Japan’s experiment module on ISS, called Kibo. (Credit: NASA)

Astronauts living on the International Space Station spend hours working out every day just to avoid losing serious muscle mass and bone density in microgravity. But will such precautions be needed to live on worlds that are simply lower in gravity than Earth, like the moon and Mars? And what effect would such gravity have on growing children? These questions are almost entirely unanswered by science, but they’re vital for humani