In space travel, “Why?” is perhaps the most important ethical question. “What’s the purpose here? What are we accomplishing?” Green asks. His own answer goes something like this: “It serves the value of knowing that we can do things—if we try really hard, we can actually accomplish our goals. It brings people together.” But those somewhat philosophical benefits must be weighed against much more concrete costs, such as which other projects—Earth science research, robotic missions to other planets or, you know, outfitting this planet with affordable housing—aren’t happening because money is going to the moon or Mars or Alpha Centauri.
You can think of flying to Mars like one of those art films where the director has to shoot the movie in a single take. Even if no scene is especially challenging, the requirement that everything go right sequentially, with no way to pause or reshoot, means that even small risks become unacceptable in the aggregate.