Mars mission bone-loss model shows risk, but relies on outdated data and untested assumptions

A mathematical model published in 2020 predicted that astronauts on three-year Mars missions could lose between 32 and 37 percent of their femoral-neck bone density, with every modeled case dropping into the osteopenic range and roughly one-third crossing into outright osteoporosis. The percentages sound alarming. They are also, in a crucial sense, not what they appear to be.

No human being has traveled to Mars. No astronaut has returned from Mars with weakened bones. The striking forecast came not from clinical observation of an interplanetary crew but from a mathematical extrapolation—researchers took bone-density measurements from 69 astronauts who had flown between 132 and 228 days, fitted a curve to that short-duration data, and extended it across mission profiles lasting 1,000 to 1,200 days. The gap between what was measured and what was projected is enormous. Almost the entire curve that runs through the Mars windows is extrapolation, not observation.

The distinction matters for how we read the warning. In microgravity, the mechanical demands on bone largely vanish. The balance between bone formation and resorption shifts. The femoral neck—the narrow region where the thighbone connects to the hip—normally bears load from standing and movement. Without that load, density declines. The 2020 study, published in PLOS One by researchers from NASA Ames and Stanford, used a nonlinear model rather than a simple linear projection. A straight line would eventually predict bone density below zero, a physical impossibility. The nonlinear approach bent toward a plateau, set at a 69 percent loss from preflight density, drawn from terrestrial evidence of prolonged bed rest and spinal-cord injury.

The researchers tested two broad mission architectures. An opposition-class profile lasted 400 to 600 days—shorter but requiring more demanding velocity changes and substantially more fuel. A conjunction-class profile lasted 1,000 to 1,200 days, roughly 2.7 to 3.3 years, using more favorable Earth-Mars geometry and generally considered easier with conventional propulsion. For the shorter option, the model projected a 15.6 to 22 percent reduction in femoral-neck density. For the longer option, the loss reached 32.4 to 36.8 percent. The authors then converted those estimates into T-scores—a measure of how far a person's bone density sits above or below a young-adult reference mean—using average preflight densities from national health surveys rather than the individual scans of actual Mars candidates.

This methodological detail reshapes what the percentages mean. The "62 percent" and "100 percent" results in headlines describe the share of modeled demographic cases crossing diagnostic thresholds under the study's assumptions. They are not measured incidence rates and should not be read as the probability that a randomly selected future astronaut will develop osteoporosis. Osteopenia and osteoporosis are not two separate diagnoses a person holds at once; osteoporosis is the lower-density category. When the abstract predicted that 100 percent of conjunction-class cases would develop osteopenia and 33 percent would be at risk of osteoporosis, the clearest reading is that all reached at least the osteopenic range, while roughly one-third crossed the more severe threshold. Even that does not translate directly into a fracture forecast. Bone mineral density contributes to bone strength, but geometry, microarchitecture, accumulated microscopic damage, and the load applied at a particular moment also matter.

The most significant limitation of the 2020 model is what it did not include. The astronaut data came from flights before the Advanced Resistive Exercise Device, or ARED, was introduced to the space station in 2009. The study did not model exercise, diet, medication or other countermeasures. It also omitted possible damage from prolonged deep-space radiation. ARED can load the body through movements resembling squats, deadlifts, and heel raises. Combined with adequate energy intake and vitamin D, it changed the bone response seen in later crews. A NASA technical summary reported that astronauts with access to ARED could return from station missions with bone mineral density close to preflight values, although bone turnover remained altered. NASA's current human-performance requirements call for countermeasures to keep the femoral neck at or above 90 percent of pre-mission density. The agency says most crewmembers on six-month missions return within those deficit limits. That is better evidence than the older input data, but it does not erase the Mars problem. A six-month station mission is not a three-year expedition. The large ARED installation benefits from the space, structure, maintenance and logistics of the ISS. NASA notes that it cannot simply fit inside an exploration spacecraft, so smaller devices must reproduce enough loading with fewer resources.

No countermeasure package has been validated in humans for 1,000 to 1,200 days away from Earth. A round-trip Mars mission is not usually three uninterrupted years of microgravity either. It contains an outbound cruise, time on the Martian surface where gravity is 38 percent of Earth's, and a return cruise. The 2020 model used total mission duration as its time variable and did not give Mars's partial gravity a separate protective term. That is understandable because there are no human data from months at 0.38 g, but it means the forecast cannot tell us whether the surface stay would slow loss, produce a new equilibrium, or be too weak to restore meaningful loading. The assumed 69 percent plateau came from Earth analogues rather than astronauts observed until their loss leveled off. Spinal-cord injury and bed rest reproduce skeletal unloading in useful ways, but they are not Mars missions. Spaceflight changes fluids, muscles, radiation exposure, daily movement and many other conditions at the same time.

The headline's percentages are real outputs of a peer-reviewed model. They are not direct observations, guarantees, or current NASA forecasts for a named mission architecture. Since the 2020 publication, current exercise and nutritional practice has supplied reasons for less pessimism. It has not supplied a three-year human dataset. Partial gravity, compact exercise hardware, deep-space radiation, autonomous medical care, and the durability of bone quality remain open parts of the problem. That makes the projection neither destiny nor a curiosity. It is a quantified description of what mission designers must prevent. A crewed Mars vehicle will carry engines, shielding and life support. It will also need to carry enough mechanical loading, in some form, to persuade the human skeleton that it still lives in a world with weight.