Inspiration Mars ship using human waste as a radiation shield: no really, it's fine
 

Dennis Tito is planning an ambitious private flyby of Mars for 2018 that will carry all kinds of logistical challenges during its proposed 501-day span, not the least of which is shielding the crew from radiation without consuming valuable resources. The team's solution is a clever one, if not especially pretty: human waste. While the walls of the Inspiration Mars spacecraft will initially be lined with water-filled bags to guard against cosmic rays, their contents will be gradually replaced with er, byproduct that will be dehydrated through the bag (possibly using polyethylene) to reclaim and purify water for drinking. As water-based materials are better at stopping radiation than metal, the approach theoretically represents the best of all worlds with less bulk, a simpler life support system and maximum room for supplies. If the Inspiration Mars group can keep the bags working at high efficiency in space, it won't have to worry about its travelers' safety; their comfort with being surrounded by their own waste may be another matter.

http://www.engadget.com/2013/03/03/i...iation-shield/