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I was listening to a Science Friday podcast in which the Martian movie was being discussed. The science contributors stated the radiation problems are the biggest issue for Mars exploration.
Reading Magnetic shield could protect spacecraft states that a 1 Tesla magnet would work:
Bamford and colleagues say these results show that a spacecraft could in fact be protected using a bubble just some 100–200 m across. This, they say, would correspond to a magnet of about 1 Tesla, which would be light enough to be transported into space.
How much does a 1 Tesla neodymium magnet weigh on earth?
Could permanent magnets be installed in the Mars habitat and in the seat of the rover?