Why Astronauts Lose Bone in Space, and How They Fight to Keep It

On Earth, every step you take is a small workout for your skeleton.  Your hips, spine, and legs carry your weight from the moment you get out of bed, and they quietly stay strong because of it.  Send a person into orbit and that load disappears.  NASA says astronauts on four- to six-month missions lose, on average, 1 to 1.5 percent of the bone density in weight-bearing bones like the hips and spine every month.  An older adult on Earth typically takes a year or more to lose that much.

In our look at why jumping builds bone, the lesson was that bone responds to sharp, sudden loads.  Spaceflight is the same experiment run in reverse: take the load away almost completely and watch what the skeleton does.  Here is why bones thin in orbit, how space agencies have fought back for six decades, what happens when astronauts come home, and why a trip to Mars makes the problem harder.

A skeleton with nothing to push against

Bone is living tissue that is rebuilt all the time.  Cells called osteoclasts dissolve old bone, cells called osteoblasts lay down new bone, and osteocytes buried inside the bone act as strain sensors that help decide which side wins.  When a bone bends slightly under a load, those sensors push the balance toward building.  When the load goes away, the balance tips the other way.

That is what happens in microgravity.  Blood and urine tests on space station crews show markers of bone breakdown rising during flight while markers of bone formation lag behind, leaving a net loss.  The damage isn’t spread evenly.  It is worst in the bones that work hardest against gravity on Earth, such as the hips, the lower spine, and the shins.  The wrists and arms, which astronauts use to pull themselves around the station, hold up much better.

The spongy bone inside the ends of long bones and in the vertebrae takes the hardest hit.  This inner bone is a lattice of thin struts called trabeculae, and in orbit those struts get thinner.  Some break apart entirely.

The calcium that leaves the skeleton doesn’t just vanish.  It moves into the blood and then the urine, which raises the risk of kidney stones, a serious problem when the nearest hospital is 250 miles straight down.  Weightlessness does one more odd thing to the skeleton: without gravity compressing it, the spine straightens and stretches, and NASA says crew members can grow as much as 3 percent taller while in space.  They shrink back once they return.

Sixty years of watching the bones

Space medicine researchers have been tracking this since the beginning.  Bone loss was seen as early as the Gemini flights of the 1960s.  The early measurements weren’t very precise, but the pattern showed up again and again, on Soyuz, Apollo, Skylab, the Salyut stations, Mir, and the International Space Station.

Skylab, America’s first space station, was the first place NASA could study the problem over weeks rather than days.  Its three crews in 1973 and 1974 stayed up for as long as 84 days, and medical tests showed them losing calcium throughout their missions.  They pedaled a stationary bicycle, and William Thornton, a physician who flew on the space shuttle, became one of the strongest voices for exercise as the main defense against bone loss.

Russian cosmonauts pushed missions much longer.  On Mir, average bone losses of 1 to 2 percent per month were recorded.  The physician-cosmonaut Valeri Polyakov spent 437 days aboard Mir in 1994 and 1995, still the longest single stay in space by anyone, partly to learn whether a human body could hold up on a trip as long as a Mars mission.  When he landed, he chose to walk the few steps from his capsule to a chair instead of being carried, to show that a crew could still function after a long flight.

The space station gym

Bicycles and the elastic bands of early missions weren’t enough.  Today’s station crews have three main machines and spend about two hours a day exercising, according to NASA.

The most important one for bone is the Advanced Resistive Exercise Device, or ARED, installed in 2008.  A barbell lifted in orbit weighs nothing, so ARED uses vacuum cylinders and flywheels to create resistance of up to 600 pounds on its bar.  That lets astronauts do squats, deadlifts, and heel raises with loads close to what they would lift in a gym on Earth.  There is also a treadmill and a bike.  On the treadmill, a harness with bungee cords pulls the runner down onto the belt, so every footstrike delivers some of the jolt that gravity would.  The Canadian Space Agency says the pull starts at about 60 percent of an astronaut’s body weight early in a mission and rises to about 85 percent.  NASA named the current model COLBERT after the comedian Stephen Colbert, whose fans flooded a 2009 online poll to name a station module after him.

The heavier loads made a measurable difference.  A 2013 study led by Adrian LeBlanc compared 11 astronauts who trained on ARED with 18 earlier crew members who used an older device that topped out at about half the resistance.  Those on ARED lost noticeably less bone, though losses at the hip and the neck of the femur, the spot that often breaks in falls, still showed up after flight.

That study also tested a drug.  Seven astronauts took alendronate, a common osteoporosis pill that slows the cells that break down bone, once a week starting three weeks before launch and continuing for their roughly five-and-a-half-month missions.  Combined with ARED workouts, it prevented the expected drops in nearly every measure the team tracked, including hip density, estimated hip strength, and the calcium flushed out in urine.  Drugs like this can cause stomach problems, which are harder to manage in orbit, so they are not a simple fix.

Coming home doesn’t fully undo it

Muscle tends to rebuild fairly quickly once astronauts are back under gravity.  Bone is slower, and it may not come all the way back.

In a 2022 study in Scientific Reports, researchers at the University of Calgary and NASA scanned the shins and wrists of 17 astronauts with a high-resolution CT scanner before their missions, right after landing, and again 6 and 12 months later.  The missions lasted four to seven months.  A year after returning, the group’s shinbone strength and density were still about 1 to 2 percent below where they started, and nine of the 17 astronauts had not fully recovered their shin bone density.  Astronauts who flew longer than six months fared worse, with shin strength still down about 3.9 percent, compared with 0.4 percent for those on shorter missions.  The authors estimated the lasting loss was equal to a decade or more of normal age-related bone loss.

The scans pointed to a likely reason.  When the tiny trabecular struts thin, they can thicken again once gravity returns.  When they break apart, they don’t seem to reconnect.  The study also found that a standard bone density scan, the kind most clinics use, made one astronaut look fully recovered even though the detailed CT showed the biggest lasting deficit in the group.  And the astronauts whose bone came back tended to be the ones who had done more deadlifts in orbit than they did before launch.

Testing on Earth: bed rest and a jumping sled

Astronauts are few and their time is expensive, so much of this research happens on the ground with volunteers who lie in bed for weeks or months.  The beds are tilted so the head sits 6 degrees below the feet, which shifts fluids toward the head and keeps weight off the legs, roughly mimicking life in orbit.  Volunteers eat, wash, and even use the toilet while lying down.  The writer Mary Roach visited one of NASA’s bed-rest studies for Packing for Mars, and her chapter on these “terranauts” is a funny, surprisingly moving look at what it takes.

Bed rest studies have also tested a solution that will sound familiar to anyone who read our jumping article.  In the Cologne RSL study, published in 2017, 23 young men spent 60 days in strict head-down bed rest at the German Aerospace Center (DLR).  Twelve of them trained five or six times a week on a sledge jump system, a machine that let them do jumps and hops while lying on their backs, pulled toward a footplate by pressurized cylinders.  Each session took only about three minutes of actual jumping.  The jumpers showed no significant loss of shin bone.  The group that didn’t train lost up to 2.6 percent of shin bone mineral, along with about 5 percent of their leg muscle mass.  The authors recommended jump training as a very time-efficient exercise for astronauts, older adults, and sedentary people.

Another approach is to bring gravity along.  In the AGBRESA study in 2019, run by DLR, NASA, and the European Space Agency, 24 volunteers spent 60 days in head-down bed rest.  Two groups spent 30 minutes a day on a short-arm human centrifuge, either all at once or in six five-minute bouts, spinning fast enough to produce about 1 g at the body’s center and around 2 g at the feet.  Published results suggest the spinning softened some bone loss at the hip and spine without stopping it, with the split sessions looking the most protective.

Animals got there first.  In 1977 the Soviet satellite Kosmos 936 flew rats in onboard centrifuges.  The spun rats came home with denser bones than rats that floated freely, though still not as dense as rats kept on Earth.

Mighty mice and a decoy molecule

Some researchers are looking for a drug that protects muscle and bone at the same time.  In December 2019, a SpaceX cargo flight carried 40 mice to the space station for an experiment called Rodent Research-19, led by Se-Jin Lee and Emily Germain-Lee of UConn Health and The Jackson Laboratory.  Lee discovered myostatin, a protein that limits muscle growth, and some of the mice were “mighty mice” bred without it, with about twice the normal muscle mass.

After 33 days in orbit, ordinary mice had lost 8 to 18 percent of the weight of individual muscles and up to 11 percent of the bone density in some bones.  The mighty mice kept most of their extra muscle.  A third group got injections of a decoy receptor that soaks up both myostatin and a related protein called activin A, and in those mice both muscle and bone actually increased, even without gravity.  The treatment is experimental, but the results, published in PNAS in 2020, point to drugs that could help both astronauts and people on Earth who lose muscle and bone from long illness or immobility.

Why Mars changes the math

Six months on the space station is one thing.  A round trip to Mars could take around three years, and the spacecraft will be far smaller than the station, with no room for a machine the size of ARED and no resupply flights bringing spare parts.  Mars itself has a little over a third of Earth’s gravity, which may help after landing, but nobody yet knows whether that is enough to hold bone steady.  A crew member who breaks a hip on Mars is a very long way from an orthopedic surgeon.

That is why the current research is pushing toward smaller, smarter countermeasures: compact machines that deliver heavy loads, short bouts of jumping instead of hours on a treadmill, drugs that slow bone breakdown, and possibly a centrifuge on board.  Bone loss is one of the human health risks NASA formally tracks, along with radiation.  It joins a long list of spaceflight hazards that also includes runaway space junk and the micrometeoroids that station shields are built to stop.

What the rest of us can take from it

Astronauts are an extreme case, but the biology is the same for anyone whose bones stop being loaded, whether from bed rest after an injury, a cast, a long illness, or simply a very sedentary life.  A few lessons carry over.  Bone responds to load, and particularly to heavy or sudden loads.  Lost bone is much harder to rebuild than to keep.  And the payoff from loading seems to come from intensity more than from hours of gentle effort.  That matches what earthbound studies have found about jumping and resistance training.

This article is for general information, not medical advice.  If you have osteoporosis, a past fracture, or another health condition, talk to a doctor or physiotherapist before adding jumps or heavy lifting to your routine.

The bottom line

Without gravity, the skeleton stops getting the signals that keep it strong, and astronauts lose roughly 1 to 1.5 percent of their hip and spine bone density each month on long missions.  Six decades of research have gone from stationary bikes to vacuum-powered weight machines, bungee-harness treadmills, and bone drugs, and the best combinations now prevent much of the loss.  But a year after landing, many astronauts still haven’t fully recovered, and a mission to Mars will need lighter and cleverer defenses, perhaps a few minutes of jumping, a decoy molecule, or a spin in a centrifuge.

Load your own bones, or read about life in the void