The prospect of colonising Mars is often viewed as a bad idea, and the reasons why are valid, to name a few: - It would be extremely expensive - It has toxic dust and no breathable atmosphere - It has high levels of radiation - It may pose other, yet unknown risks to human health. (What are the long-term effects of living with only 40% of the gravity we experience on Earth?) But it is extremely likely that a catastrophic event will eventually occur on Earth that has the potential to wipe out the human species. We need a back-up plan to preserve humanity. This idea is that we continue with the mission to populate Mars, but make it a transient population of a few hundred or thousand people, that rotate once a year. This will give us a chance to recolonise Earth in the event that something catastrophic comes and wipes out those of us that are on Earth. Plus, I'm sure many people would volunteer for the experience!