> Having a self-sustaining living space is by far the biggest problem (one where all of the waste we produce can be reused to create new drinkable water, air an

> Having a self-sustaining living space is by far the biggest problem (one where all of the waste we produce can be reused to create new drinkable water, air and food, and to reliably continue to do so between Mars launch windows). That problem gets a lot easier and cheaper when you don't have to design for margins of 1.2 to or so to save on weight. When you can throw extra weight at a problem to overbuild it for reliability you can solve these problems cheaper and more reliably. > The second biggest is probably avoiding contamination of Mars, if we actually care about exploring one of the biggest mysteries of life - has it arisen only on Earth? Are there other modes of life possible, and what do they look like? Planetary Protection is the new version of treating Earth as "gaia". It's a form of regressive environmentalism. This is not an important goal. Mars is not a very good candidate for supporting life now that we know how many geothermally active ice moons with undersea oceans exist in our solar system. If hypothetical life on Mars is similar to life on Earth then it's not that valuable of a thing to learn and anything found would vary drastically from life on Earth from billions of years of separate evolution. If it's very different from life on Earth then it will still be easy to find even with plentiful human contamination. Finally, with additional mass you can bring substantially heavier and more complex analysis and remote robotic equipment with you. (Or on a completely separate mission before humans arrive.) > The third biggest challenge is human psychology and physiology - could we actually get a group of ~100 people or however many we need initially to live and be productive for the years-long space mission (including travel time to and from)? What do we do when at least one of them inevitably gets sick on Mars? This is overrated given that there's plentiful human experience throughout the history of humanity where humans suffered significantly worse isolation and hardship. Some projects will fail from human psychological issues, and the risk from them should be minimized (bring plants along, allow spaces for people to self-isolate, etc) but a single failure should not be taken as a reason that the entire concept is doomed. For sickness, you try to minimize possible issues by bringing significant medication and medical equipment, but sometimes there will be inevitable deaths. As mentioned, having additional mass that you can bring for human comforts and medical equipment saves a lot of potential deaths and issues