Frey argues that there are four distinct sources of risk:
First, the reduced gravity environment. From almost zero to about 1/3 earth's gravity, which will cause fluid shifts in the body, loss of normal stress on the bones and muscles, and changes in stimuli to the nervous system.
Second, the environment inside the vehicle or habitat poses threats from floating particles, which might be inspired. Toxic wastes, poor illumination, loud noise and poor thermal control.
Third, the environment outside the vehicle or habitat poses threats, including radiation and meteorites or other debris.
And fourth, the psychological and psychosocial stresses will be extreme.