On of the problems that will be faced by deep explorations of space is protecting the crew from exposure to radiation. This is not an easy thing to defend against. Several approaches have been suggested to deal with this threat.
The three main schools of thought on radiation shielding are: Material, Magnetic and Electrostatic Shielding.
Material Shielding has the drawback of requiring lots mass. A major factor for a ship design. The more mass, the more fuel needed to escape gravity and propel the ship. Material shielding protects against the widest range of cosmic radiation.
Magnetic Shielding has weak points at the poles, much like the Earth does, allowing radiation in at those spots. It also requires a very strong magnetic field, which might have its own ill effects, we don’t know yet. Magnetic shielding works best against charged particles.
Electrostatic Shielding works by giving the hull a large positive charge, which repel positively charged particles but attracts negatively charged ones. A negative charge on the skin would have the opposite effect. So electrostatic shielding can only protect against particles of a single charge.
Radiation Shielding Improvements
The key might be to combine the strengths of each of these methods, while reducing their drawbacks. This can give us a reduction in mass and energy requirements over any stand-alone system. Lets start with electrostatic shielding and how it can be improved.
In electrostatic shielding a section of a ship can be given an external charge to repel like charges, but it attracts opposite charges. Both positive and negative particles are a problem in space. Instead of charging the outside of the hull around the crew where bombardment from oppositely charged particles would be a problem, move the charged areas to the ends of the ship, away from the crew.
Use both a positively and a negatively charged region at opposite ends of the ship, perhaps on extensions to increase distance from the crew area. This will draw charged particles away from the area between the poles, reducing the number headed for the crew area.
This design has an additional benefit. The ship is now an electric dipole of the type known as an Electret. This electret dipole is the electrostatic equivalent of a permanent magnet. As we are using the electrostatic system to generate the magnetic field, there is no increase in mass or energy requirements for this added protection.
A magnetic field will also help to deflect cosmic radiation away from the crew area and towards the charged poles of the ship where they can be absorbed. This magnetic field does not have to be as powerful as a stand-alone magnetic field. It only needs to help deflect a particle towards the charged areas at the poles of the ship.
Material shielding is last on our list. Its use should be limited to vital areas due to its mass. Plastics like polyethylene fit the bill of being lightweight and providing good protection. Recent work that allows plastics to carry a charge however can make material shielding more effective.
We do not want an electric charge on the skin of the main body of the ship. This does not prevent us from placing charged layers within the hull, much like a capacitor, so the electric field does not radiate into space. With this approach, any charged particles that make it past the other defenses and into the hull, will be slowed down even more than in regular material shielding by the charged layers. Cosmic radiation types which does not have a charge, like gamma rays, produce charged particles when they encounter material shielding, and this approach will minimize that danger as well.
Design Considerations
The hull should be modular in design with a large number of small sections. This will allow easy repairs to the hull I the event of damage. Also, if a micrometeorite hits the hull, an electrical discharge between the charged layers could occur. A modular design will prevent this from affecting the whole system.
The positive and negative layers within the hull can be tied into the charged sections at the end of the ship supplying them with the necessary charge. In this way all three shielding systems are hooked together and use the same energy, reducing the total energy requirements while keeping the mass at a minimum.
If an Ion Drive or other propulsion system that emits charged particles is used, this approach can give us a boost in performance as well. By placing the negative pole of the ship at the rear where the negatively charged ion stream is ejected, the ship and the ion stream will repel each other, giving a slight increase in performance.
If a high intensity radiation event is headed for the ship, the ions normally ejected can be diverted to the shielding system for an additional boost in field strength to protect the crew at these critical times.
Conclusion
So by combining all three shielding concepts into a single design, an overall reduction in mass and energy requirements for protection from cosmic radiation can be achieved.
CHARLES LUSTER
SOURCE ACADEMIA.EDU
A)MATERIALS FOR SHIELDING ASTRONAUTS FROM THE HAZARDS OF SPACE RADIATIONS
J. W. Wilson*, F. A. Cucinotta**, J. Miller***, J. L. Shinn*, S. A. Thibeault*, R. C.
Singleterry*, L. C. Simonsen*, and M. H. Kim****
*NASA Langley Research Center, Hampton, VA 23682, john.w.wilson@larc.nasa.gov
**NASA Johnson Space Center, Houston, TX 77058
***DOE Lawrence Berkeley National Laboratory, Berkeley, CA 94720
****NRC/NAS Fellow NASA Langley Research Center, Hampton, VA 23682
ABSTRACT
One major obstacle to human space exploration is the possible limitations imposed by the
adverse effects of long-term exposure to the space environment. Even before human spaceflight began, the potentially brief exposure of astronauts to the very intense random solar energetic particle (SEP) events was of great concern. A new challenge appears in deep space exploration from exposure to the low-intensity heavy-ion flux of the galactic cosmic rays (GCR) since the missions are of long duration and the accumulated exposures can be high. Because cancer induction rates increase behind low to rather large thickness of aluminum shielding according to available biological data on mammalian exposures to GCR like ions, the shield requirements for a Mars mission are prohibitively expensive in terms of mission launch costs. Preliminary studies indicate that materials with high hydrogen content and low atomic number constituents are most efficient in protecting the astronauts. This occurs for two reasons: the hydrogen is efficient in breaking up the heavy GCR ions into smaller less damaging fragments and the light constituents produce few secondary radiations (especially few biologically damaging neutrons). An overview
of the materials related issues and their impact on human space exploration will be given.
INTRODUCTION
The ionizing radiations in space affecting human operations are of three distinct sources and consist of every known particle including energetic ions formed from stripping the electrons from all of the natural elements. The radiations are described by field functions for each particle type over some spatial domain as a function of time. The three sources of radiations are associated with different origins identified as those of galactic origin (galactic cosmic rays, GCR), particles produced by the acceleration of solar plasma by strong electromotive forces in the solar surface and acceleration across the transition shock boundary of propagating coronal mass ejecta (solar energetic particles, SEP), and particles trapped within the confines of the geomagnetic field. The GCR constitutes a low level background which is time invariant outside the solar system but is modulated over the solar cycle according to changes in the interplanetary plasma which excludes
the lower energy galactic ions from the region within several AU of the sun [1]. The SEP are associated with some solar flares which produce intense burst of high energy plasma propagating into the solar system along the confines of the sectored interplanetary magnetic field [2] producing a transition region in which the SEP are accelerated. SEP have always been a primary concern for operations outside the Earth’s protective magnetic field and could deliver potentially lethal exposures over the course of several hours [3]. The trapped radiations consist mainly of protons and electrons within two bands centered on the geomagnetic equator reaching maximum intensity at an altitude of 3,600 km followed by a minimum at 7,000 km and a second very broad maximum at
10,000 km [4]. The trapped radiations have limited human operations to altitudes below several hundred kilometers and potentially lethal exposures are obtained over tens of hours in the most intense regions. Low inclination orbits are shielded from extraterrestrial radiations by the geomagnetic field and are mainly exposed to the trapped environment. Inclinations above 45° are sufficiently near the geomagnetic poles for which GCR and SEP exposures can be significant.
Indeed, about half of the expected exposures of the International Space Station (ISS) in its inclined orbit of 51.6° will be from GCR [5].
In the usual context, shielding implies an alteration of the radiations through interactions with intervening materials by which the intensity is decreased. This understanding is to some degree correct in the case of the relatively low energy particles of the SEP and the trapped radiations wherein the energy deposited in astronaut tissues can be easily reduced by adding shield material.
As one would expect, some materials are more effective than others as the physics of the
interactions differ for various materials. The high energies associated with the GCR are distinct in that the energy absorbed in astronaut tissues is at best unchanged by typical spacecraft shielding configurations and use of some materials in spacecraft construction will even increase the energy absorption by the astronaut. For GCR, one must abandon the concept of “absorbing” the radiation by use of shielding. The protection of the astronaut in this case is not directly related to energy absorption within their body tissues but rather depends on the mechanism by which each particle type transmitted through the shield results in biological injury. Even though the energy absorption by the astronaut can be little affected, the mixture of particle types is strongly affected by the choice
of the intervening shield material. Knowledge of the specific biological action of the specific
mixture of particles behind a given shield material and the modification of that mixture by choice of shield materials is then a critical issue in protecting the astronaut in future human exploration and has important implications on the design and operation of ISS.
Understanding the biological effects of GCR behind intervening material is then key to
protection in future NASA activity in either ISS or deep space. As yet no standards on protection against GCR exposures have been promulgated since insufficient information exists on biological effects of such radiations [6,7]. The most important biological effect from GCR exposure of which we are currently aware is cancer induction which relates to mutation and transformation (a specific mutation) events in astronaut tissues. Our knowledge of radiation carcinogenesis in humans is for gamma ray exposures for which excess career risk is proportional to tissue dose (energy absorbed per unit mass) accumulated at low dose rates. Although insufficient data exists to estimate astronaut cancer risks from the GCR high charge and energy (HZE) ions, there exist relatively detailed data on the biological response of several systems including survival, neoplastic
transformation, and mutation in mammalian cells and Harderian gland tumor induction in mice.
Other biological effects may come to light as exposure of living systems to high energy heavy ion beams continues to be studied. We will discuss the available response models in light of the design criteria used for ISS and the implications for materials research. For further discussion of these issues see “Shielding Strategies for Human Space Exploration” [8]. In the present paper, we review the GCR environment and discuss the issues of shield design in the context of developing a strategy for reducing the health risks of astronauts in future missions. In particular we will examine the role of materials research and development in controlling astronaut health risks from exposure to ionizing radiation in space.
GCR AND BIOLOGICAL RESPONSES
The galactic cosmic rays consist mainly of nuclei (ions) of the elements of hydrogen thru
nickel. The energy spectra are broad and extend from tens to millions of MeV (figure 1). The most important energies for protection lies near maximum intensities from a few hundred to several thousand MeV/nucleon (a nucleon is the name given to neutrons and protons of which the ions are composed). The salient feature of these radiations is that a significant number of these particles have high charge which affects the means by which energy is transferred to tissues. Their ion tracks seen in nuclear emulsion are shown in figure 2. The optical density (related to energy deposited) of the track increases as the ion charge squared and the intensity and the lateral extent of the track depend on the ion velocity. The tracks in the figure are for approximately 400 MeV/nucleon ions. Considering that the mammalian cell nucleus size is
several micrometers, it is clear that the passage of a single iron ion through the cell nucleus is a potentially devastating event. The protection standards applied to ISS are those recommended for Space Station Freedom scheduled for a low inclination orbit in which GCR exposures were minimal [6]. These standards were adapted, in part, from those used in the nuclear industry for mainly low energy radiations where ion tracks have very limited lateral extents and biological responses are characterized by mainly the energy lost per ion path length (linear energy transfer,LET). The enhanced effectiveness of high LET radiation to cause cancer for a given absorbed energy is given by an LET dependent quality factor [11] as shown in figure 3. The excess cancer risk is then assumed proportional to the dose equivalent which is the product of quality factor and
dose. Although little data exists on human exposures from HZE radiations, the limited studies in mice and mammalian cell cultures allow evaluation of the effects of track structure on shield attenuation properties and evaluation of the implications for dosimetry. The most complete HZE exposure data sets for mammalian cells have been modeled including the mouse embryo cells C3H10T1/2 for the survival and neoplastic transformation data of Yang et al.[12, 13], the hybrid hamster cells V79 for the survival and mutation data of various groups [14], and the mouse Harderian gland tumor data of Alpen et al. [15, 16]. Model results for the Harderian gland tumor data are shown in figure 4 in comparison with data from Alpen et al. [16]. The Harderian target cell initiation cross section (the initiating event in tumor formation is thought to be neoplastic transformation) is shown in figure 5 and compares closely with the transformation cross section found for the C3H10T1/2 cell transformation data of Yang et al. [13]. The most notable feature of
the cross sections in figure 5 are the multiple values for a given LET which implies the
corresponding relative biological effectiveness (RBE) is dependent not only on the LET but also the ion type. This fact is at variance with the latest ICRP recommended quality factor [11] which is a defined function of only the LET (figure 3).
Track structure related events are difficult to study in whole animals since the local environment within an animal varies across the organ under study and is modified by the surrounding tissues.
Cell cultures can be used to better control the local environment and provide an improved system for track structure studies. Among the best studied cell systems is the hybrid hamster cell V79 for survival and mutation end points. The model of the V79 system is shown in comparison with data from various groups in figure 6. As we shall see, these track
the given material compared to aluminum. For example, polyethylene is 16 percent more effective than aluminum in controlling dose equivalent with only 1.5148 g/cm2 of material. Much larger gains are achieved at 5 g/cm2 thickness. The 5 g/cm2 thickness is typical for an area within a human-occupied vehicle loaded with equipment. Modest reductions in dose equivalentare found for all three materials at the 5 g/cm2 thickness. A mouse carried on the same mission of one year duration will have an excess risk of Harderian gland tumors shown in the last column in Table I.
A substantial increase in Harderian tumor risk for shield thickness x, HG(x), is found for both thicknesses of aluminum and great amounts of aluminum are required to reduce the risk.
Polyethylene shows substantial improvements in performance for controlling Harderian gland tumor risk at both thicknesses. These findings have important implications for deep space exploration but also for ISS which receives half of its exposure from GCR ions.
MATERIALS RELATED ISSUES
The biologically based models show complex dependence on radiation quality which is
expressed in terms related to the details of the particle track as distinct from the simple LET dependence of the quality factor used in conventional radiation protection practice including ISS.
Even the cancer risk attenuation characteristics of spacecraft shield materials are found to be different for the track structure dependent and LET dependent models leading one to conclude that any useful dosimetric technique must take into account these differences as well. Most important in this respect is that LET dependent quality factors overestimate the effectiveness of most shielding materials (see figure 7) and would falsely indicate reduced cancer risk in many applications. Note also that the relative importance of material choices is clearer in the track structure models. This in part results from the fact that aluminum is inherently poor for protecting against tumors in mice induced by the GCR environment. Clearly the relative importance of design alternatives depends critically on our understanding of the biological action of specific radiation
components and our ability to evaluate the transmitted radiations through specific shield materials [7,23].
In this regard, the attenuation properties depend on the atomic/nuclear database which has undergone change in recent years. For example, the dose equivalent relative to the unshielded value H(x)/H(0) is shown in figure 8 where H(x) is the dose equivalent within an aluminum shell of thickness x. We normalize the dose equivalent to the unshielded value H(0) to minimize the effects of changing environmental models in the succession of results shown in the figure. The remaining differences are mainly in nuclear models and transport procedures. The curve labeled Letaw et al.
[24] is for the database developed by the Naval Research Laboratory in common use
materials development and one would hope to approach the range of maximal performance in figure 9 as closely as possible. Although the actual performance achievable must await an improved understanding of biological response to GCR ions [7], it is clear that high performance shield materials will greatly reduce the mass requirements to protect astronauts in future missions and thereby greatly reduce the associated launch costs.
CONCLUDING REMARKS
The estimation of shield attenuation characteristics of various materials depends on the details of the biological response model. In that the experimental biological evidence displays clear dependence on other parameters in addition to LET, the adequate shield design for future deep space missions needs to reflect this dependence on these other factors (such as track width).
Although accumulated data on biological response to heavy ion exposures allows studies of the relative advantage of material choices, the final design of future mission shielding must await a clearer understanding of the human response to GCR radiations. In the meantime, validation of the transport procedures and identification of high performance shield materials should be the focus of current materials research. In that materials with high hydrogen content yield good shield performance, polymeric materials are expected to play an important role in protecting the astronaut on future missions. As part of the consideration of new materials is the practical engineering design process in which high performance shield materials are incorporated into cost effective designs. Many of these engineering issues for use of polymeric composites are already being addressed in the development of light weight aircraft designs. Practical engineering experience
gained in the use of polymeric composites in aircraft design is expected to have an important impact on the future of spacecraft design.
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(TO BE CONTINUED)
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