What comes out of the human body and goes into a toilet is often thought of as an end product. But for future residents of Mars, human waste will need to take on a more vital role. Like becoming fertilizer for growing food. Because it costs thousands of dollars to lift every pound of stuff into space, and even more to get it to Mars, mission planners don’t want to waste a thing. Human excrement, being rich in nutrients, can be a composter’s dream. So a group of European researchers is taking on the task of figuring out just how to safely manage a high-tech Martian outhouse.