There are all kinds of dangers on a trip to Mars. Muscle atrophy, cramped quarters, social/isolation stress, being in space. Take your pick. One of the biggest though, is being exposed to all manner of radiation and cosmic rays without an o-zone layer to keep you safe. The solution? An O-zone layer of poop. Basically.
Dennis Tito has big plans to send a man and a woman skimming by Mars in 2018, and if that's going to happen, he needs a solution for the cosmic ray problem. And his team has been thinking "poop shield." Tito team member Taber MacCallum put it this way to New Scientist: "It's a little queasy sounding, but there's no place for that material to go, and it makes great radiation shielding."