It is one of the biggest hurdles NASA must overcome before astronauts embark on long duration space travel to Mars or building permanent settlements on the moon: how to minimize the heavy toll exacted on even the most physically fit human bodies in zero gravity. So to help ensure astronauts don’t arrive at their destination too weak to explore, the space agency has teamed up with Pluristem, an Israeli biotech company, to investigate whether injecting them with cells derived from a mother’s nutrient-rich placenta can increase muscle volume — a process now in trials on Earth for the elderly.