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daniel l.

Pocket Planets?

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Now suppose, what if a method was invented to generate spherical artificial gravity? I believe it would be possible to easily place one of these generators in the core of a small Asteroid like Itokawa or Dactyl, and by tuning it to 1g, effectively transform the Asteroid into a small planet, Atmospheric gasses could be easily shipped in, and any ice inside the asteroid could be used to make surface oceans.

A rubble-pile asteroid like Itokawa would undergo the transformation in a very interesting manner, it would appear to liquefy. Imagine it stretching and squeezing back and forth like silly-putty before finally coming to stability as a perfect sphere, with any large chunks of rock jutting out as mountains and continents.

I imagine it would be an awesome thing to see, and a rather interesting experience to live on. Would the sky still be blue? The atmospheric height would not extend as far as that of a true planet, certainly not enough to scatter light, so I believe it would probably remain black, a rather interesting experience indeed!

And think of the horizons! On Itokawa, for example, objects or structures just a few hundred feet away would quickly fall beneath the horizon, and the curvature of the world would be evident immediately.

But pocket-planets could have many awesome uses! Outposts? Penal colonies? Private property for rich space-gazillionaires? Not to mention good old habitation in general!

What do you guys think of this idea? 

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Posted (edited)

Vescape = sqrt(2gR)
Escape velocity of an Itokawa-sized object will be ~ sqrt(2*9.81*500) ~100 m/s. So, gasses will be happily escaping unless you put this thing into a protective bubble (or a dimensional gap, or a force field).
If put this into a plastic bubble,  sky will be black and full of stars.
Also you need a magnet, or make the bubble enough thick to withstand the radiation.

Mountains and pits will be still ~10 km high/deep, so this limits this planet with ~10 km size to keep a custom shape. The closer to 10 km - the rounder.

***

I thought about such thing long ago, inspired by Farmer's Maker of Universes / Private Cosmos book series,
Probably, the easiest way to do this is to locally increase the gravity constant (what, I believe, happened in KSP and Spore universes).

Also maybe the most useful shape of a pocket planet is a heavy torus with a wide hole inside.
Torus makes gravity with its mass. Gravity field of torus looks somewhat close to an ellipsoid.
You can select such parameters of torus big and small radii that this ellipsoid of gravity will be close to a thick flat disk and you get a near-flat, erythrocyte-shaped planet .

Such planet will have two flat sides, but you still have normal gravity walking over its edge.
Also you can place a mysterious lake near the center, having two surfaces and zero-G in mid-water.

The upper size can be a dollhouse, with flat round map. A valley or laguna surrounded by a hill ridge.
(Edge Ridge or Ridge Edge - can't decide).
It's a park to live and have pleasure.

A castle built on one side of the ridge allows to watch the whole valley/laguna and also look at stars.
Cable transport crosses the valley. A railroad runs along the ridge.

Architecture is mostly built along the ridge, leaving the valley more or less empty. Rear parts of the buildings grow inside the ridge.

The "down" side of the planet would be a heatshield. The owner would turn it to danger or to the Sun if wants a night.
The heatshield would consist of some light (antonym to "heavy") material.

Inside the downside - storehouses, etc.
As they are directed 180 degrees to the castle (antipodes), this is a great cave, with the "down" side of the lake beneath and a stone cupola (that heatshield) above the head.
It's filled with stone foam or truss structures, maybe molded basalt armored with metal, or so.


(Also this Antipode Cave is the pocket underworld where it's comfortable to make weird things.)

From afar, a spaceship would first feel its gravioty as if it were a usual material point.
Approaching to the planet it will land as onto a flat surface.

Probably the only stable orbit close to the planet lays in its equatorial plane.
But with such planet size you don't really need satellites, just towers.

Another advantage is that the owner's personal gunboat space yacht needs just 100 m/s to fly away, so it can be launched by slaves in many different ways.

Energy source - probably radioactive "hot stones" inside the Lake.
I.e. RTGs.
They keep water warm and don't need a servicing. Just put them inside thick tungsten barrels.

***

(Sounds music from "Gangsta's Flat-Earther's paradise.")
 

Edited by kerbiloid

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Posted (edited)

This isn't that great of an idea... theoretically you could make a shell world if you have a black hole and can control it, but that takes too much mass. It'd be more efficient to build Bishop rings or Mckendree Cylinders. You would need about 1.47e19 kilograms for a shell world with a ten kilometer radius. You'd only get about 1200 square kilometers. You could just put two O'Neill cylinder pairs near each and get about the same area. That'd only be about 1.6e13 kilograms. Much easier to deal with. And no black holes involved.

Although tiny planets are still cool.

The reason I mention black holes here is because that's the only known way of getting enough mass into a small enough size. Maybe neutron star density matter could work, but that's arguably more difficult. Of course, black holes have their own issues.

Edited by Bill Phil

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4 hours ago, daniel l. said:

What do you guys think of this idea? 

The gas would leave in a giant whoosh due to low escape velocity, as @kerbiloid points out.  Planets retain atmospheres not because of high surface gravity, but because of high escape velocity.

If you're positing Star Trek fantasy-science artificial gravity, then you could also posit some sort of "force field" to keep the air in, I suppose.  Or, more prosaically, a physical barrier like a plastic bubble (though it would have to be pretty darn strong to contain the pressure, unless you built it very large).

However, if you're gonna do that... it seems far more practical to build the habitat inside-out.  Take a big nickel-iron asteroid, melt it, shape it into a cylinder, inflate it, spin it up to provide "centrifugal gravity" on the inner surface, then fill it with air.  Run a "sun tube" down the axis to provide internal illumination.  Quite a few science-fiction stories have this concept in them-- it always appealed to me because it doesn't require any Star Trek technology.  It's gargantuan engineering, but doesn't require any speculative science.

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5 hours ago, kerbiloid said:

Vescape = sqrt(2gR)
Escape velocity of an Itokawa-sized object will be ~ sqrt(2*9.81*500) ~100 m/s. So, gasses will be happily escaping unless you put this thing into a protective bubble (or a dimensional gap, or a force field).
 

Even on a relatively huge Earth we generally don't see winds that fast; the fastest recorded are in that ballpark, though. Those are obviously extremes. On a tiny world like Itokawa, no such weather systems could form. Largest enclosed spaces on Earth (Boeing factory, Vehicle Assembly Building, Aerium...) are approaching the order of magnitude of the enclosure that could encompass Itokawa, yet weather patterns are barely forming there, with them being mostly just some wispy clouds.

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Posted (edited)

16 minutes ago, Shpaget said:

Even on a relatively huge Earth we generally don't see winds that fast; the fastest recorded are in that ballpark, though. Those are obviously extremes

Heat velocity of air molecules is ~500 m/s (at room temperature).

V = sqrt(3 * 8.31441 * T / MolarMass) = sqrt(3 * 8.31441 * 290 / 29e-3) = 500 m/s

Edited by kerbiloid
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Posted (edited)

It's not the wind, it's temperature. Air molecules at 20°C are fast (500m/s or so). "swoosh" *gasp* ...

But, if you can make a gravity generator :-/ you might tame molecules as well. Herding the fleas.

Edit: sorry, @kerbiloid, i didn't realize your post  !

Edited by Green Baron
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Haven't though of that. :)

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6 hours ago, Bill Phil said:

…theoretically you could make a shell world if you have a black hole and can control it…

AFAIK such small black hole would have very steep gradient rendering it even less practical.

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47 minutes ago, radonek said:

AFAIK such small black hole would have very steep gradient rendering it even less practical.

Well you just need about 1g over a few meters. 

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3 hours ago, kerbiloid said:

Heat velocity of air molecules is ~500 m/s (at room temperature).

2 hours ago, Green Baron said:

Air molecules at 20°C are fast (500m/s or so).

^ This.

And actually, it's even worse than that, if your goal is to retain gases.  Even if your escape velocity were, say, 1000 m/s, you'd still be losing air at 20°C.  Why?  Because not all molecules are going the same speed.

The "500 m/s" number is just an average-- actually, it's a Gaussian distribution around that value.  There will be some molecules going faster, and some going slower.  There will be a tiny but non-zero number of molecules going a lot faster than that.  So, what happens?  You have a system that, for whatever reason, has an equilibrium temperature of 20°C.  Let's say your escape velocity is, oh, 2000 m/s.  There will be some tiny number of molecules moving faster than that, located high enough in the atmosphere that they have a long mean path (i.e. aren't likely to bump into another molecule and lose that energy).  So those molecules go flying away into the depths of space and are gone forever.  This means that the average temperture of the atmosphere goes down just a smidgeon (because it lost its highest-velocity molecules).  Then whatever processes (sun, geothermal, whatever) that maintain the equilibrium at 20°C bring it back to that temperature again.  And repeat the process.  Over time, the atmosphere slowly, slowly leaks away.

That's why, for example, the only planets that have significant amounts of hydrogen in their atmospheres are gas giants.  Why?  because a terrestrial-sized planet simply can't hold on to them.  Hydrogen molecules are very lightweight, which means they have a higher velocity at a given temperature than heavy molecules do.  The mean speed of hydrogen molecules is lower than Earth's escape velocity... but it's not enough lower to hang on for millions or billions of years.  So it all evaporates.  Gas giants can hang on to the hydrogen because they have a much higher escape velocity, and also (in our own solar system, at least) are significantly lower temperatures.  (Presumably you could have a terrestrial planet with a hydrogen atmosphere if it were both fairly massive and very cold, but our own solar system has no such body that we know of.)

 

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Posted (edited)

56 minutes ago, Snark said:

...

The "500 m/s" number is just an average-- actually, it's a Gaussian distribution around that value.

...

 

I mean technically it's a Maxwell-Boltzmann distribution, but it's close enough :P

Edited by Steel
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On 19. 6. 2017 at 10:27 PM, Snark said:

…So those molecules go flying away into the depths of space and are gone forever. 

Well, if anything KSP taught me that flying into depths of space is not that easy :-) Applying same statistics, shouldn't most of those molecules end at orbit similar to home planet with a good chance to be recaptured?

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1 hour ago, radonek said:

Well, if anything KSP taught me that flying into depths of space is not that easy :-) Applying same statistics, shouldn't most of those molecules end at orbit similar to home planet with a good chance to be recaptured?

Yes, it can happen.  Depending on how the orbits are set up, you can end up with the planet orbiting inside a thin, hyper-attenuated "torus" of gas, with the centerline of the torus coinciding with the planet's orbital path.

(And this has been the topic of some interesting science fiction, too-- Larry Niven's The Integral Trees is a neat world-building concept.  The idea there is that there's a planet in a low orbit around a neutron star, and the planet leaks enough gas to fill up a torus thickly enough to have a human-habitable pressure near the centerline.  So it's basically a naturally-occurring "atmosphere without a planet" where people live in a zero-gravity, three-dimensional environment, without spacesuits.  But I digress.)

The thing is, though, that gas can escape even from the torus.  Plus, the torus is gargantuan compared with the size of a planet.  To put things in perspective, a torus around the Earth's orbit, with a tube radius of, say, 1% of the orbital radius, would have a volume of around 6.5e30 m3, which is roughly six billion times the volume of the Earth itself, which in turn is quite a few orders of magnitude bigger than the volume of the Earth's atmosphere.  And a gas torus would have a tube radius considerably bigger than 1% of the orbital radius.

What it boils down to is that a gas torus like that is going to have a volume that is going to be at least trillions of times the volume of the planet's atmosphere, making it, in effect, a bottomless pit.  It's basically impossible to fill up.

So when I say the molecules are "gone forever", that's not necessarily technically true-- it may be that a vanishingly tiny number of molecules eventually end up blundering back into the planet's neighborhood and get re-captured.  But if the mean expectation time for recapturing a molecule is significantly longer than the life expectancy of the solar system, it might as well be "never".

The best real-world example of the gas-torus phenomenon (that we know about) is probably Jupiter's moon Io.  It's incredibly volcanically active, spewing huge volumes of ejecta (including gas).  Io's gravity is too weak (escape velocity 2500 m/s) to hang on to much of an atmosphere; about 1 ton per second "leaks" away.  Even though the gas can escape Io, however, it has a harder time escaping Jupiter (takes another 7200 m/s to escape Jupiter, after escaping Io).  So you end up with a gas torus around Jupiter, centered on Io's orbit.

It's a significant effect.  The gas gets ionized and then whipped to a frenzy by Jupiter's brutal magnetic field, to the point that the whole area is hideously unhealthy even for unmanned probes (electronics tend to get fried, quickly).

But even with all that going on-- Io still has practically no atmosphere.  Even though it's been dumping 1 ton per second into Jupiter orbit for goodness knows how many millions (or perhaps billions?) of years, Io's "atmosphere" maxes out at 3e-4 pascals, or about three billionths of Earth's sea-level pressure. (On the night side, it goes down to under a trillionth.)  So, basically, a pretty darn hard vacuum.  If you could stand on Io, in the brief interval before the radiation fries you, you'd need a pretty darn sensitive instrument to detect that it's there, if you're going by pressure rather than radiation levels.

Think about that for a moment.  The entire mass of Io's atmosphere, over the entire globe, is something on the order of 10 million tons. That's equal to the amount of gas that it "leaks" to jovian orbit in about four months.

Which essentially proves the point.  If anywhere ought to have a torus-fed atmosphere, it ought to be Io.  Stable orbit.  Tiny torus, by interplanetary standards (it's a ring around Jupiter, not the Sun.)  High-volume, continuous gas feed that leaks a mass equivalent to its entire global atmosphere, every four months, for many millions of years.  And yet, only manages to hang on to a few billionths of an atmosphere.

So, yeah.  Might as well be "gone forever."  :wink:

 

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The earth does actually as well draw a tail of gas behind it on its orbit and thus looses a small amount of gas constantly. Atmospheric gases (as well as many other elements) are constantly renewed and exchanged in the course of the various geological cycles on earth, in this case plate tectonics play a presumably vital role in storing and releasing elements over medium- to long term cycles (e.g. Wilson cycles).

It is all not that easy ... :-)

Edit: the earth's gas tail is mostly lost because it is blown away by radiation pressure from the sun. When the earth comes next year to look for it it is up and away :-)

Edited by Green Baron

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Randall Munroe of xkcd's what-if discussion of the topic : https://what-if.xkcd.com/68/

 

Basically :

- It's hard to keep standing.

- It's hard for staying there.

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