# Two astronauts were followed for four years after long missions, and one recovered lost leg-bone structure while the other retained hidden damage

> Astronaut bones continued rebuilding for years after two crew members returned from the International Space Station, yet their recoveries followed very different paths. One person regained all measured aspects of bone density and structure within four years. The other retained damage inside...

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Published: 2026-08-20T23:10:02+00:00
Categories: Explainer, Health

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Astronaut bones continued rebuilding for years after two crew members returned from the International Space Station, yet their recoveries followed very different paths. One person regained all measured aspects of bone density and structure within four years. The other retained damage inside the spongy framework of the lower leg, even as thicker outer bone helped restore strength.

The findings come from an [open-access study](https://www.nature.com/articles/s41526-025-00511-x) published in **npj Microgravity** on July 28, 2025. Researchers led by the **University of Calgary** followed two astronauts before flight, immediately after their return and at several points during a 48-month recovery period.

Both astronauts had spent between four and seven months aboard the International Space Station. Their scans reveal how **long-duration spaceflight** affects different parts of the skeleton and why an astronaut may regain overall bone strength while fine internal structures continue to carry signs of microgravity exposure.

## Four years of bone recovery

Most earlier astronaut bone studies followed recovery for about one year. The new case studies extended that window to four years, giving the team a rare view of slow changes that continue long after an astronaut resumes walking, exercising and living under Earth's gravity.

The researchers studied a man in his 50s and a woman in her 40s. They measured the lower tibia near the ankle, the radius near the wrist, the hips, femoral neck and lower spine. Scans were performed before flight and after landing, followed by measurements at six months, 12 months, 36 months and 48 months.

Two imaging methods provided different views of recovery. High-resolution peripheral quantitative computed tomography, called **HR-pQCT**, produced detailed three-dimensional images of bone near the ankle and wrist. Dual X-ray absorptiometry, or **DXA**, measured bone mineral density at larger central sites such as the hip and spine. The full paper is also available through the [PubMed Central archive](https://pmc.ncbi.nlm.nih.gov/articles/PMC12304210/).

## The tibia suffered the greatest losses

The lower tibia showed the clearest effect after the astronauts returned to Earth. Total bone mineral density at this weight-bearing site fell by 3.09 percent in one crew member and 4.59 percent in the other. The tibia normally carries body weight during standing and walking, so months without regular gravitational loading reduced the daily forces that help maintain it.

Bone also weakened more than the density figures alone suggested. Computer simulations based on the scans estimated that the force needed to cause failure fell by about 7.5 percent in both astronauts. Changes in **bone microarchitecture**, including the arrangement of tiny internal structures, contributed to that larger decline in mechanical strength.

The wrist responded differently because the radius carries less body weight during ordinary movement. One crew member showed no detectable loss there. The second had modest declines, including a 1.26 percent reduction in total density and a 3.94 percent reduction in the density of the inner spongy bone.

## Bone strength returned before density

Recovery followed separate clocks. Estimated **failure load**, a measure of how much force the bone could withstand, returned to preflight levels within six to 12 months in both crew members. Total bone mineral density at the lower tibia took as long as 36 months to recover.

Strength can improve before every lost mineral has returned because a bone's performance depends on where tissue is placed as well as how much tissue exists. Small changes along the outer shell can add support in areas that carry high loads. Continued mineral deposits can also harden newly formed tissue over time.

The study discussion states, "Bone strength returns to pre-flight levels within 12 months." Such recovery is encouraging for astronauts returning from missions lasting several months, although strength estimates cannot describe every microscopic change or predict every future injury.

## One astronaut retained hidden damage

By the four-year scan, crew member one had recovered across all measured density and microstructural values. Crew member two followed a slower and less complete path. The inner **trabecular bone**, which resembles a fine supporting lattice, remained thinner and more widely separated than it had been before flight.

Trabecular thickness in crew member two was still 8.65 percent below the preflight value after 48 months. Bone volume fraction remained 8.23 percent lower, while the spaces between the small supporting structures were 2.67 percent wider. Trabecular density also showed no clear improvement during the recorded recovery period.

Differences between two people can arise from many influences, including their starting bone structure, mission exposure, exercise response, diet, hormone conditions and the way each skeleton responds to unloading and reloading. With only two participants, the study cannot identify which personal factors produced the separate recovery paths.

## Cortical bone helped restore strength

The body appeared to compensate for some of the lingering internal loss in crew member two. Cortical bone, the dense outer shell of the tibia, became 17.16 percent thicker than its preflight measurement by the end of the four-year period. The added shell provided mechanical support while the inner lattice remained altered.

Researchers traced the change to activity along the inner surface of the cortical layer. Bone was removed and added around the boundary between compact outer tissue and spongy inner tissue. Such **cortical thickening** may help explain how whole-bone strength returned even though several trabecular measurements stayed below their original levels.

Bone continuously renews itself through **bone remodeling**. Specialized cells remove older or damaged material, while other cells build replacement tissue. Earth's gravity and normal movement once again placed force on the astronauts' legs after landing, giving the skeleton signals that encouraged rebuilding in mechanically useful regions.

## The first six months were critical

Voxel-by-voxel scan comparisons allowed the team to estimate how quickly tissue was being removed and formed. Bone resorption was high across the period from preflight to landing, averaging 8.54 percent of total bone volume per year. Formation then rose sharply during the **first six months** back on Earth, reaching an average annualized rate of 5.45 percent.

Formation and resorption settled to much lower levels between six and 12 months. The rates remained relatively stable during the later follow-up periods. Continued increases in density after the first half-year likely came partly from mineralization, a slow process in which newly formed bone gains mineral and becomes harder.

The timing suggests that early rehabilitation deserves close attention. Exercise and nutrition plans during the first months after landing may influence the tissue that replaces bone lost in orbit. The study did not test a new treatment, so larger controlled research will be needed to determine which recovery programs produce the best long-term results.

## Why standard scans may miss deficits

DXA scans suggested that both crew members recovered fully at the lumbar spine. Crew member one also regained most of the bone density lost at the hip and femoral neck. These measurements offered a useful broad picture of mineral content across major skeletal regions.

HR-pQCT revealed finer deficits that DXA could not separate. A DXA result combines information across an area, while high-resolution scans distinguish the dense outer shell from the spongy interior. An astronaut can therefore reach a reassuring density value even while the internal framework remains changed.

The difference supports the use of detailed imaging during long follow-up periods, especially after missions that expose crews to microgravity for many months. HR-pQCT currently focuses on peripheral sites such as the wrist and ankle, yet those locations can reveal changes in structure that broad density measurements may leave unresolved.

## Lessons for longer space missions

Future crews traveling toward the Moon or Mars could experience long periods with little gravitational loading, followed by work under partial gravity and another major adjustment after returning to Earth. Recovery may last much longer than the flight itself and individual astronauts may require different schedules for monitoring and rehabilitation.

The two cases support **personalized countermeasures** based on skeletal site and individual response. Exercise equipment in orbit must load the legs strongly enough to preserve weight-bearing bone. After landing, detailed scans could help medical teams decide how quickly each astronaut should return to demanding physical tasks.

Small case studies require careful interpretation. Two people cannot represent the full astronaut population and natural aging may account for part of the change recorded across four years. The extended observations still provide a valuable timeline that larger astronaut groups can test.

As the study notes, "Complete microstructural recovery may take up to 48 months or possibly remain permanently altered after extended missions." Long-term monitoring can show whether lingering deficits stabilize, continue to heal, or raise health concerns years after a crew member comes home. Broader mission context is available from [NASA's Human Research Program](https://www.nasa.gov/hrp/), [NASA's International Space Station overview](https://www.nasa.gov/international-space-station/) and [ESA's human and robotic exploration program](https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration).