asteroid habitat with gravity

[Cigaboo]

I am working on a concept for a computer game that involves a hollowed out s-type asteroid that spins around to simulate gravity.  For storyline reasons, I wish to keep the asteroid small, the scale similar to this pic (I apologize for the roughness):
http://www.cradleofc.com/images/hab_center.jpg
At the center of the asteroid is a docking station (a cube-shaped craft is docked in the pic).  You can see a cylindrical tunnel that runs the length of the asteroid, occasionally broken up by large cylindrical rooms.

What level of gravity do you think would be comfortable for an extended stay in space?  My guess was about 1/4 to 1/5 the force of Earth’s gravity.  To determine how fast the habitat would need to rotate to generate that gravity, I used the equation:
sqrt ( gravity_force_m/s^2 / distance_from_asteroid_center_to_room )  = radians/sec
Then I converted radians to degrees: radians / 6.28 * 360 = degrees/sec

I found that an asteroid this small would have to spin too fast, making the occupants sick or dizzy.  I then thought maybe I could move the center of the spinning to the end of the asteroid, in a way seen in this pic:
http://www.cradleofc.com/images/hab_side.jpg
This would increase the distance from the asteroid’s spin center to the rooms.
But I suspect this would not work, because it is not the center of mass.  What say you, physics gurus?

I also have a side question about the upcoming NASA Orion spacecraft.  This image (from Wikipedia) shows the different parts.  http://en.wikipedia.org/wiki/Image:Cev_design.jpg  
I assume the Launch Abort System and Spacecraft Adapter disconnect when it is in space, but the Crew and Service Modules remain attached, correct?  Does the Service Module detach before reentry?

[MKremer]

First of all, I think you could be hard pressed to find an asteroid of that size and shape that isn't a rubble pile. (ie- a conglomeration of masses, from very large boulders - mini-asteroids on their own - all the way down to dust grains, mostly held together by their combined gravity. Example- Itokawa explored by JAXA's Hayabusa mission.)
Tunneling might be easy, but the main risk is bits and pieces of the asteroid starting to fly off as soon as the rotation speed increases, and maintaining a central CG and stable rotation axis.

[hmh33]

Yep - I think even pretty large bodies are held together mainly by their own gravity.  If they start spinning fast enough to produce significant centrifugal forces, those same forces will tear them apart.

[Cigaboo]

Thanks for the input - didn't realize that.  Assuming it were a larger asteroid or the "falling apart" issue didn't apply, would the second idea (rotation center on one end of the asteroid) work in terms of rotation or does it violate the laws of physics?

[Generic Username]

hmh33 - 24/10/2007  10:55 PM

Yep - I think even pretty large bodies are held together mainly by their own gravity.  If they start spinning fast enough to produce significant centrifugal forces, those same forces will tear them apart.

Back in the early 1960's, when Men were Men and Aerospace Engineers dreamed BIG, Dandridge Cole of Martin Co. suggested making arti-g colonies from nickel-iron astorids. Process was simple: drill a hole to the center of the asteroid, pumo a boatload of water in, then seal up. Then, using truly huge mirrors, refelct sunlight onto the asteroid until it nearly melted. The water inside would boil, and what with the nickel-iron alloy being soft and malleable, it'd blow up like a balloon.

Wouldn't work (the steam would find vents *long* before the metal could soften that much), but damn was it thinking big.

The two images below illustrate Cole's intended final product (the right-hand part of each image, at any rate).

http://www.fabiofeminofantascience.org/RETROFUTURE/coleanalog.jpg
http://www.fabiofeminofantascience.org/RETROFUTURE/cole1small.jpg

[Cigaboo]

That's a pretty cool idea.  But unfortunately, the main reason I would want to keep the asteroid both small and S-type is gameplay related.  You see, part of the game is the strategy of sending probes to nearbye asteroids to collect resources.  Metal is one of the most valuable resources because it is used for building more probes and structures to advance your exploration in the game.  If the habitat itself was mineable, it would conflict with that.  I wanted the present the habitat as something that was already exploited for important resources, and further mining would risk structural damage.  

But I will obviously have to consider the realism issues people are pointing out.

[hmh33]

The rotation center has to be the center of mass.  But you could stick a habitat on a long boom.
Also what rotation rates were you getting?  You'd be surprised at how fast it can be without people getting too dizzy.

[Cigaboo]

On the first design, where the center of spinning is the center of mass, the distance from the center to the furthest room would be about 15 meters.  Assuming I want a gravity of about 2.45m/s^2   (1/4 earth's gravity), I got that I would need to rotate at about 23 degrees per second.  I think that's a full revolution in about 15.5 seconds.  

[meiza]

You could of course try putting two asteroids in socks and connecting the socks with a string and then spinning them... The center of gravity would be in the middle of the string.
You will need big socks and thick strings for this, mind you. Then both asteroids would have fairly straight felt gravity field.
At one rotation per minute and 1 gee, a = v^2 / r = w^2 * r where w is angular velocity of radians per second. 2pi per 60 seconds is about 0.1 radians per second so r= a/w^2 = 10/0.01 m so the radius is 1000 m. You'd need a 2000 meter string so both ends are 1000m from the center of rotation.

A small, ten million tonne asteroid (about 100 m diameter, depends on density and hollowness) needs 100 GN of force to hold at one gee... so with a 5 GPa strength carbon fiber you'd need 20 square meter cross section wire to dangle it, or about 5 m diameter if it was one "wire". More like a pillar. If you had carbon nanotubes (developed for the space elevator) with 100 Gpa, then you'd need one square meter or about 1.2 meter diameter of solid nanotube wire (or perhaps stalk here). Or it could be 100 12 cm wires.

[hmh33]

The Mars Gravity Biosatellite project is using a spin rate of 32rpm to get Martian surface gravity (0.38g) with a smaller radius.  Ground centrifuge tests are taking place at the moment to see if there's any appreciable "dizzy" (Coriolus / gravity gradient) effects from the rotation, but it's not thought to be a big issue.  Of course mice are smaller than people, so gradient effects would be more pronounced.  I think your ~4rpm would be quite reasonable though, maybe take a little getting used to but I wouldn't write it off.
Structural integrity of the asteroid is more of a problem.

[kkattula2]

IIRC 1 rpm is preferable, but 2 won't affect most people. 4 rpm will adversely affect many people. 8 to 10 is only tolerated by a small percentage.

Re: nickel-iron asteroid: Melt the whole asteroid to seal the cracks, then insert a pipe and pump nitrogen into the centre. Blow it like a glass bulb.

[TyMoore]

I have to agree that most asteroids are going to be rubble piles, especially S-type silicate bodies--I would seek a nickel-iron asteroid that already has a high spin rate (in excess of what is needed to centrifically disassemble it.) Also, I think that any potentially 'colonizable' body will have plenty of useful materials in it...

Have you considered the idea of building a rotating habitat within a chamber hollowed out of the asteroid instead of spinning the whole asteroid? In this way, the rest of the bulk of the asteroid can provide the necessary reaction forces for an internal habitat to spin 'against.' In this way, you can have a decent habitat, something to spin against (i.e., using electric motors for spin/despin,) and scads of radiation shielding, plus you won't have the trouble of having an asteroid with a shifty spin axis due to changes in the asteroid's moment of inertia due to mining.

Anyways, Dr. Theodore W. Hall has done some excellent mathematical and biometric studies of
'simulated' gravity rotating habitats. And his results are very, very interesting, especially those
concerning comfort levels in a spinning habitat. It seems that dizziness will set in for humans if the spin rate exceeds two rpm. So a spin rate no greater than 1 rpm is what we should be aiming for--design a habitat around a 1 rpm rotation with the desired level of 'g' and this will tell you how big it should be!

Theodore Hall's Curriculum Vitae contains many interesting, down-loadable papers (in PDF format) at:

http://www.twhall.com/

Another interesting one can be found here:

http://www.spacefuture.com/archive/artificial_gravity_and_the_architecture_of_orbital_habitats.shtml


This is just about the most extensive study I have ever seen on spinning habitats and comfort levels for humans, and it is done in a pretty rigorous style. Space Architecture can't just be about what is possible, it must be also about what is physiologically practical for the human beings who will live there!

[publiusr]

Generic Username - 25/10/2007  12:29 AM

[Back in the early 1960's, when Men were Men and Aerospace Engineers dreamed BIG, Dandridge Cole of Martin Co. suggested making arti-g colonies from nickel-iron astorids.
Wouldn't work (the steam would find vents *long* before the metal could soften that much), but damn was it thinking big.

 I was wondering about molten asteroids close to the sun with some type of explosive (surrounded by meters of tile) to explode a molten blob into a rough sphere all at once.

We need men like Dandridge Cole. Sadly, however, we have men opposed to them who hardly have any vision for space--men who need motion sickness tablets because continental drift makes them dizzy.

[Generic Username]

publiusr - 26/10/2007  10:01 AM
 I was wondering about molten asteroids close to the sun with some type of explosive (surrounded by meters of tile) to explode a molten blob into a rough sphere all at once.

Two things:

A: You need not be close to the sun, you just need sufficiently big mirrors. Should be easy to do if you're up to melting asteroids.
B: You don;t want to detonate, you want to slowly inflate. Detonations will simply shred the thing.

[Tom Ligon]

I've thought about some of these ideas, and even used a "bottle-blowing" scheme in a story once.

Our initial efforts will almost certainly involve small-scale milling and smelting operations, nothing so grand as an all-at-once fabrication of a large habitat.  The catch will be that we'll have to start out with facilities we can haul up economically.  

I would expect the first efforts at shell-blowing, if it is ever done, will start with refined bulk material.  In my story, I supposed this was a byproduct of extracting whatever high-value "unobtainium" got us out there.  Scandium had gone sky-high in price when I wrote the story, and so I supposed scandium and the rare earths.  By using refined material, you avoid all sorts of slag inclusions which could result in leaks, stress concentrations, etc.  

Bubbles would also be a potential problem, as would chill cracks due to non-uniform cooling.  It is not that the technique could never work, but we'll start by making smaller pieces due to starting with small fabrication equipment and lowered risk of making a big piece of scrap metal.

One facility that would probably help would be a "vacuum bottle".  The environment is naturally a vacuum, so the primary heat loss from molten metal will be radiation.  To keep from losing heat during the melting process, a reflective envelope (not necessarily massive) would be helpful.  Very light gas pressure could even keep a foil "bag" inflated.