Radiation environment onboard spacecraft at LEO and in deep space

It is well known that outside the Earth's protective atmosphere and magnetosphere, the environment is very harsh and unfriendly for any living organism, due to the micro gravity, lack of oxygen and protection from high energetic ionizing cosmic radiation, as well as from powerful solar energetic particles (SEPs). The space radiation exposure leads to increased health risks, including tumor lethality, circulatory diseases and damages on the central nervous systems. In case of SEP events, exposures of spacecraft crews may be lethal. Space radiation hazards are therefore recognized as a key concern for human space flight. For long-term interplanetary missions, they constitute a limiting factor since current protection limits might be approached or even exceeded. Better risk assessment requires knowledge of the radiation quality, as well as equivalent doses in critical radiosensitive organs, and different risk coefficient for different radiation caused illnesses and diseases must be developed. The use of human phantoms, simulating an astronaut's body, provides detailed information of the depth-dose distributions, and radiation quality, inside the human body. In this paper we will therefore review the major phantom experiments performed at Low Earth Orbits (LEO) [1]. However, the radiation environment in deep space is different from LEO. Based on fundamental physics principles, it is clear that hydrogen rich, light and neutron deficient materials have the best shielding properties against Galactic Cosmic Rays (GCR) [2,3]. It has also been shown [4,5] that water shielding material can reduce the dose from Trapped Particles (TP), the low energetic part of GCR, and from low energetic SEP events. However, the total dose from GCR, for moderate shielding thicknesses, is actually increasing when increasing the shielding thickness due to the buildup of secondary fragments, protons and neutrons [5]. Examples of promising shielding materials are polyethylene and hydrogen rich carbon composite materials. Nevertheless, not even these shielding materials have been proven to significantly reduce the radiation health risks compared to e.g. aluminum shielding due to the high energetic GCR particles, the created fragments, and the large radiobiological uncertainties in the GCR risk projection [6,7]. A better understanding of the radiobiological effects of GCR are therefore needed, as well as better cancer risk models, and models for estimating the risks for circulatory diseases and damages on the central nervous systems. To reduce the health risks, a combination of passive and active shielding might be a realistic option for long term interplanetary missions, in combination with means to minimizing the time in deep space and to perform the missions during solar maximum to minimize the flux of GCR. Suitable radioprotectors, e.g. agents that act directly to protect cellular component and oppose the action of radiation induced free radicals, and reactive oxygen species, as well as radiomitigators, e.g. agents that accelerate post-radiation recovery and prevent complications, could also be developed. There might also be a need to accept an increased risk for carcinogenesis than what is stated by current dose limits.