|Fig. 1: Schematic of the Van Allen Radiation Belts. Source: Wikimedia Commons.|
Life on earth is protected from the hazards of the deep space environment by the solar winds, the Earth's atmosphere and the Earth's magnetic field. As human spaceflight refocuses to explore deep-space locations of interest, including the lunar surface, near-earth-objects (NEO), and Mars, crews will leave behind the shelter provided by the Earth and face prolonged exposure to three major sources of radiation:
Galactic Cosmic Radiation (GCR): High energy GCR particles (~25MeV - ~20GeV) of all atomic numbers are showered into the galaxy when stars undergo supernova. Approximately 88% of all GCR particles are hydrogen, 10% are helium, and the remaining percentage consists of heavier ions.  The flux of high-energy particles into the solar system is isotropic, arriving at any point in deep space with equal intensity from all directions, but is affected by the Sun's natural 11 year cycle. During periods of solar maximum, GCR intensity is reduced by the deflection of lower energy cosmic rays by the increased volume of plasma in the solar wind. Differences in GCR flux between solar maximum and solar minimum can be as great as a factor of five. 
Solar Cosmic Radiation (SCR): SCR is composed of two categories of radiation, low energy solar-wind particles that are constantly emitted from the sun (generally considered not to be dangerous), and highly energetic solar particle events (SPE). SPE-based radiation is a consequence of coronal mass ejections that originate from disturbed magnetic regions on the sun's surface. The typical 11 year cycle of the sun is characterized by a period of four years of relative inactivity, followed by seven years with increased numbers of SPE's. These ejections of high energy particles are highly directed, affecting only small regions of space, but are characterized by very high particle fluxes and can be extremely hazardous to space systems and crewed space vehicles.
Van Allen Radiation Belts: The Earth's magnetic field, generated by the motion of the molten iron core of the planet, collect and trap SCR and low-energy GCR ionized particles, creating bands of increased radiation in the vicinity of Earth. Typically, crewed missions spend a small amount of time in the influence of the Van Allen belts, and their contribution to overall crew radiation exposure is comparably small to GCR and SPE sources.
The deep space radiation environment is a primary health concern for astronauts embarking on long duration exploration beyond the Earth's magnetosphere. Health risks of radiation exposure can be grouped into acute and delayed categories based on the timeframe of the observed symptoms. Rapid exposure to large quantities of radiation lead to symptoms of acute radiation syndrome, usually developing within minutes or days of the exposure. Delayed effects, including cataracts and cancer, can develop when aggregate exposure to low-level radiation is substantial, but the dosage levels and dose rates are not sufficient to cause the onset of acute affects that lead to death. Tables 1 and 2 show the dosages required to cause the onset of delayed and acute symptoms of radiation sickness.
|Table 1: Radiation effects in humans after chronic whole body irradiation. |
|Table 2: Radiation effects in humans after single acute whole body irradiation. |
Tables 3 and 4 show typical radiation dosages for GCR and SPE events. Entries in the tables are simulated dosages to crew members behind aluminum shielding of increasing areal density. Shield thickness can be determined by dividing areal density by the material density.
|Table 3: Annual skin and bone marrow dose and dose equivalent for solar maximum and solar minimum periods as a function of aluminum shielding areal density. |
|Table 4: Annual skin and bone marrow dose and dose equivalent for several large SPEs as a function of aluminum shield areal density. |
Radiation dosage from GCR sources alone contribute to an aggregate 0.2-0.4 Sv/year, which places the dosage level high enough to cause concerns for long-term delayed radiation effects and increased cancer risk in astronauts.  Unfortunately, the mechanisms leading the development of these delayed effects are not well known and the magnitude of the assumed risk is not well characterized at this time, necessitating additional study of these particles on human physiology.
In contrast to GCR, the burst of directed radiation accompanying a SPE can result in high radiation dosages over short time periods. As shown in Table 3, the levels of exposure during a SPE event can be life threatening without adequate shielding. Projections for radiation exposure can be as high as 5-15 Gy, more than enough to induce the onset of acute radiation syndrome and create an immediate medical crisis. [1,4]
Dosage from a radiative environment can be diminished via the use of radiation shielding. These shields consist of material that absorb or scatter incoming high-energy particles, protecting the personnel and equipment from ionizing radiation. In principle, these shields can be made of any material, but some exhibit better absorption and scattering properties than others. Typically, dense materials like depleted uranium and lead make better shields than their low density alternatives.
Desiging a vehicle to provide adequate radiation protection to crews exploring deep space is a difficult endeavor. Tab. (3) shows the insensitivity of radiation dosages from GCR sources to shield thicknesses, this is due to the highly energetic nature of the incoming particles. As a consequence, shielding thickness and weight are prohibitively large to measurably protect crews from cosmic rays. Tab. (4) shows radiation from SPE events is highly attenuated by the presence of absorbing material, but the amount of directed radiation is large enough to require as much as 9 cm of solid aluminum to provide sufficient protection. [5,6]
Future concepts for deep space vehicles pose several potential solutions addressing the challenges of providing a safe habitat for crews during intense radiation storms, including: hull shielding, deployable water shielding, integrated seat shielding, and a deployable high-density polyethlyene (HDPE) slabs that can be erected into shelters inside the cabin of the vehicle. 
The deep space radiation environment remains one of the primary challenges and concerns facing the next generation of space explorers. Additional work is required to better characterize the risks of delayed radiation effects, including, cancer, cataracts and leukemia and to establish acceptable exposure limits. Vehicle design necessitates the use of advanced radiation absorbing materials and special protocol in the event of intense solar activity. These obstacles must be cleared to successfully and safely venture away from the Earth's protective care.
© Sean R. Copeland. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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