How big of an issue is bone density loss when traveling to Mars?

Posted on September 14, 2016


I think space is awesome. I think going to Mars is awesome. I think it’s important to do. I’m not going to make an argument why, but you can see one here.

A problem for human spaceflight is countering the negative impacts of moving in an environment where gravity is less than Earth’s. This is the predominant impetus for some space posts I’ll have, as there is a ton here which can be applied to everyday people on Earth, and vice versa.

Sometimes I’ll hit on current treatments; sometimes I’ll hit on ways I think things could potentially be improved. Maybe this can be my small way to help the Mars movement.

Finally, I want to stress, even if you’re not interested in space, this can be valuable to learn about. This post has application to appreciating the significant differences activity plays in bone density, and understanding why preparing for rare events, like a fall (which can break your hip), are where exercise becomes indispensable.

Bone responds to load extraordinarily specifically. Load it more; it gets more dense. Load it less; less dense. Too little density and we run into terms like osteoporosis.

In microgravity we don’t load bone at all (without intervention, such as exercise or artificial gravity). On Mars, gravity is 38% of Earth’s. So transit time to Mars -microgravity- and time spent on Mars -Mars gravity- give the concern of bone density issues.

We don’t want to get off from a six month trip in microgravity and immediately break a hip. That’d be catastrophic.


Femoral neck fracture. Credit:

-> Transit times vary. Six months is what Mars Direct uses, and is a common enough number we’ll use it here.

-> We’ll be focusing on the lower body. Upper body isn’t a concern from a bone density standpoint.

Some natural questions then are how much bone do we lose? And how much can we lose without concern?

I don’t know much about engineering, but one thing I know is you want to plan for worst case scenarios. That’s something we’ll try our best to do here. We’ll also try to use fancy terms like “factor of safety.”

According to this document from Dava Newman’s MIT course -she’s currently the deputy administrator at NASA- bone loss in microgravity is 1-2% per month. This reference is at least 10 years old and doesn’t account for a solid exercise program, it’s basically an interventionless number (our exercise programs on the International Space Station were futile until recently), but we’ll assume 2%.

Unfortunately, nobody knows what bone loss on Mars will be. Some think –delusionally extremely optimistically- it could be nothing. That 38% Earth gravity could be enough to stop any loss. We’ll assume much worse than that. If bone loss is 2% in microgravity, let’s say on Mars it’s 62% of that.

2 * 0.62 = 1.24% per month on Mars

Now we need bone density numbers to start with. We’ll look at what is arguably the most crucial site- the femoral neck. While bad on Mars, we can likely deal with a broken fibula or broken foot. A broken hip is not something we want to fuck with.

While females should make up most of our astronauts to start, we’ll use males.

  1. The likelihood is males will dominate the crews
    1. Culture / sexism is obviously a factor, but males like dangerous activities / risk taking more too. The pool of candidates will likely be disproportionately male, at least initially. Could be a way to boost one’s status. After all, the Origin of Species does state women prefer men with the middle name danger.
  2. Being female throws all kinds of wrenches into the mix with bone density. Pre menopause, post, apparently regularity of period can be a factor (then there’s the question if you’re a former athlete did you have amenorrhea), etc.

Plus, I think the chart from below is just a nice place to start, because of the segmented populations and the long follow up.


Baseline is 17 years old. Credit: Rates of bone loss in young adult males

The likelihood is astronauts are going to have an athletic background, one way or another. That by their mid to late 20s, they’ll probably qualify as former athletes, per above. For instance, former high school athletes who are still active, but not like they were.

Former athletes in this case are hockey and badminton players. Not exactly people who load the hell out of their bones like football players or powerlifters, but that helps with our lean-towards-assuming-worst-case-scenario.

Per the chart above, let’s say our numbers at 29 years old are:

  • Former athlete =1.17 g/cm^2
  • Control = 1.10 g/cm^2

Now we need to extrapolate to the average astronaut age, 48.

After the former athletes hit peak bone density, call it 1.33 g/cm^2 at two years follow up, they go down to 1.17 at 12 years follow up.

  • 1.33 – 1.17 = 0.16 g/cm^2 / 10 years = 0.016 g/cm^2 per year


  • 1.20 – 1.10 = 0.10 g/cm^2 / 10 years = 0.01 g/cm^2 per year

Extrapolating like this isn’t always accurate. (It’s probably more often wrong than right.) If you look at the green line below, between 7 and 12 years you can see it leveling off. So the above may be assuming a faster rate of loss than actually would happen.


But that’s ok. Again, fitting with our worst case scenario model.

So the above people are 29 years old; we need to get to 48.

  • Former athletes lose per year 0.016 * 19 years = 0.304
    • 29 years old => 1.17 – 0.304 = 0.866 g/cm^2 <= 48 years old
  • Controls lose per year 0.01 * 19 years = 0.19
    • 29 years old => 1.10 – 0.19 =0.91 g/cm^2 <= 48 years old

This clearly doesn’t work, because the former athletes end up with less bone density! Yet we know former athletes tend to maintain some of their edge years and years later. (Example.) Certain things stick with you to some degree. We’ll come back to this.

Now we need to look at what forces bone can deal with:

From Dava Newman's course.

From Dava Newman’s course.

Then, what forces do we need to be able to handle? Here are the two scenarios perhaps most relevant for Mars exploration / colonization-

  • Running
  • Falling

It’s unlikely an astronaut is going to be sprinting on Mars, but shit happens on Earth, and we don’t know what Mars shit is like. Maybe two astronauts are off exploring, an accident happens, and one has to run to get help.

Falling is more obvious. Britney Spears appears out of nowhere scaring the crap out of you,


Been on Mars since circa 2000.

a slip and suddenly all your weight could land on your hip.

Or on your knee, impacting your hip.

Considering how rocky Mars is; the cubersomeness of moving in a spacesuit; the issues one will have with balance if not working on it (exercise) during the six month journey, this is perhaps the most pressing issue of anything orthopedic.

In fast sprinting, the body has to handle 2-3 times its bodyweight. This

  • weight = mass * gravity

Let’s go with an 82 kilogram (180lb) male. Too heavy for what we’d want the first astronauts to be, but


  • 82kg * 9.8 m/s = 802N

Add really fast running -running an average astronaut likely couldn’t do-

-> If our bones get this weak, our muscles will be significantly weaker too, where they aren’t going to be able to generate this amount of force anymore.

  • 802N * 3 = 2406N

That puts us at:


0.43 gm/cm^2 as the threshold. Essentially, if we all out sprint (and hit top speed (not the same as running as fast as you can for 20 meters)) with that bone density, we’re breaking our hip. But on Mars, we don’t have the same amount of gravity.

  • 2406 * 0.38 = 914 N

Butttt, on Mars we do have a spacesuit to wear. On the International Space Station these are 136 kg. While unlikely they’ll be that heavy by the time we get on Mars, or that we’ll be able to generate the same level of force wearing one as we can on Earth without wearing one, we’ll roll with it.

  • 82kg = mass of astronaut
  • 136kg = mass of suit
  • 9.8m/s = Earth gravity
  • 0.38 = percentage of Earth gravity on Mars
  • 3 = extra force due to very fast running
    • => (82kg + 136kg) * 9.8 m/s * 0.38 * 3 = 2435N

Pretty much back to this,


What about falling? This study:

Theoretical Implications of the Biomechanical Fracture Threshold

puts bone strength needed at 4000 N minimum, to feel confident the hip won’t break when falling on it. If we worry about something like falling while holding some rocks, which would increase our weight, or falling off an elevated surface (standing on some rocks), which would increase the energy we hit the ground with, then we can add a factor of safety of two:

  • 2 * 4000N = 8000N

Then on Mars,

  • 8000 * 0.38 = 3040N

Thus, on Mars our femoral neck bones being able to handle a fall is more important than being able to handle sprinting.

This is true for Earthlings too. If you’re someone worried about bone density, one way or another you can prevent yourself from running to where you’ll break your hip. However, preventing a fall isn’t something you can always do.

Putting us at,


0.57 g/cm^2. This is the number we don’t want to be getting towards when going and being on Mars.


Bone loss in transit and on Mars

There is an embedded spreadsheet below. Used numbers are,

  • 2% bone loss relative to baseline per month, when in transit to and from Mars
  • 1.24% bone loss relative to baseline per month, when on Mars
  • Starting bone density of 0.91 g/cm^2
    • From our first chart and calculations i.e. non-athlete bone density.
  • Six months to Mars, 18 months there, six months back

Even in this scenario, we leave Mars with 0.60 g/cm^2 bone density. Close, but above, our 0.57 g/cm^2 threshold.

It’s worth recapping the assumptions,

  • Worst bone loss rate in transit
  • Zero bone gain when on Mars
    • Going from microgravity to Mars gravity, we might gain some of our bone loss back.
  • A starting bone density of a sedentary person
  • Zero intervention
  • A fracture threshold that would likely lessen based on wearing a spacesuit, due to padding the hips.
    • To do a full proper analysis, you’d need to look at all this. Mass of the suit, padding of the suit (we could even add padding), if the angle of falling would change, etc. Not sure it’s possible to simulate this in vitro.
    • The falling values calculated above would be closer to what might happen in a Hab. (No suit on.)
    • An underappreciated benefit of maintaining muscle mass is having extra padding around the hip(s), dampening a potential fall.
  • No plateauing of bone loss

For the last point, spinal cord injury patients are a decent analog for this. While bone loss can persist longer than sometimes stated (bolding mine; references at end)-

“This bone loss took place gradually, reaching a significant plateau at 19 years post injury and then started improving.”

It can take a while to be significant:

“The decreasing bone parameters reached new steady states after 3-8 years, depending on the parameter. Bone mass loss in the epiphyses was approximately 50% in the femur and 60% in the tibia, while the shafts lost only approximately 35% in the femur and 25% in the tibia.”

“Although femoral BMDs of both paraplegic and quadriplegic patients 40-59 and 60+ years of age decreased over time, none showed significant bone loss in this regions until 10 years after their injury.”

Long story short- we likely assumed too fast a rate of bone loss. In the spreadsheet above, we get to nearly 50% of original bone density in 30 months. It takes paraplegics at least three years to get there.

What is a problem in the spreadsheet is coming back to Earth. Fracture thresholds at the femoral neck have been put at 0.66 g/cm^2.

Bone density in masters -40-64 year olds- speed and power athletes has been measured at 1.30 g/cm^2 in the pelvis.

Using that as our baseline:

And now we’re above even that threshold. Or if we use the paraplegic number of ~50% femoral bone loss after three years, that would put us at 0.65 g/cm^3.

When it comes to Mars exploration with Earth return, it’s tough to see this being a showstopper. Send out of shape, older people who’ve barely ever moved? Yeah, not the best. However,

  • some exercise before we go
    • which we want people doing anyways (you’re not getting a 1.30 g/cm^2 bone density without exercise!)
  • some exercise on the trip
    • which we want people doing anyways (artificial gravity doesn’t help us be ready to run nor are we likely to have access to it once on Mars)
  • some exercise while there
    • which we want people doing anyways (older people, like astronauts, need it more than anybody)

and this is likely the least of our concerns.

The more interesting question then is Mars colonization. What happens to bone after staying in Martian gravity for years? At some point, bone loss will plateau, but is that plateau high enough to not be a fracture concern? In one regard, you may think “Sure it will be. The bone will get strong enough to handle the forces placed on it.” In another regard, this isn’t always true for people on Earth!

Example scenario- You’re a person doing fine for years, then one day you fall and break something. You get checked out and realize you have low bone density. Your bone density might be fine for the majority of daily life, but it’s not fine for those unexpected scenarios, which is really where the fears stem from.

How can you then get your bones (/ tendons / muscles) prepared to handle an unexpected load during daily life? How can you get your body better able to prevent those unexpected scenarios to begin with? Exercise is your best intervention.

If you’re roaming around on Mars for a while but that’s all your activity, you still run the risk of those unexpected scenarios, which are more likely on unfamiliar terrain. Scenarios which could much more easily debilitate or kill you in the abscence of modern healthcare. (Dealing with hip fractures in the youth is even tough; having a significant chance of being life altering.)

-> Considering the potential side effects of bone density drugs, it’s debatable if drugs are even an entertainable intervention on Mars.

Maybe we’ll have to build a track for Martians to run around on.

Densitometric patterns of spinal cord injury associated bone loss.

Relationship between the duration of paralysis and bone structure

Demineralization in tetraplegic and paraplegic man over time.

Bone mineral density and bone turnover in male masters athletes aged 40–64


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