I appreciate S. Fred Singer's comments1 on my article, "Next Stop, Mars."2 He is correct that a focus on improved propulsion could reduce travel time to Mars and therefore radiation exposure. Also, his suggestion to use low atomic weight materials within shields is a time-honored and reasonable approach – although these shields would need to be quite thick to reduce radiation to terrestrial levels. I disagree, however, that magnetic shielding would not be effective.

The effectiveness of Earth's magnetic field decreases with increasing latitude, so that people living near the equator receive much more protection from the field than those living close to the poles. Also, the Earth's magnetic field is weak. But an active shielding system for use in space could use whatever magnetic field strength is needed to provide the desired protection. Also, the spacecraft could be positioned within the protected area (i.e., at the "equator"...

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