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."
The plan would be about as simple as it sounds: create an outside layer for the capsule where the people are, and fill it with feces. But poop isn't the only effective radiation deflector. Food does a pretty good job too, and without actually becoming radioactive itself, so chances are it could be incorporated into the shielding too. Hopefully far away from the waste. Likewise, water can help too, both drinking water and the waste water it turns into.
The details aren't finalized yet, of course. And the trip—if it happens—is years away. But human waste isn't going to become any worse of a radiation shield between now and then, and it's always going to be something that's produced by astronauts, so you can bet this'll happen sooner or later. Let's just hope we never have to adopt it here on Earth. [New Scientist]