The Deep Space Radiation Environment

Sean Copeland
March 26, 2012

Submitted as coursework for PH241, Stanford University, Winter 2012

Fig. 1: Schematic of the Van Allen Radiation Belts. Source: Wikimedia Commons.

The Space Radiation Environment

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. [1] 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. [2]
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 Health Risks of Space Exploration

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.

Chronic Dose	Risk
~0.2 Sv	First evidence of increased cancer risk as late effect from protracted radiation.
2-4 Sv/year	Chronic radiation syndrome with complex clinical symptoms

Table 1: Radiation effects in humans after chronic whole body irradiation. [2]

Acute Single Dose	Effect	Outcome
~0.2 Sv	First evidence of increased cancer risk as late effect.
< 0.25 Sv	No obvious direct clinical effects.
> 0.5 Sv	Nausea, vomiting.	No early death anticipated
(> 0.7) 3-5 Sv	Bone marrow syndrome: Symptoms include internal bleeding, fatiuge, bacterial infections and fever.	Death rate for this syndrome peaks at 30 days, but continues out to 60 days. Death occurs from sepsis.
5-12 Sv	Gastronintestinal tract syndrome: Symptoms include nausea, vomiting, diarrhea, dehydration, electrolytic imbalance, loss of digestion ability, bleeding ulcers.	Deaths from this syndrome occur between 3 and 10 days post exposure. Death occurs from sepsis.
>20 Sv	Central nervous system syndrome: Symptoms include loss of coordination, confusion, coma, convulsions, shock, and the symptoms of the blood forming organ and gastrointestinal tract syndromes.	No survivors expected.

Table 2: Radiation effects in humans after single acute whole body irradiation. [2]

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.

Solar Environment	Al thickness (g cm²)	Skin annual dose (cGy)	Skin annual dose equivalent (cSv)	Bone marrow annual dose (cGy)	Bone marrow annual dose equivalent (cSv)	Annual effective dose (cSv)
Solar Min	1	6.2	27.4	5.7	16.7	17.9
	5	6.4	24.6	5.8	15.6	16.7
	10	6.5	21.8	5.8	14.6	15.4
Solar Max	1	18.4	79.8	16.4	44.5	48.8
	5	18.3	66.9	16.3	40.5	43.7
	10	18.0	56.2	16.1	37.0	39.3

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. [1]

SPE event	Al thickness (g cm²)	Skin annual dose (cGy)	Skin annual dose equivalent (cSv)	Bone marrow annual dose (cGy)	Bone marrow annual dose equivalent (cSv)	Annual effective dose (cSv)
Aug. 1972 SPE	1	1501	2824	79	111	338
	5	227	351	40	56	89
	10	77	115	21	31	40
Oct. 1989 SPE	1	825	1535	49	69	193
	5	136	208	26	36	55
	10	48	72	14	20	26
Jul. 2000 SPE	1	886	1753	25	36	171
	5	87	138	10	15	28
	10	23	34	5	7	10

Table 4: Annual skin and bone marrow dose and dose equivalent for several large SPEs as a function of aluminum shield areal density. [1]

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. [3] 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]

Radiation Shielding

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. [7]

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.

References

[1] L. W. Townsend, "Implications of the Space Radiation Environment for Human Exploration in Deep Space," Radiat. Prot. Dosimetry 115,44 (2005).

[2] C. E. Hellweg and C. Baumstark-Khan, "Getting Ready for the Manned Mission to Mars: The Astronauts' Risk from Space Radiation," Naturwiss. 94, 517 (2007).

[3] L. E. Peterson and F. A. Cucinotta, "Monte Carlo Mixture Model of Lifetime Cancer Incidence Risk from Radiation Exposure on Shuttle and International Space Station," Mut. Res. 430, 327 (1999).

[4] S. Trovati et al., "Human Exposure to Space Radiation: Role of Primary and Secondary Particles," Radiat. Prot. Dosimetry, 122, 362 (2006).

[5] F. Ballarini et al., "Modelling the Influence of Shielding on Physical and Biological Organ Doses," J. Radiat. Res. 43, S99 (2002).

[6] L. W. Townsend, J. E. Neal and J. W. Wilson, "Large Solar Flare Radiation Shielding Requirements for Manned Interplanetary Missions," J. Spacecraft, 26, 126 (1989).

[7] Managing Space radiation Risk in the New Era of Space Exploration (National Academices Press, 2008).