Earth's magnetosphere, which shields us from deadly solar radiation is decaying rapidly. When it collapses, as Mars's did billions of years ago, Earth will be stripped of its atmosphere, water and life. This episode explores new evidence that the Earth's magnetic poles are on the verge of reversing. As it does, Earth's magnetosphere will weaken greatly for thousands of years, allowing solar radiation to penetrate to the Earth's surface. This will threaten many species, including man, with extinction. In 2010 three satellites will be launched to detect minute variations in field strength and identify any new areas of pole reversal. For mankind to survive, we must develop technological methods of protecting ourselves before this happens.