We have been going to space for decades now, but until the year 2000, those stays in orbit were always temporary. However, when three astronauts moved into the International Space Station (ISS) for a four-month stay, it marked the beginning of a decade and a half of a permanent human presence in space. The arrival of these three astronauts at the ISS on Nov. 2, 2000, sparked one NASA official to remark, "We're going into space forever with people first circling this globe, and then we're going to Mars...."

Why would we ever want to go to Mars? As pictures beamed back from planetary probes and rovers since 1964 have shown, Mars is a desolate, lifeless planet with seemingly little to offer humans. It has a very thin atmosphere and no signs of existing life -- but Mars does hold some promise for the continuation of the human race. There are more than six billion people on Earth, and that number continues to grow unabated. This overcrowding, or the possibility of planetary disaster, will force us to eventually consider new homes in our solar system, and Mars may have more to offer us than the photos of its barren landscape now show.

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How Terraforming Mars Will Work | HowStuffWorks