I just listened to the Mars Astronomy Cast episodes and I have a question about the bone-weakening that may result from low gravity conditions on Mars.

If low gravity does in fact weaken ones bones, would it only be an issue when the astronaut returns to Earth from an extended Mars mission? Would such an astronaut suffer the health effects of low gravity while still on Mars? Would this mean that a person living on Mars for an extended period, could not return to Earth because the change back to higher gravity would be too much for the body to endure?

(This is my first post - I love the podcast)

Even a few days in space is a problem.

The lack of gravity causes blood to 'rise up' in the body and head. The compensation mechanism? Pump plasma out of the blood and into the chest cavity.

When you get back to earth, instant low blood pressure.

Concerning bones, I'm not sure we know whether the stay in 0.4g Martian gravity would result in calcium loss in the bones. The thing we know is trouble is the long ride to and from. A spinning habitat can be good for the bones. The cosmic ray issue is another problem, but you didn't ask about that.

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Calcium in healthy interior bone is in constant turnover. It is pulled out into the blood for metabolic functions and redeposited where called for. An important signal for deposition is damage to the bone.

Physical stress to bones increases deposition over time and increases bone strength as seen in those engaging in athletic activity. Normally active adults are able to just keep their bone density. A lack of bone stress due to reduced gravity will definately lower bone density over time. This is a fairly slow process to be sure but would effect anyone in low gravity for years.

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I've been waiting for movement on the Mars Biogravity mission for a while now. Looks like they only just got to the PDR stage late last year though so have a while to wait.

If on mars you jog three times faster than on earth and get into a habit of carrying three times as much, you may be putting enough pressure on your bones to prevent decay. But I'd be worried about injuries resulting from everything being 38% the weight, but 100% the mass. I imagine that people would learn to cope with it, but it would be a danger, particularly if their bones are weakened by a long space trip or low gravity.

Not a problem!

Here's how/why:

For simplicity sake, let's do some gross rounding, and say they'll spend Year 1 getting there, they'll spend Year 2 on Mars, and Year 3 returning.

That's 1 yr in zero G, 1 yr in 1/3 G, and 1 yr in zero G.

Is that a problem?

YES! It's a HUGE problem. By the time they returned, they'd have lost roughly half the density in their bones, which is NOT how I want to begin my 45th birthday...

Yes, bungee exercising equipment can ameliorate that somewhat, but even with 2 hrs of exercise a day, they're still spending more than 90% of their time in the zero G while travelling, and more than 90% of their time in 1/3 G while on Mars.

They will loose serious amounts of bone mass.

Enter the two-part solution:

1. Space travel: Attach a small weight to a non-conducting tether and extend the tether a short distance away from the Earth-Mars shuttle. Immediately fire a thuster perpedicular to both the tether and an imaginary line from Earth to the shuttle until the shuttle has performed a 360 circuit around the center of mass between the weight and the shuttle (this brings the sum of all course changes to zero). The appropriate intermediate tether length to accomplish this given the total mass and the thruster's specific impulse can be easily calculated.

Slowly extend the tether to it's full design length. Fire the thruster again, but this time in equal bursts, balanced 180 deg apart from one another along the circle of rotation, until 1 G is reached.

Many studies have proven the optimal rotation velocity is approximately 1 rotation per minute. It's a simple task to make that a standard, then use the right mass for the counterweight and tether length to make that happen.

Before arriving at Mars, simply reverse the process.

This provides a constant 1 G imparted to the astronauts via centripetal force, thereby solving the bone density problem on the out and back trips.

2. Mars' gravity is a bit more than 1/3 G. It's 0.376 G, to be precise. What we need to do is create a lightweight, inclined track, such that a runner on that track would be able to run 1.5 miles in 13 minutes (13 min is roughly halfway between the men's good-excellent time (12 min) and the women's goo-excellent time (14 min) - please, no flak! The US Air Force set these, not me!)

Anyone who has studied the first three chapters of a Dynamics text can solve for the radius and the required bank angle of the circular running track to achieve 1 G under Mars' 0.376 G and a velocity over the track of 6.9323 mph (let's make it simple and just call it 7 mph).

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."

This is why I wished we returned to the Moon soon after Apollo. We have far to little data on the effects of reduced gravity over long time periods. Most of the research has gone into zero-G studies.

This question is pertinent to any discussion of long term bases or colonization. If someone is born on Mars could they live out their life on Mars without resorting to daily centrifuge visits? (assuming they never plan to go to Earth)

There is almost no data on the effects of low gravity on humans; the great mass of data cited in these cases is all derived from microgravity environments, particularly freefall in orbit. This cannot be extrapolated to Martian gravity.

Bed rest experiments might give some useful data, but short of actually going to Mars, only a rotating environment in Earth Orbit would give an accurate picture of the effects.
The Mars gravity proposal Loglo mentioned is a good way of looking into this
http://www.marsgravity.org/main/mission.html

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The next two modules delivered to the ISS international space station could each have a ball baring ring several meters in diameter to attach a connecting tether perhaps 100 meters long. One of the modules would need complete life support for one or two astronauts. It would be sort of like a prison cell, but I suppose there would be volunteers. Since the ISS is much heavier, the center of rotation would likely be approximately on the ball baring ring of the modual fastened to the ISS. The ISS would wobble slightly, perhaps enough to mess up the microgravity experiments. Likely we could rotate the life support module fast enough to produce 1/10g without damaging the ISS, but we would have to test for metal fatige and end the rotation, if the ISS or the tether showed signs of possible failure.
My guess is 1/10 th g would not be significantly different from zero gravity as far as bone loss and other zero gravity health problems are concerned, but we would learn how much the corriellis effect harmed humans. Model two might need a lot longer tether than 100 meters (to reduce corriellis effect) plus some beefing up of the ISS where there was even slight evidence of metal fatige. Model 2 could possibly produce 1/6 g Moon or even 0.38g Mars. By now we would have spent perhaps 20 billion dollars, so you can understand NASA is not anxious to run a test like this. Worse, If a micro meterorite or bit of space junk severs the tether abruptly, the life support module would be flung away from the ISS, possibly with eventual fatal results for the occupants of the life support module.
Coerriellus effect will be severe on the mice planned for the MIT bio satellite. Perhaps hamsters or squirrels tolerate (as in hamster wheel) tolerate corrillis effect better than mice. Neil

Tall thin skinny people on earth who weigh only 38% as much as the average person in the United States usually have okay bones.

But a 38 year old, 38 kilogram, two meter tall adult who gained 68 kilograms would be likely to break a bone. 100 kilogram is close to the average weight for two meters tall. Are astronauts typically lighter or heavier when they return from months in zero g? Neil

They would be weirdly thin to begin with. But an anorexic who gradually gained weight until they were 106 kilos would probably be okay as their bones would grow stronger as they gained weight. If they gained it all at once then they would be at high risk of falling and breaking bones.

I was wondering why Mars' surface has 0,37G, in spite of it's mass only being about 10% of Earth's.
What's the relationship between the two variables?

Mars's density is 3.934, considerably less than earth's 5.515. However, when you are on mars you're standing closer to the center of mass than you are on earth. For example if there were a solid lead planet the same mass as earth, when you stood on the surface you would feel more than earth normal gravity because you would be closer to the center of mass than when you're standing on earth.

Well, yes and no.

Their bones would be thinner due to the reduced gravity, as would their muscles, potentially. It could be they just lift three times as much on a regular basis, in which case their bones could be nearly as strong and dense as our own.

However, if they don't routinely lift three times as much as we do, if general muscle mass and bone density is more a factor of gravity than it is genetics, then their bones could be as low as one-third the strength of our own. I suspect, however, that genetics plays at least some role, and that's reflected in the current studies of prolonged weightlessness, where the astronauts were living in zero-G for more than a year, but even with moderate exercise less than 10% of each day, their bone density was slowed dramatically.

If I had to swag it, I'd say both bone density and muscle mass after long-term (five to ten years or more) of Earth-born humans on Mars would settle in around 2/3 of what it is here on Earth.

Unless you managed to find a bunch of astronauts who were weight-lifting fanatics. I dare say that doing 600-lb (mass) bench presses and similar exercises a couple hours a day would keep both their bones and their muscles Earth-ready.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."

A perfectly acceptable alternative would be to make the anchor relatively light, and simply extend the tether. A large, dead satellite retrofitted with a simple remote-controlled, solar-powered ion engine would work nicely.

Your guess is correct.

The studies were done 40 years ago. We already know. There is absolutely no reason whatsoever to repeat previously accomplished, extensive, and expensive testing, particularly when doing so in space probably costs 10,000 to 100,000 times more, in constant dollars (today's dollars), than the testing (and results) which are already on the books, and have been for four decades.

In fact, the first studies of the Coriolis illusion (more appropriately called the Coriolis effect, as per what's in most psychological texts) were conducted prior to WWII, to discover why even experienced fighter pilots sometimes augered in for no apparent reason.

In the 60's and 70's, additional tests were conducted to determine the optimum rate of revolution for orbital colonies.

Why merely reduce it when you can eliminate it simply by ensuring the axis of rotation of the body and the counterweight is parallel to the Earth's axis of rotation?

Beefing up is expensive.

Exactly. Particularly when NASA itself already did the studies and has the answers on some shelf. What NASA needs is a feather duster, not a $20 Billion tether experiment.

For the ISS/module pair, that wouldn't be very good for either, particularly the model.

For a small counterweight at the end of a long tether, it wouldn't be so bad, and a recovery is easier than you might imagine, provided the unit had fuel and a thrust. We're not talking escape velocity, here. We're talking tangential delta-V which would correspond to an angular velocity of 1 rpm and a radius r which would impart a 1 G acceleration to the unit at that angular velocity.

Terrific. Watching mice puke at $100 Million a heave.

What did NASA do? Hide the plans and play dumb to get funding for something that can be (alreay has been) easily simulated on the Earth (just like they did 40 years ago...)?

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."

How can you know that? There haven't been any experiments in 0.1 gee yet. However, if I had to guess, I'd say that living in 1/10th gravity would produce sufficient bone density to thrive in that environment, as the bones form density as a reaction to stress; only if the subject tried to return to Earth would there be a problem.

No, that wouldn't work. You would get coriolis effects in rotating habitats wherever they are, orbiting a planet or just orbiting the Sun, or even in deep space between the stars.

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Orion's Arm . The Starlark . Voices: Future Tense . OA Flickr set

Thank you, Ronald!
I also found a nice little 'planetary weight converter' (courtesy of 01101001)