Home base on the moon. Boot prints on Mars. Visits to asteroids. With the world’s spacefaring nations looking beyond the space station to envision human missions to increasingly distant destinations, scientists have already begun to tackle the many challenges of sending humans farther and farther from our home planet. Missions to the space station have made substantial contributions to our knowledge of how the human body adapts to microgravity for 3, 6 or even 12 months; however, taking steps deeper into our solar system will require much longer expeditions. A human mission to Mars, for instance, will likely mean a 6-month journey each way, coupled with a stay of about 18 months on the surface of the planet.