Spinning up rocks

[Nuke]

i finished "reading" leviathan wakes (it was an audiobook). anyway i had seen the show and had been wondering how gravity worked on some of the rocks, not really being high enough to allow for any health benefits or even getting around. i just assumed they used their mag boots on ceres, eros, etc. the show really didnt elaborate very much. but in the book these rocks are spun up so they have backwards gravity from what you would expect (would explain why the airlock was in the floor). but wouldnt spinning up a rock such as ceres just cause it to fly apart? would it be possible re-inforce them so they dont do that?

[Starman4308]

A rubble pile asteroid would indeed fly apart. Even more sturdy bodies would likely suffer a huge loss of mass if spun up so far above surface gravity, unless you artificially reinforced them.

However, by the time your civilization can spin Ceres up to that speed, it's clearly extremely high tech. 10^20 kg of asteroid does not change its rate of rotation without a gigantic input of energy/rotational momentum.

EDIT: Argh out of likes. Still, an insightful question.

Edited March 6, 2018 by Starman4308

[Scotius]

Wait. Backwards gravity? As in - working not towards the center of mass but outwards? How is that supposed to work without some piece of magic tech?

[Starman4308]

33 minutes ago, Scotius said:

Wait. Backwards gravity? As in - working not towards the center of mass but outwards? How is that supposed to work without some piece of magic tech?

Simulated artificial gravity via a centrifuge that is a hollowed out asteroid. Or a hollowed out dwarf planet.

It'd get much weaker towards the poles, now that I think about it.

If you're thinking the concept is absurd: it is.

[Nuke]

11 minutes ago, Starman4308 said:

Simulated artificial gravity via a centrifuge that is a hollowed out asteroid. Or a hollowed out dwarf planet.

It'd get much weaker towards the poles, now that I think about it.

If you're thinking the concept is absurd: it is.

its so absurd that i never figured it out watching the expanse that this was going on. i just assumed it was too nuts an idea for a hard(ish) sci-fi. my brain just automatically filtered out that possibility. i had to read the book to understand that this was the case and that i didnt catch the cues there were in the show.

Edited March 6, 2018 by Nuke

[Bill Phil]

Yeah. Once you can spin up dwarf planets and keep them together, you've reached clarketech, end of.

[YNM]

2 hours ago, Starman4308 said:

If you're thinking the concept is absurd: it is.

You'd have to strengthen the outside of the rock.

[Shpaget]

A civilization capable and considering the asteroid spin up endeavor would certainly be capable of building a habitat of the same size from scratch. That sounds much cheaper, simpler and would result in better final product. For starters, you don't need the huge structural elements that carry billions of tons of useless rocks above your head. Entire structure would be much simpler to build, require less energy to spin up, offer better control of structural integrity, present fewer surprises (space station geology is not really a thing), and could be built in a shape more sensible than a sphere, such as a cylinder.

[Nuke]

2 hours ago, YNM said:

You'd have to strengthen the outside of the rock.

some kind of metal netting perhaps. big 6 inch cables in some kind of woven lattice and tensioned around it. maybe driving structural steel through the core and welded to support panels to hold large areas of surface in place, and thats a lot of structural steel. just putting that through would likely put extra stress fractures into the rock making the task of holding it all together even more problematic. on ice rocks maybe some kind of systematic melting and re-crystallization while simultaneously introducing a rebar lattice to the mix. thinking of the kind of engineering that would be involved here is rather mind boggling.

Edited March 6, 2018 by Nuke

[NSEP]

Yes, an asteroid is mostly just a bunch of gravel and dust, so it wil rip apart.

You could just hollow out an asteroid and put a spinning cylinder inside, so you wont need to spin the asteroid itself.

I think Isaac Arthur made a video on it.

[YNM]

14 minutes ago, Nuke said:

some kind of metal netting perhaps.

Was thinking so, but I guess we have better materials.

[Nuke]

3 minutes ago, NSEP said:

Yes, an asteroid is mostly just a bunch of gravel and dust, so it wil rip apart.

You could just hollow out an asteroid and put a spinning cylinder inside, so you wont need to spin the asteroid itself.

I think Isaac Arthur made a video on it.

i always imagine huge but not impossible conical (or perhaps cylendrical with staggerd slanted decks) centrifuges on any planet, moon or asteroid that has significant gravity but not enough to support human life. the centrifuge would start as a vertical pit dug deep into the surface. the centrifuge would be built on magnetic berings inside the pit with the top domed over and covered with regolith. decks are angled so that the combined natural gravity and artificial gravity is 1 g. i think thats how i would go about colonizing places like ceres. 

[Bill Phil]

3 hours ago, Nuke said:

i always imagine huge but not impossible conical (or perhaps cylendrical with staggerd slanted decks) centrifuges on any planet, moon or asteroid that has significant gravity but not enough to support human life. the centrifuge would start as a vertical pit dug deep into the surface. the centrifuge would be built on magnetic berings inside the pit with the top domed over and covered with regolith. decks are angled so that the combined natural gravity and artificial gravity is 1 g. i think thats how i would go about colonizing places like ceres. 

Ceres's gravity is so small that your floors/walls will be almost vertical.

5 hours ago, Shpaget said:

A civilization capable and considering the asteroid spin up endeavor would certainly be capable of building a habitat of the same size from scratch. That sounds much cheaper, simpler and would result in better final product. For starters, you don't need the huge structural elements that carry billions of tons of useless rocks above your head. Entire structure would be much simpler to build, require less energy to spin up, offer better control of structural integrity, present fewer surprises (space station geology is not really a thing), and could be built in a shape more sensible than a sphere, such as a cylinder.

See: McKendree cylinder. Just one would have much more area than Ceres. Then we can use Ceres as a source of shielding material.

Or just build a cluster of smaller habitats.

 

Edited March 6, 2018 by Bill Phil

[YNM]

11 minutes ago, Bill Phil said:

Then we can use Ceres as a source of shielding material.

I guess this is the reason why such hollowed-out habitats are suggested. Unless entirely processing the whole asteroid into usable raw materials are cheaper.

[Bill Phil]

48 minutes ago, YNM said:

I guess this is the reason why such hollowed-out habitats are suggested. Unless entirely processing the whole asteroid into usable raw materials are cheaper.

Probably cheaper than spinning the entire thing. 

Doing some back of the napkin math, it would take 100 years and 10^17 watts of power to spin up Ceres to a speed for 1/3 g, not accounting for losses. For comparison, we generate about 18 terawatts (10^12) on average. Breaking apart Ceres, based on more back of the napkin math, takes about 10^16 watts over 100 years, assuming no losses. Assuming 10 tonnes per square meter for shielding, and that shielding dominates (over 95%) mass for orbital habitats, then we get almost 10^17 square meters of land. Similar energy requirements, but breaking apart the asteroid gives us more land and we can spin that land up to provide 1g. Not to mention that Ceres would probably break apart if put under that much rotation, since it'd be rotating at over 1 km/s.

If all we wanted was equivalent land area to Ceres, the energy requirement would be even less.

[YNM]

7 minutes ago, Bill Phil said:

back of the napkin math

Well, I hope we're talking of the right Ceres...

[Nuke]

9 hours ago, Bill Phil said:

Ceres's gravity is so small that your floors/walls will be almost vertical.

See: McKendree cylinder. Just one would have much more area than Ceres. Then we can use Ceres as a source of shielding material.

Or just build a cluster of smaller habitats.

 

did some math after i posted that and it would only be like 1.7 degree slope on the inside of the centrifuge. 

Edited March 6, 2018 by Nuke

[roboslacker]

In Babylon's Ashes it was mentioned that the process of spinning up an asteroid involved melting down the whole thing to strengthen it. But it's still probably easier than trying to build Rama.

[Nuke]

1 minute ago, roboslacker said:

In Babylon's Ashes it was mentioned that the process of spinning up an asteroid involved melting down the whole thing to strengthen it. But it's still probably easier than trying to build Rama.

im curious at this point how far i can read into the books without spoiling the next season of the expanse.

[mikegarrison]

1 hour ago, Nuke said:

im curious at this point how far i can read into the books without spoiling the next season of the expanse.

Not far. On the TV show they have really only covered the first book and maybe half of the second one.

[mikegarrison]

It's clearly plot-driven: the origination of the story was a role-playing game where you had three factions (Earth, Mars, and Belt). The Belters had to live somewhere, and they knew there wasn't enough gravity on even the minor planets. Plus, they had a backstory that said Mars and Earth had already mined all the water off the asteroids, leaving them riddled with tunnels. So they were sealed off and "spun up" to make them sort of natural space stations.

I think they hadn't considered the structural integrity of the asteroids, or even what axis you would spin something like Eros on, but for the purposes  of the story it's just handwaved away as "the asteroids were reinforced first".

[kerbiloid]

Spoiler

19 hours ago, YNM said:

Was thinking so, but I guess we have better materials.

Tar?! Make an asphalt asteroid?

It was suggested an idea: find two asteroids going to pass near each other at close distance, reinforce them with polymer/nanotube patterns, and connect them with a cable on their flyby.
One will be a habitat, another one - a counterweight.

Edited March 7, 2018 by kerbiloid

[YNM]

20 minutes ago, kerbiloid said:

Tar?! Make an asphalt asteroid?

No, it's FRP. Often used to strenghten columns, basically to stop them from crumbling apart.

[Nuke]

20 hours ago, mikegarrison said:

Not far. On the TV show they have really only covered the first book and maybe half of the second one.

thanks for the heads up. i like to watch the show first and then read the book, that way i'm not disappointed in either. i was sort of a late comer to the party, didnt expect anything good out of syfy ever again, but they supprised me. 

Edited March 7, 2018 by Nuke

[Bill Phil]

On 3/6/2018 at 4:38 PM, roboslacker said:

In Babylon's Ashes it was mentioned that the process of spinning up an asteroid involved melting down the whole thing to strengthen it. But it's still probably easier than trying to build Rama.

Not really. If they could spin up Ceres within a generation they are at the point where they have so much energy to throw around that it doesn't matter. If they just wanted habitable area, they'd get more bang for the buck if they built separate rotating sections to live in. Could be 1g, too. It's a matter of engineering, and they could probably pull it off. 

The population of Ceres is, what - 6 million? That's less than the maximum population of an O'Neill cylinder. Not only would it be easier to build an O'Neill cylinder than spin up Ceres, life would probably be better in the cylinder than in Ceres. After all, the cylinder could easily have blue skies.

Edited March 8, 2018 by Bill Phil