Astronauts Face Debilitating Bone Density Loss After Just 6 Months in Space

In light of NASA's knowledge that long-term exposure to space can cause considerable bone density loss, is it inhumane to send astronauts into space beyond six months?
bone density

A new study of 17 astronauts shows that even after a year back on Earth, their bones are not as strong as they were before they went into space.

Living in a gravity-free environment weakens the bones by decreasing bone density. Although this density is usually restored within a year of returning from a four-month or more extended space mission, this is not always the case.

When astronauts are in space for six months or longer, their bones can lose as much bone density as they would in 20 years on Earth. Researchers reported that a year on Earth helps rebuild about half of the lost bone strength.


According to the study’s co-author Dr. Steven Boyd, some astronauts have had trouble walking due to weakness and lack of balance after returning from spaceflight. In contrast, others could happily ride their bikes. Boyd says, “When astronauts return to Earth, they react in many different ways.”

948px Soyuz TMA 08M Chris Cassidy shortly after landing
Astronaut struggles to walk after returning from ISS by NASA

Dr. Leigh Gabel, an exercise scientist at the University of Calgary, the study’s lead author, says, “People lose bone when they get older, get hurt, or can’t move their bodies … It’s very rare to know what happens to astronauts and how they get better.”

Before the 17 astronauts went to space (14 men and 3 women with an average age of 47), researchers scanned their shin and forearm bones. The astronauts then spent between 4 and 7 months in space. 


When the astronauts returned, the researchers repeated the same scans as soon as they returned, then again six and 12 months later. With these scans, the researchers could tell about the bone’s mineral density, how likely the bones might break, and how thick the tissue was.

The research team used high-resolution peripheral quantitative computed tomography, or HR-pQCT, which can measure 3-D bone microarchitecture on scales of 61 microns, which is smaller than the width of a human hair, to take pictures of the bone structure of the tibia in the lower leg and the radius in the lower arm. 

The results of this study can reveal what’s going on in the body in such a short amount of time. Usually, researchers would have to follow someone for decades to see the same amount of bone loss on Earth.


Gable advised, “We found that most astronauts’ weight-bearing bones had only partially healed a year after being in space,” Suggesting that the permanent bone loss caused by space travel is about the same as the bone loss caused by aging on Earth for a decade.

shutterstock 752558830
Bone density loss by Shutterstock

The missions of all the astronauts lasted between four and seven months. The bone density of the eight astronauts in space for more than six months did not recover either.

Try and stay in space for less than six months

The bone strength of astronauts who spent less than six months in space can be restored to its pre-spacetime state. In contrast, those who had spent more time in space experienced bone density loss in their tibias, which was the same as aging ten years. Their radii, or lower arm bones, were nearly unbreakable. Gabel believes this is because these bones do not bear weight.


Researchers want to look at astronauts’ bone density who have been in space for longer

Gabel, Boyd, and their colleagues want to find out how bone density changes after being in space for more than seven months. They are part of a NASA project to study what happens to more than a dozen body systems after a year in space. Boyd says, “We really hope that people reach a plateau and stop losing bone over time.”

Gabel says that bones are “a living organ.” “They are alive and moving, and they are always changing.” But bones get weaker without gravity due to loss in bone density.

Exercising in space is the solution that could help prevent bone loss

Boyd states lifting more weights in space could help stop bone loss. Your bone’s strength comes from tissues called trabeculae, like a bunch of struts and beams held together. When astronauts go into space, they lose trabeculae. Boyd advised that you can’t replace trabeculae once they are gone, but you can strengthen the ones still there. The researchers found that the bone that was left got thicker when it went back to Earth’s gravity.

STS 44 crewmembers exercise using treadmill rowing device on OV 104s middeck
STS-44 crewmembers exercise to prevent bone density loss by NASA

Some exercises that astronauts did while in space seemed to slow down this loss of bone density and speed up recovery a little. Astronauts who did more resistance training, like deadlifting, while in space were better at getting their bone density back to normal.

The results also show that different astronauts react differently to the same amount of time in space, with some recovering much better than others.

“Just as the body has to get used to space travel at the beginning of a mission, it also has to get used to the gravity of Earth at the end,” says Dr. Robert Thirsk, a former UCalgary chancellor and astronaut. He explained that when he got back, he had trouble right away with feeling tired, dizzy, and out of balance.


A big challenge to future space missions

Laurence Vico, a physiologist at the University of Saint-Étienne in France who was not involved in the study, is worried about the toll on the human body that a future mission to Mars might have as it would take at least two years. 

With more prolonged spaceflight, astronauts can expect to experience more bone loss and probably more trouble getting back to normal. Vico advises that space agencies should research ways to keep bones healthy, like nutrition, to stop bones from breaking down and make more bones. There will need to be a mix of ways to help bones regain strength.

What will this research mean for Musk’s plan to send millions of space colonists to Mars? Many experts already think Musk’s plan is insane.


However, researchers are always coming up with new ideas to reduce the risks to astronauts in space, such as studies on methods to reduce astronaut exposure to radiation and further investigations into laser propulsion technology that will massively reduce the travel time in space and could get astronauts to Mars in 45 days.

Feature image credit: ISS-66 Astronaut Mark Vande by NASA

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