Spending extended periods in near-zero gravity (microgravity) during the trip to Mars, or in Mars’ reduced gravity (38% of Earth’s) once there, has significant effects on the human body. Over a year—or a 3-year round trip with time on Mars—these effects can pile up, challenging health and requiring countermeasures.

Effects of Microgravity (Near-Zero Gravity) During the Flight In microgravity, like the 6-9 month journey to Mars, the body adaptation to weightlessness leads to:

1. Muscle Atrophy Without resistance from gravity, muscles (especially in the legs and back) weaken and shrink. Studies from the International Space Station (ISS) show muscle mass can drop by 20% in just a few months without exercise. Over a year, losses could exceed 30-40% without intense countermeasures ( resistance training).

2. Bone Loss

   • Bones lose density because they’re not bearing weight. Astronauts lose about 1-2% of bone mass per month in microgravity, particularly in the hips and spine.
   • After a year, that’s 12-24% loss; over 3 years, it could approach 36% in some areas, increasing fracture risk even after returning to gravity.

3. Fluid Shifts Fluids like blood shift upward, causing a puffy face and reduced leg volume ("chicken legs"). This strains the cardiovascular system, reducing blood volume by 10-20% over time. Long-term, this can weaken the heart and lead to vision issues (e.g., Spaceflight-Associated Neuroocular Syndrome, or SANS), where pressure on the optic nerve blurs eyesight. Up to 60% of ISS astronauts report this after 6 months.

4. Cardiovascular Deconditioning

   • The heart doesn’t work as hard in microgravity, shrinking slightly and losing efficiency. After a year, returning to Earth’s gravity (or even Mars’) could cause dizziness or fainting due to orthostatic intolerance.

5. Immune System and Radiation Microgravity suppresses immune function, making infections more likely. Combined with higher cosmic radiation exposure (about 0.6 mSv/day outside Earth’s magnetic field), this could weaken defenses over a year or more. Over 3 years, radiation doses could reach 1-2 Sv, raising cancer risk significantly (NASA limits lifetime exposure to 1 Sv).

6. Mental Health Isolation, confinement, and sensory monotony in a spacecraft for 6-9 months (or longer) can lead to stress, depression, or cognitive decline. A year or more amplifies this—think cabin fever in space.

Effects of Mars Gravity (38% of Earth’s) Over Time On Mars, the partial gravity helps mitigate some microgravity issues but introduces its own challenges:

1. Muscle and Bone Adaptation Mars’ gravity slows muscle and bone loss compared to microgravity, but it’s still insufficient to maintain Earth-level strength. Studies suggest bone loss might stabilize at 0.5-1% per month—half the microgravity rate—but over a year, that’s 6-12%, and over 3 years, 18-36%.

   • Muscles would weaken less than in space but still degrade, maybe 10-20% over a year without rigorous exercise.

2. *Cardiovascular Strain

   • The heart and blood vessels adapt to 0.38g, but it’s unclear if this fully prevents deconditioning. After a year, transitioning back to Earth’s gravity could still cause issues, though less severe than from microgravity.

3. Joint and Posture Problems

   • Lower gravity reduces strain on joints, but over time, weaker muscles and bones could lead to poor posture or injury. Walking feels easier, but the body might not fully adjust, risking strain after a year or more.

4. Radiation Exposure

Mars lacks a strong magnetic field or thick atmosphere, so surface radiation is about 0.2-0.3 mSv/day (vs. 0.6 mSv/day in transit). Over a year, that’s 73-110 mSv; over 3 years, 219-329 mSv. This boosts cancer risk, though less than in deep space.

5. Fatigue

Combined Impact Over 1 or 3 Years 1 Year (e.g., 6 months transit, 6 months on Mars):

Significant muscle and bone loss (10-20% and 6-12%, respectively), mild cardiovascular decline, vision risks, and moderate radiation exposure (around 200-300 mSv total). Mental strain could be noticeable but manageable with support.

3 Years (e.g., 9 months out, 18 months on Mars, 9 months back):

Losses could double or triple—muscle atrophy up to 30-40%, bone loss nearing 18-36%, serious cardiovascular risks, and radiation doses (600-900 mSv) approaching or exceeding safe limits. Mental health could degrade significantly without robust countermeasures.

Spacecraft and habitats need thick walls or water layers; Mars bases might use underground sites.

In short, a year is tough but survivable with effort; 3 years pushes human limits, risking long-term damage

Zero Gravity During the Flight

Once a human spacecraft leaves Earth’s atmosphere and stops accelerating (after the initial launch), it enters a state of free fall toward Mars, following its orbital trajectory. This creates a microgravity environment inside the spacecraft, where astronauts float because they’re falling at the same rate as the ship. The term "zero gravity" is a bit of a misnomer—it’s really the absence of a noticeable gravitational force acting on the body relative to the surroundings.

During the 6-9 month journey, the spacecraft isn’t generating artificial gravity (with current human tech). So, astronauts experience this weightless sensation the whole way.

The Sun’s gravity and Mars’ gravity do act on the spacecraft, but their effects are so gradual and weak during transit that they don’t create a perceptible "down" for the crew.

Gravity on Mars

Once you land on Mars, gravity is noticeably different from Earth’s. Mars has a surface gravity of about 3.72 m/s² which is roughly 38% of Earth’s gravity (Earth’s is 9.8 m/s²).

This reduced gravity has big implications for long-term stays—muscle and bone loss are concerns for astronauts, though less severe than in microgravity. It’s a weird middle ground: not weightless, but not quite home either.

In the end, there is no oxygen, no breathable air, atmosphere, no stable sustainable food supply and heating with violent storms, radiation, rapid temperature changes will not your life on mars or anywhere sustainable. Clip from guardian of the galaxy vol 3