Human Factor — Physical
The Body Under Pressure
Without gravity, your body starts dismantling itself. Bones thin, muscles waste away, and the clock starts on day one.
The Degradation Rates
Bone Density Loss
1-2% / month
Mostly in bones that normally carry your weight on Earth: hips, spine, and legs. On a 6-month trip, crew can lose 6-12% of bone density. That's decades of aging on Earth compressed into months. Recovery takes years and may never be complete.
Muscle Atrophy
Up to ~10–20% loss
The muscles that hold you upright can lose ~10% in just 1-2 weeks. Without countermeasures, that reaches 15-20% in the early weeks of flight. Even with exercise, overall muscle volume drops 5-10% in the first month. Some people lose more. And the strength loss keeps going for the entire mission.
Exercise Requirement (ISS)
2 hrs/day
Every ISS crew member exercises 2 hours per day on resistance machines and a treadmill with a harness. This slows the loss. It doesn't stop it. The full daily cost of fighting physical decline is covered in The Anti-Decay System.
Vision Impairment
~60% affected
Called SANS (Spaceflight Associated Neuro-ocular Syndrome). Pressure shifts inside the skull physically flatten the eyeball. Over 60% of long-duration ISS crew report some degree of vision change, though that's based on fewer than 50 people studied so far, so the true rate may shift. Some of the changes are permanent.
Additional Physical Effects
The heart muscle weakens. Blood volume redistributes. When astronauts return to gravity, many can't stand up without fainting.
Your immune cells get weaker. Viruses that were dormant in your body can reactivate. In a confined space with 100+ people sharing the same air, that becomes a risk for everyone.
Outside Earth's magnetic shield, cosmic radiation drives up cancer risk. A single Mars mission could approach or exceed NASA's current 600 mSv career limit, with most of that dose hitting during the transit.
Without gravity pulling fluid down, it pools in your head. You get a puffy face, constant congestion, and increased pressure inside the skull. This is part of what causes the vision damage.
The Exercise Infrastructure Problem
6 crew (ISS)
12 hrs/day
Total exercise time. 2-3 stations. Scheduling is tight but manageable.
25 crew
50 hrs/day
Requires 5-6 exercise stations running continuously. Maintenance, space, and power all scale.
100 crew
200 hrs/day
A dedicated fitness facility. 20+ stations. Constant maintenance. Significant volume and mass allocation.
Every day in microgravity
is a day of measurable loss.
Exercise slows the decline but doesn't stop it. Artificial gravity would solve most of this, but nobody has built one yet. Until then, every crew arrives weaker than they left. Some of that weakness is permanent. And while their bodies are degrading, their psychology is too.