High-Voltage Orbiting Long Tether (HiVOLT):
A System for Remediation of the Van Allen Radiation Belts

Background: The Van Allen Radiation Belts
The Van Allen Belts are concentrations of energetic particles near the Earth that pose significant danger to humans and spacecraft operating in those regions. The Earth’s atmosphere is continually bombarded by the interstellar flux of cosmic rays. These cosmic rays collide with molecules and atoms in the upper atmosphere and generate high-energy charged particles. A significant portion of these high-energy electrons and ions are trapped by the magnetic mirror formed by the Earth’s magnetic field, as shown in Figure 2. In addition, the Sun continually ejects high-energy charged particles into the space around it. As these solar wind particles speed past the Earth, some of them find their way inside the Earth’s magnetopause and also become trapped by the magnetic mirror formed by the Earth’s dipole magnetic field. This combination of cosmic-ray induced particles and solar wind particles trapped by the geomagnetic field form regions of intense radiation flux in Earth space. These particles bounce rapidly back and forth between mirror points above the Earth’s atmosphere, and also drift azimuthally around the Earth several times per hour. The altitude of the mirror point of a particle depends upon the pitch angle of the particle’s velocity with respect to the magnetic field, and only those particles with pitch angles greater than a certain level are trapped; particles with lower pitch angles will be lost through impact with the atmosphere. The trajectories of the trapped particles are usually described using the McIlwain "L-shell" coordinate system defined using an integral equation involving adiabatic invariants of the trapped particle motion. This coordinate system is a nested set of toroids having an "egg-shaped" cross-section. The higher energy particles become concentrated into three major radiation belts, a broad proton belt, an inner electron belt, and an outer electron belt. The existence of the gap between the inner and outer electron belts indicates that there are certain L-shells that do not trap significant numbers of electrons of any energy for long periods of time.

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Figure 2. Trapping of radiation particles by the Earth's magnetic field. (Figure courtesy AFRL)

Radiation Belt Effects on Spacecraft and Personnel:
Energetic particles with energies greater than about 1 MeV pose a severe threat to spacecraft systems in those regions. These energetic particles will steadily degrade electronics, optics, solar panels, and other critical systems by breaking chemical bonds, disrupting crystalline and molecular structures, and by causing localized charge effects. Higher energy particles in solar events and cosmic rays can cause single-event disruptions or damage to electronics. Spacecraft systems operating in Earth orbit must be hardened to withstand this radiation environment, and typically their electronics must be designed with several layers of redundancy, incurring significant expense and additional mass. The radiation particles also pose a significant threat to personnel and other biological systems in Earth orbit. As they pass through tissue, they can deposit their energy by ionizing water and proteins, causing cellular damage, modifying DNA, RNA, and proteins in ways that can lead to cancers, immune system disorders, and other maladies. Protecting personnel in space from energetic particles in the MeV range requires a great deal of extra mass for shielding; a 1996 NRC study concluded that the shielding mass required to protect astronauts during a Mars expedition could add $10B to $30B to the cost of the mission. The presence of the high radiation fluxes in the Van Allen belts limits long-duration manned missions to operation below about 1200 km altitude. Spacecraft passing through the radiation belts will experience rapid degradation of solar panels, electronics, and other systems, and thus transit times through the belts must be minimized; this effect of the radiation belts is one of the primary obstacles preventing Solar-Electric Propulsion based transfer vehicles from being used to provide lower-cost orbit transfer for manned and robotic missions.

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please contact TUI at information@tethers.com

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