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deflector shield
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Shields on real spacecraft
Spacecraft such as Giotto and Stardust, designed to pass through the dusty tails of comets, are equipped with passive, material deflector shields called Whipple shields.
Another kind of deflector that uses a bubble of charged particles, or plasma, has been proposed for use of future manned spacecraft as a way of protecting the crew from exposure to high-energy solar radiation while en route to distant destinations such as Mars (see radiation protection in space).
The first spacecraft to deploy a plasma cloud was one of the satellites in a mission called AMPTE (Active Magnetospheric Particle Tracer Explorer), launched in 1984. A plasma cloud thousands of kilometers across was released in order to learn more about the interaction of solar wind particles with Earth's magnetosphere and to serve as an artificial comet. However, as the cloud wasn't contained it completely dispersed over time.
In 2006, researchers at the University of Washington, funded by NASA, examined the potential for a plasma cloud to serve as a spacecraft shield. The spacecraft would release hydrogen gas and ionize it with a high voltage device. The resulting protons and electrons would form a cloud and be prevented from dissipating by means of a charged wire mesh surrounding the spacecraft. The plasma's magnetic field would serve as a powerful deflector of charged particles in cosmic rays (from the Sun and elsewhere in space), equivalent to aluminum shielding several inches thick. The larger the cloud, the more effectively it would deflect cosmic rays. But a larger cloud would need a bigger cage to contain it, and would therefore increase the spacecraft's mass.
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According to one estimate the optimum size of the plasma bubble might be about 100 meters across. At that size, the containing mesh would have to be stowed for launch and deployed once the craft was in space. The mesh would need to be made of superconducting material and be able to operate at relatively high temperatures, since it would be heated by sunlight. Future spacecraft might be powered by advanced engines that use plasma as a propellant. In that case, the discarded plasma could be recycled for use as a radiation shield, so that, effectively, the spacecraft would be protected by its own exhaust.
Natural deflector shields
The Earth and other planets have natural deflector shields in the form of magnetospheres. Without its magnetosphere, the Earth would be subjected to intense bombardment by charged particles from the Sun during episodes of solar activity. The Solar System as a whole also has a magnetsopheric deflector shield known as the heliosphere.
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