And here's another: What happens if one of the humans has a medical emergency and needs surgery? As Carnegie Mellon professor James Antaki told New Scientist, "Based on statistical probability, there is a high likelihood of trauma or a medical emergency on a deep space mission."
This is not just a matter of whether you'll have the expertise on board to carry out such a task. Surgery in zero gravity presents its own set of potentially deadly complications.
Think about how hard it is to pee in zero gravity: You need a funnel and a tube that siphons your urine to a sewage tank. Without those tools: Urine everywhere. Consider the difficulties of brushing your teeth. It took astronaut Leroy Chiao three paragraphs to explain that process, and it involved bungee cords, drink bags, and velcro.
In zero gravity, blood and bodily fluids will not just stay put, in the body where they belong. Instead, they could contaminate the entire cabin, threatening everybody on board.
This week, NASA is testing a device known as the Aqueous Immersion Surgical System (AISS) that could possibly make space surgery possible. Designed by researchers at Carnegie Mellon and the University of Louisville, AISS is a domed box that can fit over a wound. When filled with a sterile saline solution, a water-tight seal is created that prevents fluids from escaping. It can also be used to collect blood for possible reuse. "You won't have a blood bank in space, so if there is bleeding you want to save as much blood as you can," one of the researchers, James Burgess, told New Scientist.
If it works well enough, that will be one more thing ticked off the pre-Mars to-do list.