Presented at the Tenth Biennial SSI/Princeton Conference on Space
Manufacturing
May 15-19, 1991, Princeton, N.J.
posted with permission of author
Paper available in the volume
Space Manufacturing 8: Energy and Materials from Space, 383-386 (AIAA,
1991)
One solution to the problem of shielding crew from particulate radiation in space is to use active electromagnetic shielding. Practical types of shield include the magnetic shield, in which a strong magnetic field diverts charged particles from the crew region, and the magnetic/electrostatic plasma shield, in which an electrostatic field shields the crew from positively charged particles, while a magnetic field confines electrons from the space plasma to provide charge neutrality. Advances in technology include high-strength composite materials, high temperature superconductors, numerical computational solutions to particle transport in electromagnetic fields, and a technology base for construction and operation of large superconducting magnets. These advances make electromagnetic shielding a practical alternative for near-term future missions.
A significant difficulty for a manned missions outside of the Earth's magnetosphere, including Mars missions, asteroid exploration, and space-based mining and manufacturing, is the hazard of crew exposure to particulate radiation. With the recent resurgence of interest in manned Mars missions, crew radiation shielding has again become an active problem for investigation [1].
Two types of radiation are particularly significant: solar flare protons, and high-energy galactic cosmic rays (GCR). Solar flare protons come in bursts, lasting a day or so, following an energetic solar event. The proton flux is omnidirectional; although the source of the radiation is solar, the actual radiation comes from all directions, and hence the spacecraft must be shielded in all directions, and not just in the direction of the sun. In the absence of shielding, a single large solar flare would likely be fatal to the crew, either immediately or as a result of cancers induced by the radiation dose. Cosmic rays are a continuous background consisting of extremely high energy heavy nuclei, and are also omnidirectional. While the GCR background is not immediately fatal, the integrated GCR dose over long (>1yr) missions will approach or exceed the recommended maximum allowable whole-body radiation dose, and also may result in other significant health problems to the crew.
On the Apollo missions, the approach to crew protection was simple: on notification of a large solar flare, the mission would be aborted to Earth. Since the missions were short, the cumulative fluence of galactic cosmic rays was not significant. This approach, however, is not possible for a Mars mission, where return to Earth times will be many months, not significantly shorter than the total mission duration; and would be unlikely for a space colony or manufacturing facility in Earth orbit, with the goal of continuous occupation.
The dangerous components in both solar flare and GCR radiation consists of positively charged particles. Neutral radiation (gammas, neutrons) are a negligible component of the radiation ambient; negative particles (electrons), while present, can be easily shielded. The positive particles, however, are extremely penetrating, and require massive shields. In the case of GCR, a small amount of mass shielding has no benefit, or even negative benefit over no shielding at all, since the impact of GCR nuclei on a light shield will produce secondary radiation considerably more intense than the original GCR.
Since the particles involved are charged, an alternative solution to the problem of shielding is the use of active electromagnetic shields. The simplest such device is the magnetic dipole shield. The magnetic field of the Earth is a good example of a magnetic shield, and is responsible for the relatively benign radiation environment on Earth. A magnetic shield makes use of the fact that a charge particle's trajectory in a magnetic field is curved. As a particle enters the region of high magnetic field, its trajectory will curve away from the region to be protected. In essence, the principle is exactly the reverse of that involved in a magnetic bottle; in this case the intent is to trap the particles outside the region of interest, instead of inside. The advantages of a magnetic shield to crew safety and health are obvious.
An additional advantage of the magnetic shield is that no secondary radiation is produced by interaction of the shield with the incident radiation.
The concept of magnetic shielding using superconducting coils for space vehicles was first discussed by Levy [2] and analyzed in some detail in the 1960's and early 1970's [3,4,5,16,17], but was not pursued, primarily because there were no plans for long-duration manned flights which would benefit from such a shield. Magnetic radiation shielding received a brief resurgence of attention in the late 1970's, when the concept of large space colonies was introduced by O'Neill, as an alternative to simple mass shielding for the large populations envisioned [6,7].
The limit to the mass required to produce a magnetic field is set by the tensile strength of materials required to withstand the magnetic self-force on the conductors [8]. For the min-imum structure, all the structural elements are in tension, and from the virial theorem, the mass required to withstand magnetic force can be estimated as [9]:
M = (rho/S) (B^2 V)/(2 mu) (1)
where rho is the density of the structural material, S is the tensile strength, B the magnetic field, V the characteristic volume of the field, and mu the permeability of vacuum.
An alternative to the magnetic shielding is to use an electrostatic shield. Since both solar flare protons and heavy nuclei in GCR radiation are positively charged, it would be thought that shielding would simply require adding a positive charge to the object to be shielded sufficient to repel the particles. To shield against solar flare protons would require an electrostatic potential of on the order of 1 E8 volts; shielding against cosmic ray nuclei as well would require as much as 1 E10 volts.
Unfortunately, the situation is complicated by the interplanetary plasma. Attempting to simply charge a vehicle in Earth orbit or in inter-planetary space would result in electrons being attracted to the vehicle. This would discharge the vehicle very quickly. Levy and Janes [10] estimate that to maintain a charge of 2.E8 volts would require a power of 1 E7 kilowatts.
Con-centric spheres of opposing charge, proposed by Birch [7] to repel both positive and negative particles, would reduce the discharging, but maintaining the required voltage difference over a small distance is beyond the range of current technology.
A solution proposed by Levy and Janes [10, 11] is a hybrid of the magnetic shield and the electrostatic shield, the "plasma shield." An electrostatic charge is applied to the vehicle (or habitat) shell to repel the positively charged radiation; a magnetic field then prevents the plasma electrons from discharging the vehicle. At large distances the shield is charge neutral, since the magnetically confined electrons exactly neutralize the charge on the shell (see figure 1). Since electrons are lighter than protons by a factor of 1860, the magnetic field required for the plasma shield is reduced over that for a simple magnetic shield by the same ratio, and hence the weight associated with the field generation and structure. This ratio is even more favorable for shielding from cosmic rays.
The required electron confinement time tau to maintain the charge, assuming a maximum allowable energy expenditure of 10 kW (for the 5m. radius torus assumed), is 100 minutes. With the assumed plasma density n of 2.E9 e-/cm3, the confinement product n tau is 1 E13/cm3-sec. Magnetic containment systems for fusion applications have demonstrated n tau products in excess of E14 /cm3-sec at considerably higher temperature, so maintaining the charge is not unreasonable. They calculate that the system could likely work if the outgassing rate from the surfaces and the incident micrometeroid flux is low enough that the vehicle is not discharged by the ionized particles.
French [12] notes that the plasma shield is made even more effective if it is used in combination with a passive mass shield, since the mass shield is most effective for low energy particles. The system is also discussed by Hannah [13], who notes that there are some advantages in confining the magnetic field (and hence also the electron cloud) to a small "core" region of the toroidal habitat.
Four technical advances in recent years make magnetic shielding much closer to practicality than the early studies twenty years ago.
First, computers are now in universal use for solving particle trajectory problems by direct numerical integration. Previous solutions for the effective shielding produced by magnetic fields [2,3,4,5,6,7,16,17] relied on approximations, resulting in solutions in which the shield was completely effective for particles up to some cut-off energy and completely ineffective above this energy. In reality, the particle flux is reduced, although not eliminated, for energies above the cut-off, and this reduction can be significant in the overall shielding effectiveness. Solving the problem of the shielding produced by any given magnetic field configuration for any energy spectrum is now straightforward, even for systems in which the magnetic field is of complicated geometry. This problem has been solved for other applications [14]. In terms of plasma shielding, the field of particle transport in a plasma, once nearly unknown, is now well understood from the work done in the fusion energy program.
Second, recent advances in high-temperature superconductors mean that it is reasonable to consider superconductors operating at 77K or higher [9]. This is a range which will allow use of passive cooling, where the temperature is achieved by directly radiating excess heat to space [15], a considerable advantage over use of superconductors requiring liquid He temperatures.
Third, there is now a large body of experience in fabricating large superconducting magnets. Superconducting magnets are now standard technology on large particle colliders, such as the Fermilab "Tevatron" and the planned [now cancelled] Superconducting Supercollider, as well as for large magnetic fusion experiments.
Finally, extremely high strength to weight composite materials are now available. Since the limit to magnetic field strength is produced by the tensile strength of materials required [9], composite materials with strength to weight ratios five times (Kevlar 49) to 7 times (PBO) higher than that of steel allow considerable weight reduction in the tension members.
These advances make magnetic shielding an extremely attractive option for long space missions.
The most important enabling technology is the ability to form high-temperature superconductors into wires. While magnetic shielding can be done using conventional (low temperature) superconductors, the pay-off in simplicity and weight of higher temperatures is so great as to be mission-enabling. Clearly, advances in the critical current and the transition temperature also allow significant gains to be made as well. A second required technology which must be domonstrated is the cooling of wires to the superconducting transition temperature using passive cooling, essentially shielding the wire from the sun and allowing it to radiate to deep space.
An as-yet unresolved question on the use of magnetic shields involves the question of whether the field region can be allowed to penetrate into the crew area. Almost all existing magnetic shield designs have been designed with the requirement that little or no magnetic field penetrate into the inhabited region, due to concern about the (unknown) hazardous effects of long-term exposure to magnetic fields. However, considerably simpler engineering designs can be made if some magnetic field is allowed to penetrate into the shielded region. While magnetic fields in general are not hazardous to humans, it is not known what effect high steady-state magnetic fields have.
Magnetic radiation shielding and magnetic/ electrostatic "plasma" shielding are concepts for shielding against the space radiation environment which have reached new levels of practicality due to advances in technology in the decades since they were first proposed. Magnetic shielding is an idea whose time has returned.