Sending six smaller women into space for months or years could be significantly less expensive than sending six muscular men, and lower body weights are just a small part of it. The rest of the difference comes from the amount of food, oxygen, and other resources needed to keep smaller humans alive. For a short-duration trip, the difference might be negligible. But if you’re aiming for Mars – or the stars – the contrast between sending enough food for a large man versus a small woman could end up being substantial because, on average, men require 15 to 25 percent more calories a day than women. It is a difference that Kate Greene observed in 2013 while participating in a four-month-long simulated mission in a Mars habitat. Part of Greene’s assignment was to monitor the metabolic output of her crewmates – and on average, she reported, females expended less than half the calories of their male counterparts, despite similar activity levels. On top of that, smaller people produce less waste (think carbon dioxide and other bodily excretions), which translates to lower demands on spacecraft systems designed to recycle and remove that junk.