In The Martian, without spoiling anything, there’s a point where our stranded astronaut has to use human feces to fertilize his crop in order to actually live on the planet.
The question some folks had following that revelation in print and on the screen was pretty common: is that even possible? Well, yes.
In an asked and answered article in Popular Science, Utah State University Professor Bruce Bugbee explains that it is, in fact, possible. It’s just that it’ll take a long time.
As the article sums up, the problem with human poop is that it contains both good and bad bacteria. The good stuff will help fertilize whatever these Mars-bound explorers want to grow, while the bad stuff could actually make the astronauts quite sick. Simply dumping human feces on the plants would introduce both kinds of bacteria, and that would not be so good.
The solution? Long term composting
Bugbee offers, instead, that the best way to make this happen would be to compost the excrement “over several months” in order to slowly eliminate the bad bacteria. It will work, but the astronauts need other means of eating before it takes off.