Ask the rocket man: it’s lonely out in space. Turns out it’s more than a little unhealthy too. It’s not just the difficulty of finding a pharmacy on the International Space Station or a doctor’s office on the moon. Astronauts have to contend with the absence of gravity, the threat of gamma rays, the lack of physical activity, and the mental strain of months away from solid ground. All of it combines to make being a deep space explorer one of the most hazardous jobs on Earth. Or off it.