Earth protects itself with a magnetic field - why cant we do the same to a space vehicle?
Magnetic Shielding
Consider, then, the possibility of generating a magnetic field analogous to Earth’s to protect a spacecraft and its
inhabitants. Because of the small dimensions of a spacecraft compared to a planet, a magnetic field around a
spacecraft must be very intense, of the order of 20 T—400,000 times more intense than the static magnetic field
of Earth.
Recent engineering studies indicate that adequately shielding a modest cabin for the astronauts can be achieved
in this manner, but only if the system is constructed with superconducting wires to carry the electric current
without loss. That scenario requires a system with a total weight of about 9 t—still too heavy to land on Mars
and take off again—with most of that weight in the cryogenic system that maintains the cold, superconducting
wires and in the rigid structure necessary to hold the current-carrying wires in place.
The nature of magnetic fields poses an additional complication. A certain number of particles will leak through
even the most efficient magnetic shield. Once inside, the particles will be confined for a time for the same
reason that they had difficulty getting in. Basic laws of physics tell us that without absorption, the magnetized
space will eventually become filled with particles up to the same intensity as the cosmic rays in the surrounding
space. That means a magnetically shielded spacecraft must also serve as a particle absorber to scavenge the
accumulation of leakage. That is to say, even the best of ships must pump the bilges occasionally.
Some engineers think that advances in technology might eventually make it possible to generate a magnetic
shield with a lighter system, and the spacecraft itself may well provide enough absorbing material to keep the
leaking particles from accumulating in the shield field and exposing the astronauts. But even without these
limiting factors, residing inside such a strong magnetic field may pose a serious health risk.
The biological consequences of long-term exposure to strong, static magnetic fields are inconclusive, but one
reason for concern arises from a single data point that John Marshall mentioned to me many years ago. Marshall
was referring to an informal experiment in which a magnetic field of 0.5 T—only about one fortieth of the
required shielding field—caused electrolysis in the saliva (a lemony taste in the mouth) and scintillations in the
retinas, indicating that any motion in such fields interferes with the normal chemistry of the human body.
Those findings alone suggest that any magnetic shielding system must cancel out most of the magnetic field
over the living space of the astronauts. This field reversal could be accomplished by inducing two electrical
currents around the living quarters to generate a magnetic field with the opposite polarity as the main shielding
dipole (see Figure 1). The downside is that the spacecraft would be weighed down further by the additional steel
it would take to hold those reverse currents in place.
To exactly what degree the shielding field must be weakened, however, is still unknown. Perhaps the human
body can tolerate fields of 0.5 T or more. If so, the engineering challenges would be reduced in other words,
the main shielding field would not have to be cancelled with great precision. More research is vital to determine
what level of effort is truly necessary.
http://engineering.dartmouth.edu/~d76205x/research/Shielding/docs/Parker_05.pdf
