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.