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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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.
For further
information on Radiation Belt Remediation with the HiVOLT System,
please contact
TUI at information@tethers.com