The 0.67g Problem: Why Mars's Gravity May Not Be Enough to Save Human Muscles

A landmark study published in Science Advances has identified a critical gravity threshold for preserving muscle function in space—and Mars falls well below it. As NASA allocates more than $1 billion toward human Mars exploration in fiscal year 2026, the findings raise urgent questions about whether long-duration settlement on the Red Planet is biologically viable without radical new countermeasures.

The Experiment That Changed the Conversation

In March 2023, twenty-four mice were launched aboard a SpaceX Falcon 9 rocket to the International Space Station. Over the following four weeks, they were placed inside Japan Aerospace Exploration Agency's Multiple Artificial-gravity Research System (MARS)—a centrifuge capable of simulating different gravitational loads in orbit. The mice were divided into four groups, each exposed to a different gravity condition: microgravity (near zero-g), 0.33g (roughly lunar gravity), 0.67g, and a full 1g control [1].

When the surviving 23 mice returned to Earth in April 2023, what researchers found in their soleus muscles—a leg muscle particularly sensitive to gravitational loading—redefined the scientific understanding of how gravity affects biology.

At 0.67g, the mice demonstrated what co-author Mary Bouxsein of Harvard Medical School described as "full protection of muscle function," with grip strength comparable to the 1g control group. But at 0.33g, while muscle size remained roughly normal, grip strength decreased significantly. And in microgravity, deterioration was dramatic and swift [1] [2].

The takeaway was stark: 0.67g appears to be the minimum gravitational threshold needed to preserve muscle function. Mars, at just 0.38g, falls well below it [1].

Why Mars Gravity Is Not Enough

Mars's gravitational pull is approximately 38% that of Earth's—a figure that sounds substantial compared to the near-weightlessness of orbital spaceflight. But the new research suggests this partial gravity exists in a dangerous middle zone: strong enough to give a false sense of security, yet too weak to prevent the slow degradation of the musculoskeletal system.

"We know nothing about how much gravity exposure is necessary to halt deconditioning," Mark Shelhamer of Johns Hopkins University told Gizmodo, describing the state of the field before this study [1]. The finding of a discrete threshold was itself a breakthrough. Prior research by Lori Ploutz-Snyder at the University of Michigan, which exposed humans to short bursts of reduced gravity on parabolic flights, had suggested a similar range of 0.5g to 0.75g—broadly consistent with the mouse data [1].

The implications are profound. An astronaut arriving on Mars after a six-to-nine-month transit in microgravity would already be significantly deconditioned. The expectation had been that Mars's own gravity, combined with surface activity, would help rebuild strength. This study suggests otherwise: the reduced mechanical forces from walking and running in Martian gravity are "probably not sufficient to maintain terrestrial mineral density and muscle mass in the long-term" [3] [4].

The Scale of the Problem

The human body's response to microgravity is well-documented but still alarming. Astronauts aboard the ISS lose bone mass at a rate of 1–1.5% per month [5]. Skeletal muscle can atrophy by up to 20% in mass and 30% in strength within a single month of spaceflight [6]. Without the intensive daily exercise regimen mandated on the space station, a six-month stay in microgravity would leave an astronaut's musculature resembling that of an 80-year-old [6].

Source: NASA / npj Microgravity / Aurora Scientific

Data as of Mar 14, 2026CSV

These changes occur at the molecular level. Recent proteomic analysis of astronaut muscle tissue has revealed compartment-resolved changes in the mitochondrial proteome, remodeling of the extracellular matrix, and impaired antioxidant responses during spaceflight [7]. Studies of muscle precursor cells found that key microRNAs—miR-1, miR-133a, miR-133b, and miR-206—were downregulated under microgravity conditions, suggesting a fundamental impairment of muscle cell functionality [8].

A round-trip Mars mission, using current propulsion technology, would involve approximately 18 months in transit alone, plus a surface stay of up to a year—a total mission duration of roughly 1,000 to 1,200 days. No human has ever spent this long in conditions of reduced gravity. The current record for continuous spaceflight is Valeri Polyakov's 437 days aboard the Mir space station. A Mars mission would nearly triple that exposure [5].

The Two-Hour Daily Battle

Astronauts aboard the ISS currently spend an average of two hours per day exercising, using three primary pieces of equipment: the Advanced Resistive Exercise Device (ARED), which simulates free weights with loads up to 272 kg via vacuum cylinders; the T2 treadmill, a modified Woodway Path unit that supports speeds up to 19.3 km/h; and the Cycle Ergometer with Vibration Isolation and Stabilization System (CEVIS), a computer-controlled cycling machine upgraded in 2023 [9].

These countermeasures help, but they do not fully solve the problem. Research published in npj Microgravity found that "the muscle atrophy experienced by crew members on board the International Space Station is not efficiently prevented and counteracted by the mandatory execution of resistance physical exercises several hours a day" [7]. Astronauts who exercised more intensively showed better preservation of muscle mass and force at landing—but even the most diligent exercisers showed measurable deterioration [7].

On Mars, the challenge compounds. The exercise equipment currently used on the ISS weighs hundreds of kilograms and requires significant power. A Mars habitat would need to support similar or more advanced equipment while operating under severe mass and energy constraints. And unlike the ISS, where astronauts can return to Earth's full gravity within hours of departure, Mars settlers would have no such quick escape from deconditioning [5].

The Race for Countermeasures

The scientific community is pursuing multiple strategies to bridge the gap between what gravity provides and what human biology demands.

Pharmaceutical approaches are showing promise. Myostatin inhibitors—drugs that block a protein responsible for limiting muscle growth—have demonstrated effectiveness in preventing both bone and muscle loss in animal models during spaceflight [3]. NASA's Microgravity Associated Bone Loss-B (MABL-B) investigation, which launched aboard SpaceX CRS-33, is examining whether blocking the inflammatory protein IL-6 can prevent the bone degradation pathway that microgravity appears to activate [3].

Bioengineered solutions are emerging from university laboratories. Researchers at the University of Florida, led by Siobhan Malany, are studying three-dimensional bioengineered muscle tissues aboard the ISS, examining how they respond to electrical stimulation in microgravity to better understand the mechanisms of space-related atrophy [10]. At the University of Bristol, scientists have developed a soft, wearable exosuit powered by inflatable "bubble muscles" designed to provide artificial resistance during daily activities [11].

Digital twin technology represents a more personalized approach. Scientists at West Virginia University are building AI-powered virtual models of individual astronauts that could predict physiological responses in real-time, enabling customized countermeasure prescriptions that adapt as a crew member's condition changes during a multi-year mission [5].

Artificial gravity remains the most conceptually elegant solution but also the most technically challenging. The Orbital Assembly Corporation's planned rotating space station, designed with a 200-foot diameter ring generating approximately 0.4g, would serve as a testbed for studying the effects of sustained partial gravity on human biology [12]. The Austria-based Artificial Gravity Orbital Station (AGOS) concept envisions a spinning successor to the ISS specifically designed to mitigate microgravity health effects [12]. But engineering a rotating section into an interplanetary spacecraft introduces enormous complexity: maintaining stability, managing Coriolis forces, and dealing with the mass and energy costs of spinning a habitat section large enough to be physiologically effective [12].

Source: Science Advances / Gizmodo / Scientific American

Data as of Mar 14, 2026CSV

The Transit Problem

Even if Mars's surface gravity proves manageable with aggressive countermeasures, the transit itself remains a critical bottleneck. A conventional Hohmann transfer orbit to Mars takes approximately six to nine months each way—up to 18 months of near-total weightlessness [13].

A 2025 study published in Scientific Reports examined the feasibility of reducing Starship transit time to approximately 90 days using optimized trajectories [13]. This shorter journey would substantially reduce both radiation exposure and microgravity-related deconditioning. But even three months of weightlessness produces measurable muscle and bone loss, and the accelerated trajectory requires significantly more fuel—compounding an already staggering propellant logistics challenge [13].

Radiation adds another layer of damage. Mars-bound astronauts would be exposed to an estimated 870 to 1,200 millisieverts of radiation over the course of a mission—compared to 50 to 100 millisieverts for a six-month ISS stay [14]. Galactic cosmic rays, which are nearly impossible to shield against with current technology, "slice through cells and fracture DNA in ways that biology on Earth was never built to repair," as one analysis put it [5]. Research has shown that space radiation compounds the catabolic effects of mechanical unloading on bone, creating a synergistic damage pathway that exercise alone cannot fully counteract [14].

A Question of Biology vs. Ambition

NASA's FY2026 budget allocates more than $7 billion for lunar exploration and introduces $1 billion in new investments for Mars-focused programs, including $350 million to accelerate Mars technology development and $200 million for commercial Mars payload deliveries [15]. SpaceX has outlined plans to land the first uncrewed Starships on Mars as early as 2028, with human missions potentially following by the early 2030s [13].

But the 0.67g threshold finding introduces a fundamental biological constraint that no amount of funding can simply engineer away. As Bouxsein of Harvard noted, there is a potential silver lining: astronauts on Mars would need less strength to perform tasks in lower gravity, meaning the functional consequences of some muscle loss might be less severe than on Earth [2]. The real danger may come when astronauts attempt to return home—arriving back on Earth with muscles adapted to 0.38g after years of deconditioning.

Se-Jin Lee of the University of Connecticut, who was not involved in the study, highlighted the critical open question: whether the 0.67g threshold identified in mice translates directly to humans [2]. Mouse muscles respond to mechanical loading in broadly similar ways to human muscles, but the specific threshold could be different for a species that evolved to walk upright under full Earth gravity.

What the science makes clear is that sending humans to Mars is not just an engineering challenge—it is a biological one. The human body evolved over millions of years for precisely 1g. Every fraction of gravity subtracted from that baseline triggers a cascade of deconditioning that current countermeasures can slow but not stop. The 0.67g threshold gives researchers a concrete target for the first time. Whether the Mars exploration community can meet it—through artificial gravity, pharmaceuticals, bioengineering, or some combination yet to be invented—will determine whether humanity's ambitions for the Red Planet remain aspirational or become achievable.