|Fig. 1: Artist's rendition of a torus-shaped space habitat. (Source: Wikimedia Commons. Courtesy of NASA)|
The space environment contains a plethora of radiation from both within our solar system, from the sun, and from without our solar system, called galactic cosmic radiation. Solar radiation is composed of alpha and beta particles anywhere from keV to GeV, and galactic cosmic radiation is composed of high-energy hydrogen (88%), helium (10%), and heavier nuclei (1%). [1,2] Currently, estimates for planned future human spaceflight missions to Mars push the limits of NASA's lifetime radiation restrictions, and technological advances must be made before these types of human missions are possible. An article by a previous PH241 student goes into more detail about the sources of this radiation and the impact it can have on human health, but it begs the question: what can be done about it? This paper focuses on spacecraft radiation reduction strategies. 
There are two main categories of approaches for shielding humans from radiation in space: passive shielding and active shielding.
Passive space radiation shielding consists of placing some sort of physical material in between a person and the source of radiation. Its main advantage over other forms of radiation shielding is its ability to shield against any form of radiation, be it positively charged, negatively charged, or neutral, and it is widely-employed in Earth-based shielding applications, since weight is not an issue. For space applications, however, every kilogram of mass has a significant impact upon the mission cost and feasibility. While dense materials or thick layers of material are great at attenuating the energy levels of incoming radiation, they come with a significant amount of mass. Additionally, in Earth-based scenarios, engineers usually need to shield a radioactive item and prevent radiation from leaking out. In space- based scenarios, however, engineers seek to keep radiation from getting in to the area where the astronauts are located. For long duration space missions, the livable area that the astronauts occupy can become quite a bit larger than most Earth-based radiation sources. Thus, not only is mass an issue in space, but the volume enclosed by the shield is much larger and therefore requires even more shielding material. This can get prohibitively heavy when used as the sole method of radiation protection for long duration space flights.
Active space radiation shielding is inspired by the Earth's magnetic field, which serves both to deflect and to trap portions of the incoming space radiation. Many spacecraft designers have proposed innovative designs that form large electromagnetic fields around a spacecraft, in order to mimic the protection of Earth's magnetosphere. To the best of the author's knowledge, none of these designs have been tested in the space environment, but represent the future directions that technology must take in order to have a safe method of interplanetary space travel. Since the field is still under development, a multitude of suggested approaches exist:
Electrostatic Shielding: This approach creates an electric field around an astronaut habitat, with the negative potential facing outwards to slow down negatively charged radiation.  Engineering trade-offs to consider when designing such a system include the dielectric breakdown strength of the electrostatic material, the maximum voltage capabilities of the power supply, and the mechanical limits of the support structure in comparison to the internal coulomb forces generated by the charged components of the shield. Finally, there are no known major physiological issues associated with humans in large electrostatic fields, but further investigations are required in order to verify astronaut safety with a sufficient degree of certainty.
Magnetic Shielding. Magnetic shielding consists of forming a large magnetic field around the spacecraft, usually through the use of superconducting solenoids.  Unlike with electric fields, there are known and suspected physiological effects of moving within a strong magnetic field. In order to use this approach for space radiation shielding, the design must allow for a habitable region without significant magnetic field strengths. Usually, this is done by using a torus-shaped design that has a shielded region internal to the torus. These layouts allow for a small region between the solenoids that is free of magnetic fields, while still generating a magnetic field that is comparable to an ideal dipole at large distances. Charged particles are either deflected by the magnetic field, or trapped along the magnetic field lines, well before they approach the internal shielded region of the torus.
Plasma Shielding. Plasma shielding is a field of ongoing research, fundamentally consisting of a mass of ionized particles that is entrapped by electromagnetic fields, swirling around a spacecraft enclosure and serving to deflect or ensnare charged particles.  The protection is threefold: first, an electrostatic field with a positive potential repels positively charged radiation. Next, a magnetic field is added to ensnare the negatively charged particles that are drawn to the positive potential. Finally, these negatively charged particles would be drawn towards the positively charged surface, which could neutralize the surface; thus, a passive current-absorbing shield is placed at the magnetic poles, to absorb the negatively charged particles before they impact the positive surface. While the system is more complex, it leverages the strengths of passive, electrostatic, and magnetic shielding, and combines them into a highly effective solution.
Comparing the three types of active shields, electrostatic shields are relatively lightweight when compared to magnetic shields, but since they only repel negative particles, they also pull in positive particles, which creates a current influx that must be counteracted. Additionally, electrostatic shields are limited by voltage level, which in turn limits the energy level of the particles that they can deflect. Magnetic shields, on the other hand, do not collect currents and can achieve effective shielding for all expected radiation levels. Finally, plasma shields, are the most lightweight and the least power-consuming of all three approaches, but are also the least mature design approach. They have the potential to outperform the other two designs in terms of radiation shielding capability, but are experimentally unproven, and their functionality is hotly debated.
Examining the different methods of space radiation shielding, it is clear that no single good solution currently exists to adequately protect astronauts from the radiation environment of space. If we wish to travel to Mars, asteroids, or even to the moon for long duration missions, major developments must be made. However, the ideas being examined now lay a solid foundation to grow into feasible solutions in the future.
© Ashley Clark. 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.
 T. I. Gombosi, Physics of the Space Environment (Cambridge U. Press, 2004).
 S. Copeland, "The Deep Space Radiation Environment," Physics 241, Stanford University, Spring 2012.
 C. R. Buhler, "Analysis of a Lunar Base Electrostatic Radiation Shield Concept," ASRC Aerospace Corporation, 15 Dec 04.
 S. G. Shephard, B. T. Kress, "Störmer Theory Applied to Magnetic Spacecraft Shielding," Space Weather 5, No. 4, S04001 (2007).
 J. H. Adams et. al., "Revolutionary Concepts of Radiation Shielding for Human Exploration of Space," U.S. National Aeronautics and Space Administration, NASA Technical Report NASA/TM-2005-213688, March 2005.