Friday, December 20, 2013

What's the minimum spin hab?

This post was prompted by Robert Walker's comment: "I wonder what anyone here thinks about my idea for rotating carousels to provide gravity in the lunar colony? Not rotating entire hab, but just a thin shell of living quarters inside it, in a bigger hab if say a couple of hundred meters across, greenhouse domed, have like the living habs around the outside rotating continuously - perhaps on a track or something like that - at just the right speed for 1 g for the inhabitants. Smaller habs just rotate the entire room - and easier to construct than e.g. fairground rides on Earth because of the low gravity."

We know 0 g results in bone loss and other problems. We need gravity, but how much?

Is 2/5 g (Mars gravity) sufficient to keep us healthy? Or 1/6 g (moon gravity)? This is still not known. Our only data points are 0 g and 1 g. If a full g is needed, people on Luna or Mars bases would indeed need living quarters on rotating carousels.

On the other hand, if lunar gravity is sufficient, no carousel is needed on the moon or Mars. That would also drasticly cut the minimum sized hab needed to keep workers healthy in a microgravity environment like on an asteroid.

The amount of artificial gravity felt in a spin hab is ω2r where ω is angular velocity in radians and r is spin hab radius. Obviously if 1/6 g does the job, the hab radius can be cut by a factor of 6.

Another quantity to look at is ω, angular velocity. Earlier it was believed 1 revolution per minute was the top angular velocity humans could comfortably endure. This combined with assuming a full gravity resulted in proposals like the behemoth Stanford Torus. (2 * pi / 60 seconds) * 894 meters = ~9.8 meters/second^2 or about 1 gravity.

If spinning doughnuts nearly 2 kilometers across are a prerequisite for asteroid miners, I wouldn't expect asteroid mining habs in this century or the next.

But is 1 revolution per minute really the top ω workers can endure? Research by James Lackner and Paul DiZio suggests workers could become acclimated to higher angular velocities. If workers can get used to 2 rpm that would cut needed radius four fold. 3 rpms would cut radius 9 fold. Here is a table showing hab radius (in meters) that be needed for various angular velocities and gravities:

1 g 5/6 g 2/3 g 1/2 g 1/3 g 1/6 g
1 rpm
894
745
596
447
298
149
2 rpm
223
186
149
112
75
37
3 rpm
99
83
66
49
33
16
4 rpm
56
46
37
28
19
9

If lunar gravity is sufficient and workers can get used to 4 rpms, a 9 meter radius hab does the job!

Obviously a 9 meter radius spin hab is more doable than a 900 meter radius Stanford Torus.

While we're talking about effects of microgravity, let's take a look at cosmonaut Valeri Polyakov. From a SpaceDaily article: "Polyakov's space flight had lasted 438 days (bettering a year by more than two-and-a-half months). Yet upon return, his health was not much different than other cosmonauts' after a long flight. After those first steps, he completely readapted to gravity within two months. Moreover, his bone loss had been very low, only around 7 percent in some of his weight-bearing bones," 

 Granted, only a small fraction of us have the self discipline to adhere to Polyakov's exercise regimen. But he demonsrates that exercise can mitigate microgravity bone loss.


If you hope for humans on surface of other planets or in asteroidal habs, it would be good to know what gravity humans need and what angular velocity they could get used to.

19 comments:

Anonymous said...

The problem is exactly that - we have no datapoints between 0 and 1g. However, i think there is no "sufficient" - living in constant 0.99g will have a slightly different long term effect on body than 1g.

The question is - what sort of relationship do we have between bone loss and gravity ? Linear ? Exponential ? Logarithmic ? To figure that out with any confidence, you would need at least 1, preferrably 2 more data points.

BTW, a 1 mile radius spinning station does not need to be a stanford torus - it just needs to be a long enough tether or cable with two backup cables for triple-redundancy ..

Hollister David said...

"Sufficient" as in sufficient to keep us healthy? I suppose that's how you define healthy. Someone living in .99 g might have a femur a few micrometers thinner what he'd have in a full g. If that's the only effect, I would call the person "healthy".

I just added a mention of Valeri Polyakov, a cosmonaut who was able to enjoy fairly good health in zero g!

You're right there are alternatives to a torus. A bolo or baton could be less massive.

Andrew Platzer said...

Do we even need gravity full time? Perhaps spending 8 hours a day sitting or lying down sleeping might be enough to reduce the worst effects of 0g. Lying down, you wouldn't experience disorientation from the Coriolis effects meaning even higher rotation rates or a smaller radius could be used.

Jon Goff said...

Hop,

I've been trying to hammer on this point (that we need to get data points between 0 and 1g) for a long time. It's why we need some sort of variable gravity research setup. I've been working on ideas for how to do this affordably...

~Jon

Jon Goff said...
This comment has been removed by a blog administrator.
Jon Goff said...

Hop,

Not sure if you got my previous comment or not. But I'm glad you're making this point (about our lack of knowledge of what happens between 0 and 1G). I've been harping on it at Selenian Boondocks for several years now. I've seen a few ideas for how to start answering the question, but none of them have been affordable enough to realistically happen. I'm working on some ideas for how to get preliminary answers (ie at least with small mammals/rodents) for an affordable price.

~Jon

Hollister David said...

Jon, thanks for your input! Sorry that comments don't show up right away. I don't have an adequate spam filter so each comment gets reviewed before publishing. Sometimes things get hectic and I don't have time to review comments.

Other forums seem to able to publish comments immediately but don't suffer from spam. Hopefully I'll learn how to do that.

Nydoc said...

How expensive would it be to launch mice to LEO and spin them at 0.9 G? Could this be done as a secondary payload on a commercial launch?

Robert Walker said...

One simple idea, I thought of,m is to just schedule a supply vessel for the ISS at the same time as a passenger vessel, and before (or after) docking - tether them to each other with a long tether and set them spinning enough to get low g. It's an experiment that was tried out with the early Gemini 11 and has never been tried again.

They just generated a tiny amount of artificial g for Gemini, too small to feel, and the experiment had to be cut off short.

I don't know what is actually practical e.g. strength of tether and securing the spaceships - to do it safely. But surely we can improve on Gemini 11 which is our only data point so far on either true artificial gravity or ultra low g.

One other thing that could be tested on Earth is whether we can adapt to the coriolis forces. Seems not at all impossible - after all just about everyone gets seasick after a few hours in a rough sea on their first sea voyage - but sailors adapt to it and can spend months at sea without getting seasick. Might be same for coriolis force.

Also no experiment on Earth can duplicate the effect of even normal g. + coriolis effect. Because on Earth - the artificial g is always combined with true g. So for significant coriolis, is always going to be greater than 1 g, which will itself have some effect on the body + the direction of the effect is also different as the artificial g. in space is felt directly towards the centre of rotation - and we can't simulate this on Earth at all.

So - first of all should at least duplicate the Gemini experiment + longer duration and more noticeable artificial g, can't see how that can be hard to do at all if we could do it in 1966.
http://en.wikipedia.org/wiki/Gemini_11

If it is practical, I'd propose, send a supply ship + passenger vehicle to the ISS together. Tether them together - this can be done at any time e.g. after delivery of supplies - when the supply ship is filled with waste products is also fine - and some volunteer (or two) gets into the passenger ship along with lots of supplies, and then set them spinning around the tether - and gradually spin up. Try short tethers first then can try longer ones. Spin faster and faster - up to full g if possible - but if the astronauts report sickness, then of course slow down the spin or keep it steady until they adapt - and keep pushing the envelope. May well find that after several days of that, they can tolerate full g with a short tether no problem at all.

Whatever happens, should get lots of data on artificial gravity and effects of coriolis force and tolerance of humans for all this, data we don't have at all.

The only thing this wouldn't let you test is the effect of doing full g for a short period of time like a few hours a day. That would need other experiments such as the idea to send a centrifugal sleepinq quarters module to the ISS.


What do you all think, is this a feasible experiment, and cheap to do, or am I missing something?

Doug Plata said...

I would like to suggest a three-step process to getting two data points between 0 and 1 gee. I think we need two because we don't know if the biological response curve is linear, up-sloping, down-sloping, or s-shaped.

First, purchase two pygmie marmoset ( aka finger monkies). This is an essential element to the fundraising plan because they're so cute! Also, make a couple of A4H jumpsuits for them because that would be so adorable and because A4H deserves our support.

Next, conduct garage development and experimentation until you have an apparatus that can simulate the experiment planned on a Zero Gee flight. This would be a contraption with two tethers with radii appropriate for a Zero Gee cabin, with a variable-speed motor, two small capsules at the far ends, and the ability to spool out the tether as the motor accelerates.

Next, conduct development in the garage until one gets the contraption spun up to 1/6th gee at one end and some other gee at the other (best if 2/3 gee but could be less than 1/6th if the Coreolius is too great). Also, the finger monkies need to be trained to eat Gummie Bears upon prompting immediately after being spun up.

Secondly, launch a Kickstarter campaign to raise money for the Zero Gee experiment. Include video of the garage experiment and feature the finger monkies prominently. Take advantage of every media opportunity to be interviewed along with the finger monkies.

Immediately seek co-sponsors for the Kickstarter campaign in exchange for their presence during the Zero Gee flight and some media appearances with the finger monkies. Might Branson, Allen, Diamandis, etc be willing to be associated with this important effort in exchange for their matching contributions up to a certain amount well beyond that being asked for the Kickstarter campaign?

The cost for the Zero Gee flight experiment would be modest and I believe easily reached. But the Zero Gee Kickstarter campaign is only the excuse for launching the fundraising effort for the real deal which would be a primate free-flyer experiment.

This would cost millions. It would be perhaps either a ride-shared payload on a dedicated F9 launch or sent to and launched from the ISS. It would be two capsules tethered to each other with consumable mass placed asymmetrically in order to give 1/6th and 2/3rd gee. There would have to be enough consumables and H2O and O2 recycling equipment to last the monkies (perhaps small finger monkies) for about three months.

After three months, the cables are cut and the capsules reenter and the monies are collected and analyzed for many biomedical indicators.

Doug Plata said...

A couple of things more.

A centrifuge swing arm can be as thin as a tether in free-space and also under the surface of an asteroid or moon of Mars. Next up would be a lunar centrifuge which, for the same length would need to be thicker (force x lever arm you know). The same length on Mars would need to be thicker yet. Yet, the spin rate to achieve the same total gee would be less on Mars than the Moon than free space. So there's a bit of complexity in figuring out just how big and expensive achieving the necessary gees would be for the various locations.

As for how much of a gee is needed and how often one needs to get it, like everyone else, I really don't know. But, guessing along with everyone else, I believe that the ISS experience suggests that gravity is much like a sedentary lifestyle such as bed rest. Recent non-space research indicates that it is not just how much exercise one gets in a day but actually how much sitting one does. If one sits around a lot not getting up to walk around casually, then the sitting alone creates a cardiac risk. So, what this suggests to me is that occasional gravity in a centrifuge will not be the same as getting that amount of gravity spread throughout the day.

Also, exercise studies show that most of the cardiac benefits come from just walking. There's a diminishing returns for intensity.

Putting it together, my guess is that the biological response curve is down-sloping (decelerating) where a little bit of gravity will be what's most important. And also that brief daily centrifuge sessions will not be as beneficial as ongoing levels of gravity.

Put together, I speculate that:
- free-space full artificial gravity will be easiest and best,
- Mars gravity with daily centrifuge sessions and a good exercise routine will be second easiest,
- Lunar gravity will be noticeably better than zero gravity and that daily (or nightly) centrifuges will not be as good as something similar on Mars and that such lunar living will result in long-term sedentary-like conditions perhaps becoming significant after about year 5 or so.

Again, just an educated guess.

Thomas Lee Elifritz said...

My personal criteria for minimum gravity is enough to make water flow past surface adhesion, self cohesion and tension such that gravity fed hydroponics will work on reasonable water feed schedules, and such that the plants can orient themselves along the gravity and lighting flux gradients. That alone solves a lot of problems with human space flight.

Doug Plata said...

Thomas, Why did you choose that criteria instead of a human biomedical criteria such as bone mineral density, muscular or cardiac function?

Hollister David said...

Doug, see my mention of Valeri Polyakov above. In my opinion Polyakov demonstrated atrophy could be mitigated with exercise.

But there are a lot of the body's housekeeping chores that rely at least in part on gravity. Sinuses draining for example. I suspect drainage and edema issues would be largely mitigated if Thomas' criteria were met.

There are a number of routine hygiene chores that become hard in microgravity. Enough gravity so that water flow overcomes surface tension might be enough to enable flush toilets. I believe this alone would considerably boost morale. And with better morale comes a better immune system.

But the above is speculation on my part. We won't really known until we get actual data.

Robert Walker said...

I've done a new blog post on my science20.com column on this topic,
see
an Humans Stay Healthy In Low g or Artificial Gravity to Travel to Other Planets? Why Nobody Knows and How to Find Out

Hollister David said...

Robert, thank you for posting a link to that. As usual, your articles are thoughtful and informative.

glenn robinson said...

A modest proposal: The statement that we have only two data points is well taken, but not precisely true. Here on earth, we have small gravity variations. The populations of Longyearbyen, Norway (high latitude, low elevation) and La Rinconada, Peru (low latitude, high elevation) experience about 0.6% difference in gravity. A comparative study of bone density in the two populations (corrected or controlled for vitamin D deficiency, gender and age) might bound the slope of the curve near 1g.

Hollister David said...

Glenn, interesting point. Besides having slightly less gravity, La Rinconada has a much lower atmospheric pressure, close to the limits of what humans can endure. People wouldn't subject themselves to these conditions if there weren't rich gold deposits there. There are Mars and moon advocates who propose building and expanding underground pressurized habs. La Rinconada would be a great place to test this notion. Pressurized habs would be a wonderful thing for the people of that city.

glenn robinson said...

I imagine at that altitude that they have reliable sunshine, too. One could also test solar thermal autoreduction to produce oxygen to feed the pressurized habs. Basically dry run an orbital ore processing facility in a location that is used to supporting ore processing machinery.