Such a shield could leave Mars in the relatively protected magnetotail of the magnetic field created by the object, allowing the Red Planet to slowly restore its atmosphere. About 90 percent of Mars's atmosphere was stripped away by solar particles in the lifetime of the planet, which was likely temperate and had surface water about 3.5 billion years ago. According to simulation models, such a shield could help Mars achieve half the atmospheric pressure of Earth in a matter of years. With protection from solar winds, frozen CO2 at Mars's polar ice caps would start to sublimate, or turn directly...