Cosmic ray particles, especially those over 40 MeV, are the major radiation danger in outer space. These are almost entirely positively charged protons, alpha particles, and heavier nuclei. Negatively charged beta particles – electrons – make up a fraction of 1% and can be blocked by mechanical shielding. Some of the positive particles are Solar Energetic Particles (SEP) generated by solar flares or by coronal mass ejections with energies up to a few MeV. The rest are galactic or extra-galactic particles with energies ranging to over 3 x 1020 eV. Fortunately the frequency of these high energy particles falls off in a steep power law such that well over 99.9% of cosmic rays have a kinetic energy of less than 20GeV.
The Apollo missions were of a few days duration and were flown during a quiet sun period. The astronauts returned directly to earth, where they were again protected by its atmosphere and magnetic field. The ISS orbit is within the earth’s magnetic field which eliminates particles under 10 GeV. In both cases crew exposure is strongly limited even without protection. By contrast the situation is critical for long duration space flights or space habitats away from earth where the exposure duration may be decades. The crew will be subject to the full range of solar weather and may never return to the protected earth environment. On flights beyond the heliopause the situation is much worse because of very high levels of additional particles below 2 GeV.
Various magnetic shield topologies have been investigated. It has been proposed to give the spacecraft a net positive charge. This is difficult to maintain as it attracts every free electron in the universe to neutralize it. An alternative would be a neutral electrostatic shield as is presented here, using current technology. The main engineering problem for sufficient field strength would be the size required to avoid breakdown; however, light objects can be very large in microgravity. The neutral structure would consist of a positively charged central hull surrounded by a negatively charged grid with the entire assembly spun to generate gravity within the hull and to balance the Coulomb force between the hull and the grid. For long duration flights or habitats, spin gravity is the only current practical solution to microgravity induced health problems. When passing through an area of higher radiation the grid could be pumped up to a higher voltage and the spin increased. The crew would move inboard to lower gravity areas and would have more of the vessel structure between them and the outside environment.
The first architecture would be an elliptical hull 3 km long and 500 meters in diameter. Three km is sufficient space for an axial 50 GeV linear electron accelerator driven by klystrons. A 2 km dielectric support strut would extend on axis from the cold end of the hull. A 2 km beam tube inside another axial support strut would extend from the hot end of the LINAC. This tube would be provided with quadrapole focusing magnets to keep the beam collimated. The outer end of the beam tube would be an electron capture and secondary emission suppression structure. The grid would extend between the outboard ends of the two support struts and when fully expanded would have a radius of 1.5 km, yielding a clearance of at least 1 km from the hull. A good material for the grid might be metalized ultra-high-molecular-weight polyethylene fiber, which has an extremely high specific strength. The amount of metal should be minimized as the Coulomb force is proportional to the area of the metal in the grid, as is the amount of charge required to reach a given voltage. The 50 GeV LINAC is required to charge the grid relative to the hull.
The second architecture would be an elliptical hull 2 km long and 1 km in diameter. In the 1 km diameter waist would be a 50 GeV electron synchrotron. A separate 2 km dielectric support strut would extend on axis from each end of the hull. Up to 3 tangential beam tubes would extend from pick-offs at the synchrotron out to a 2 km radius conductive ring. Balancing tangential dielectric support struts would also support the ring. The beam tubes would be provided with quadrapole focusing magnets to keep the beam collimated. The outer end of the beam tubes would have an electron capture and secondary emission suppression structure. The grid would extend between the outboard ends of the two support struts and the ring. When fully expanded it would have a radius of about 2 km, yielding a clearance of at least 1 km from the hull. A good material for the grid could again be metalized ultra-high-molecular-weight polyethylene fiber, which has an extremely high specific strength. The amount of metal should be minimized as the Coulomb force is proportional to the area of the metal in the grid, as is the amount of charge required to reach a given voltage. The 50 GeV synchrotron is required to charge the grid relative to the hull.
It is proposed that the nominal hull-grid differential be around 20 GeV. This will suppress SEP completely and other particle radiation by three orders of magnitude, yet is low enough to preclude vacuum breakdown over 1 km. As mentioned above, this could be increased in high radiation areas with an increased risk of breakdown. Remotely controlled electron emitters facing the hull would be provided on the grid supports to discharge the grid gracefully in order to allow repairs or such activities as docking with other spacecraft. At such times the off duty crew would retire to a protected core area. The electron cathode emitters could be thermionic devices consisting of ceramic tubes or plates with passages under the surface and covered with thoriated tungsten. These would be heated by burning fuel and oxidizer stored in the pylons. Preferably these would be either hypergolic fuel pairs such as dinitrogen tetroxide / hydrazine or catalytically ignited fuels such as hydrogen mixtures using platinum black to eliminate ignition hardware. The burning fuel would be passed through the cathodes’ internal passages with topology designed for even heat distribution, and exhausted outward in a balanced manner. Spinning a spacecraft creates problems, but the human issues have to take precedence in decades long missions. Communications, for instance, could be handled by antennas mounted on counter-rotating donuts on the hull around the bases of the axial struts. For habitats and astrometry level accuracy, instruments could be mounted on external platforms.
