Decades of research aboard the ISS and its predecessors (Mir, Skylab) have produced a detailed understanding of how the human body responds to microgravity. The primary concerns for Mars-class missions are bone mineral density loss, skeletal muscle atrophy, cardiovascular deconditioning, neuro-vestibular dysfunction, fluid redistribution effects (including Spaceflight-Associated Neuro-ocular Syndrome, or SANS), and immune system dysregulation.

Microgravity deconditioning and countermeasures:

ISS crew members on 6-month rotations experience measurable bone loss (approximately 1-2% per month in load-bearing bones without countermeasures), muscle atrophy (particularly in postural and anti-gravity muscles), and cardiovascular deconditioning. Current ISS countermeasure protocols, including approximately 2.5 hours of daily exercise using the Advanced Resistive Exercise Device (ARED) and cycle ergometer, have proven effective at significantly reducing but not eliminating these effects. Pharmaceutical interventions such as bisphosphonates have shown promise in reducing bone resorption rates.

A Mars transit phase of 6-9 months in microgravity is comparable to a standard ISS rotation, and existing countermeasure protocols can be adapted. The concern is that upon Mars arrival, crew must be operationally capable immediately, unlike ISS returnees who undergo a terrestrial reconditioning period. Mars surface gravity (0.38g) may assist recovery, but this is unverified.

The partial gravity knowledge gap:

One of the most significant unknowns in Mars mission planning is the human physiological response to 0.38g over periods of months to years. All existing long-duration data comes from either 1g (Earth) or near-0g (ISS). There is no empirical data on whether 0.38g is sufficient to maintain bone density, cardiovascular tone, and neuromuscular function, or whether it merely slows deconditioning without preventing it. Short-duration parabolic flight data and centrifuge studies provide only fragmentary insight.

Artificial gravity:

Rotating spacecraft concepts (either spinning the entire transit vehicle around its longitudinal axis, using a tethered counterweight configuration, or incorporating a short-radius onboard centrifuge) could provide partial or full gravity during transit, potentially eliminating microgravity deconditioning entirely. Engineering challenges include structural dynamics, rotational speed and Coriolis effects on crew vestibular systems, and mechanical complexity. These concepts are technically feasible but add mass and design complexity to the transit vehicle.

Research priorities:

A variable-gravity research facility, either in low Earth orbit or on the lunar surface (0.16g), would provide critical data on the dose-response curve of gravity and human physiology. Understanding whether 0.38g is "enough" gravity to maintain health is arguably one of the highest-priority open questions for Mars mission planning. Mars analog research, including extended-duration missions at facilities such as the Mars Desert Research Station (MDRS), provides valuable data on the human factors, crew dynamics, and operational demands of surface exploration that inform physiological countermeasure requirements.