"We could send senior-citizen volunteers to the Red Planet, where they could spend their final months conducting experiments, laying the groundwork for future permanent settlements and digging their own graves." That's from this great piece at Scientific American in response to the op-ed Lawrence Krauss (a Scientific American columnist) wrote in Monday's Times about sending astronauts on one-way trips to Mars. Apparently, the greatest obstacle to man's visiting the red planet lies not in launch capabilities or guidance systems or the threat of Martian attack, but rather in the dangers posed by exposure to the sun's powerful radiation on the return flight. Building a strong enough shield is [...]