Many Astronauts Never Recover all of their Bone Density after Returning to Earth

In his book, Endurance, astronaut Scott Kelly described the arduous task of readjusting to life on Earth after spending a year in space. As part of NASA’s Twins Study, Kelly lived and worked aboard the International Space Station (ISS) while his identical twin (astronaut Mark Kelly) remained on Earth. While the results of this study revealed how prolonged exposure to microgravity could lead to all manner of physiological changes, the long and painful recovery Kelly described in his book painted a much more personal and candid picture.

As it turns out, astronauts who spend extended periods in space may never fully recover. At least, that is the conclusion reached by an international team led by the University of Calgary after they assessed the bone strength of multiple astronauts before and after they went to space. They found that after twelve months of recovery, the astronaut’s bones had not regenerated completely. These findings could have significant implications for proposed future missions, many of which involve long-duration stays in space, on the Moon, and Mars.

The study was led by Leigh Gabel, an assistant professor of kinesiology at the University of Calgary, a member of the McCaig Institute for Bone and Joint Health and the Alberta Children’s Hospital Research Institute. She was joined by researchers from the Human Performance Laboratory (HPL) at UofC, the German Centre for Immunotherapy (DZI) at the University of Erlangen–Nuremberg, the University of Bonn, the University of Texas Medical Branch, and the Human Health and Performance Directorate (HH&P) at the NASA Johnson Space Center. The study that describes their research recently appeared in Science Reports, a scientific journal published by Nature.

According to ongoing research conducted aboard the ISS (including the NASA Twins Study), long-term exposure to microgravity can have a considerable impact on the health and well-being of astronauts. These include the loss of muscle mass and bone density, eyesight, cardiovascular health, organ function, psychological effects, and gene expression. Moreover, reacclimating to Earth’s gravity is painful as (literally) every cell in the body is forced to readapt to a constant 9.8 m/s² acceleration towards Earth’s center.

Ultimately, the effects depend upon the duration of an astronaut’s stay in orbit. As a result, space agencies are eager to determine how long astronauts can stay in space, the toll this will have on their health, and whether or not they can fully recover once they’re home. Steven K. Boyd, the Director of the McCaig Institute and a co-author of the paper, explained to Universe Today via email:

“The effects of space flight varied depending on the time astronauts spent in space in our study, which ranged from 4 to 7 months. Those who spent more time in space lost more bone and couldn’t recover it after 12 months back on Earth. The concern is that with anticipated missions to Mars, which could take years, bone loss could be substantial. We don’t know if at some point bone loss will taper and stabilize. The next phase of our study is to participate in NASA’s CIPHER project which monitors several aspects of human health (including bone health) in longer-term missions of up to a year. We hope that we won’t see much worse bone loss after a year compared to ~6 months, but we don’t know…”

Ordinarily, bone density is measured using Dual X-ray Absorptiometry (DXA), which uses very small doses of ionizing radiation to measure the amount of bone loss. For the sake of their study, the team examined the bone health of seventeen astronauts (strength, density, and microarchitecture) using High-Resolution peripheral Quantitative Computed Tomography (HR-pQCT). This allowed the team to produce three-dimensional bone density measurements with a resolution of 61 nanometers (µm).

The team conducted these measurements before the astronauts went to space, upon their return to Earth, and after 6 and 12 months of recovery to assess biomarkers of bone turnover and exercise. As Boyd explained using a structural metaphor, some of the loss is never fully recovered:

“If you used the metaphor of the Eiffel Tour, the metal rods that comprise the tour are akin to the bone structural elements that we can see at high resolution. During space flight, some of those ‘rods’ become disconnected as bone is resorbed. Back on Earth, some bone is recovered, but the new bone is laid onto the remaining structure. The body cannot magically reconnect those ‘rods’ and this results in a permanently different structure, even if the bone density is partially or even fully recovered. It’s like the Eiffel Tour had nearly the same amount of metal, but fewer rods and therefore not as strong.”

In short, their analysis showed that astronauts experienced a 0.9% to 2.1% loss in Bone Mineral Density (BMD). Similar disparities when noted when comparing the astronauts’ overall BMD, as well as their trabeculae and cortical BMD values. These types of bone consist of the spongy, porous microarchitecture that makes up the interior structure of bones and the dense outer bone layer that makes up nearly 80% of skeletal mass (respectively). To put it another way, the astronauts suffered a degree of bone loss commensurate with a decade or more of regular living on Earth.

This confirms previous research findings that showed how extended stays in orbit had an aging effect on the human body and its functions. Said Boyd, these findings have significant implications for the future of spaceflight and demand that further research is conducted:

“We need to understand the long-term effects of microgravity on bone health so that during longer missions we can be sure that the astronaut’s bones don’t become too weak. We also need to know whether the astronauts can recover once back on Earth so that they can go about doing their normal activities. The amount of bone loss deficit at 12 months after return to Earth doesn’t mean astronauts will start breaking their bones, but if they lose even more in long-duration flights, it could become a major concern.”

By 2033, NASA and China hope to start sending crewed missions to Mars every 26 months (when the two planets are closest to each other in their orbits). Using conventional technology, a one-way trip to Mars will take six to nine months, followed by a few months of science operations on the planet’s surface. This effectively means that astronauts headed for Mars will have to spend up to a year and a half in microgravity and several months in Martian gravity (38.5% of Earth’s gravity) before they make it home.

Closer to home, NASA, the ESA, China, Russia, and several commercial entities are gearing up exploration programs that will – to quote NASA’s on the goal of its Artemis Program – create a “sustained program of lunar exploration.” This will include the creation of surface habitats that can accommodate rotating crews and ongoing science operations – like NASA’s Artemis Base Camp, the ESA’s International Moon Village, and the Sino-Russian International Lunar Research Station (ILRS). This means that astronauts (and possibly civilians) will spend extended periods in lunar gravity (16.5% that of Earth’s).

When you consider how much planning is dedicated to the exploration and commercialization of space, there appears to be little doubt that the future of humanity is out there. But before we embark on this bold vision, we need to know all we can about the risks and opportunities. While the potential for scientific breakthroughs is immeasurable (and the commercial opportunities may also be immense), so are the hazards.

Further Reading: Nature