kennedy [70] and reproduced with permission from Life Sciences in Space Research.