Spaceflight osteopenia

Astronauts lose an average of more than 1% bone mass per month spent in space.

[1] There is concern that during long-duration flights, excessive bone loss and the associated increase in serum calcium ion levels will interfere with execution of mission tasks and result in irreversible skeletal damage.

[3] William E. Thornton, an astronaut and physician, was one of the biggest proponents of exercise as a way of preventing bone loss.

On Mars, where gravity is about one-third that of earth, the gravitational forces acting on astronauts' bodies would be much lower, causing bones to decrease in mass and density.

Data from immobilization studies and from patients with spinal cord injuries support this.