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#1 2013-03-27 08:15:14

Terraformer
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From: Logres
Registered: 2007-08-27
Posts: 3,362
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Rogue super-Earths/mini-Neptunes...

... or, perhaps, ones that are gravitationally bound to the Sun but deep in the Oort cloud.

It seems likely, judging by the examples of Uranus and Neptune, that any planets of about Earth mass in the Oort cloud will be more like mini-Neptunes, possessing crushing atmospheres of hydrogen, helium, ammonia and methane over an ice mantle with a rocky core. Given the scarcity of hydrogen where they formed, though, I'd expect them to have a much greater proportion of methane and ammonia. Which is good news for anyone looking to colonise the planet itself, since it means there's both more carbon available for building and more buoyancy can be obtained from our hydrogen aerostats (and more lift from our wings). Given the upwelling of heat from the interior that we've seen in the ones we know about (Uranus and Neptune), we might not even need fusion power to live there (and, if they're only a light month away, fission would suffice to get us there).

But, I'm not so much interested in colonising the world itself as much as I am the system of moons it would likely have, because I'm a Sci-Fi writer and we like to have extensive systems to set our adventurers loose in, and this is why I've made this post in Terraforming.

First question - do we need a worldhouse? Given the extremely low temperatures we could expect the exosphere to reach, and the lack of any appreciable solar wind, do we need a structure to curb atmospheric escape? If the planet (okay, I'm calling it a planet, in the same way I'd call Titan or Europa planets) is about the size of Titan, we wouldn't need one, but how small could we go? It seems possible that a very small (1-2% surface gravity) world wouldn't actually need anything, since a nitrogen atmosphere would start to condense out rather than escaping (driving an interesting case of atmospheric convection...?).

Second question - power. This is where a worldhouse would really shine (as well as allowing us to make the atmosphere breathable in this millennia), pun not initially intended. Power could be delivered to the worldhouse, which would probably have an awful lot of LED lighting embedded in it, which would sustain the world below. Alternatively, a sunline could be constructed around the primary, which would deliver light to the planets orbiting it. The sunline could be powered by fusion, or it could be powered by the primaries own thermal stockpile.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#2 2013-03-27 18:20:48

Void
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Registered: 2011-12-29
Posts: 3,884

Re: Rogue super-Earths/mini-Neptunes...

Well I found this, which is related.

http://news.nationalpost.com/2012/11/14 … smic-void/

http://www.cfa.harvard.edu/news/2012/pr201212.html

http://www.livescience.com/19753-rogue- … stars.html

But you are talking about mini-Neptunes formed from the parent stars materials, and I am just killing time.

Otherwise I would suggest gravitational flexiing and magnetic fields as a source of energy.

Other than that, if you like space elevators that only reach down into the upper atmposphere, and want to link more than one in a ring around the planet, there is the possibility of gathering electrostatic energy (lightning), from the weather systems of your mini-Neptunes.

I have a big problem in such a system seeing how you get metals to work with.  They are burried under deep ice.

The exception could be a moon like Enceladus of Saturn.  A case like that might permit access to core materials.  Or perhaps a ancient collision might have laid a core somewhat exposed by blowing the ice shell off of a moon.

Alternately a Veus ejected to the outer solar system might collect Hydrogen, cool off, develop plates and have volcanism, and mid-ocean ridges where a canyon between ice sheets might reach to rock, maybe warm/hot rock.

Last edited by Void (2013-03-27 18:48:50)


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#3 2013-03-28 06:12:47

Terraformer
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From: Logres
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Re: Rogue super-Earths/mini-Neptunes...

Well, I'm presuming that the trace metals we need we'd mine from the debris (asteroids) orbiting the rogue planet. But most of what we need (Hydrogen, Carbon, Nitrogen, Oxygen, Sulphur and Phosphorus) we can collect from the atmosphere (I think Phosphorus forms gaseous compounds?). We can use Carbon for most of our building needs. And for our electronics. And for our catalysts. And for pretty much everything else we need it for...

It will be a lot easier if we master fusion first.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#4 2013-03-28 08:06:43

Void
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Re: Rogue super-Earths/mini-Neptunes...

OK, I guess you are likely right.  The mix-master would have ejected some small dry bodies from the inner solar system to the outer solar system.

Good point, it makes your plan workable.


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#5 2013-03-31 10:16:54

karov
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From: Bulgaria
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Re: Rogue super-Earths/mini-Neptunes...

Terraformer,

I think we do not need fusion there.
There is plenty of geothermal energy.
Also "geo"magneto-dynamic one.
Neither global tenting necessary, cause really huge colonial rafts could float in the atmosphere by just containing 20 C atmosphere. ( Like Cloud Nine tensegrity spheres of Buckminster Fuller's ). Given the mega-trend of hyper-uranization, I doubt that the colonist will bother to rectrate Earth-like wilderness.
The cold outside will be a real blessing cause it provides for really deep heat sink hence thermal engines will work with higher Carnot efficiency.
I think your story will be much more solid and interesting if you use only present day proven tech.
If the colonist decide to really terraform the body some day ( which I doubt because they'll lose the advantages and wealth of the cryo-fluid ready environment ), they could use the local space resources to build a solar concentrator.
On distance of "about a light month" which is roughly 1 000 000 000 000 km or 5500 AU, the aperture of this giant soleta or lense between the star and planet would have to be "only" 5000 times the planetary diametre.

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#6 2013-04-02 08:03:24

Void
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Re: Rogue super-Earths/mini-Neptunes...

I am wondering, working with floating habitats (Which both of you seem to be indicating) at 20 degC, is there a energy potential to tap just from air currents?

Differing wind flows at differing levels?  Windmills.  Maybe nano-windmills all over the outer surface of such a habitat, and perhaps "Sea anchors" hung below the device, to catch a differential air current below?  I think I have seen that planets like Neptune and Uranus have significant winds.

Last edited by Void (2013-04-02 08:04:40)


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#7 2013-04-02 10:47:42

karov
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Re: Rogue super-Earths/mini-Neptunes...

Void,
Absolutelly - tapping the atmospheric kinethic energy is obigatory.

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#8 2013-04-02 12:39:25

Terraformer
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From: Logres
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Re: Rogue super-Earths/mini-Neptunes...

When I said terraforming, I was referring to the bodies that are likely to orbit such worlds - my thinking is that power could be beamed to them from the central body, which should have a surplus of energy if similar worlds in our system are reliable indicators.

karov, I'm not sure the pressure difference between a trimix atmosphere at 293K will give that much lift in a Helium/Hydrogen/Methane atmosphere, even when said atmosphere is at 50K (would there even be much methane at such a temperature?). We probably will need additional lift from hydrogen gas bags. Still, I can quite easily see large sky islands on such worlds, measuring many square kilometres. Carbon aerogel will suffice for insulation.

It would be an interesting place to live.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#9 2013-04-02 22:25:08

Void
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Re: Rogue super-Earths/mini-Neptunes...

A real challenge for human habitation, but who can say never?

If not then maybe a robotic system that beams power to the moons of such a world.

If not floating in the atmosphere, then maybe tethers dangled "Space Elivator" like, to dip windmills into the winds?  Not easy either.

Even more a challenge, but could the "Slushy" ices of silicates and water, methane, etc, somehow be accessed?  I know that heat, very very high heat is an issue, but maybe at those pressures, machines of a new type of new unknown materials, to compose robots/machines for the purpose?

Lots of materials.  I know that some would say that a space elivator is a challenge, a nasty of even for Earth, but the graviation for Saturns and Neptunes are not that far from our 1 gee.  Maybe a mini-Neptune would have even less gravitation?


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#10 2013-04-03 05:33:13

karov
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From: Bulgaria
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Re: Rogue super-Earths/mini-Neptunes...

Terraformer,

I thought you meant the "neptune" itself, cause they have pretty much Earth-like surface gravity ( though much higher escape velocity ).

About the lifting force of a 300k breathable air bubble vs. 100k gas environment ... it must float. There must be some calculator reflecting temperature, density, chemical compsition ( molar masses ) within the expanses of the vast Net.

Perhaps Midoshi can help here.

Carbon foams - yes: insulation, structural material ... There is plenty of carbon in such atmosphere to be condensed and grown into nano-3d-grid structures by some coral-like replicators.

http://en.wikipedia.org/wiki/Cloud_nine … ty_sphere)

... and, yes, Void is right - it is mini-neptunes not rogue 'neptunes'. So escape velocity won't be an issue.

The satelites of such planet are like massive water dams containing astronomical quantities of potential energy. They could be downloaded - in rough or processed in ready goods state - thus delivering the necessary energy for the biosphere to run for million / billion years.

http://archive.is/5in9  -- Paul Birch's archive.

Last edited by karov (2013-04-03 05:45:56)

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#11 2013-04-03 10:11:15

Terraformer
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From: Logres
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Re: Rogue super-Earths/mini-Neptunes...

I'm glad to see there's an archive of Paul Birch's stuff. Sad to hear of his death.

The density difference between a gas at 100K and 300K is 9 times. If the atmosphere is about twice the density of hydrogen (so the density of helium), it would be ~0.178kgm^-3 at 300K, so ~1.6kgm^-3. Air at 300K is about 1.25kgm^-3... we could use it as a lifting gas, yes, but we'd need lots of ballrooms and palatial accommodation to get enough lifting force for our habitat.

If we take it down further, however, to where the air pressure is say 4 bars, and use a trimix atmosphere... the outside air density might be about 6kgm^-3, and the interior air density 3-4kgm^-3, giving us 2-3kg lift for each cubic meter of air we have. Meaning a hectare of parkland with a 500m ceiling could lift 10-15 kilotonnes. Obviously, we're going to need something to grow our flowers, crops and trees in, but I doubt that would take up a full tonne - that would be a pool of water 1m deep. I reckon we could get it down to 1-200kg per square meter, leaving us with quite a lot of lift for our other things. Like houses, and industry.

I do suspect that there will be such worlds in abundance in the Oort cloud. Any Mars/Earth mass world is likely to be surrounded by such an atmosphere as to make it a gas dwarf.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#12 2013-04-03 14:21:08

Void
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Re: Rogue super-Earths/mini-Neptunes...

I had some further thoughts.

Such worlds might come in a variety of types. 

Actually like Neptune,  or like an Earth with a thick atmpsphere with a bottom pressure such as at the bottom of the Ocean, and a continum between the two extreems.

I base this on other peoples speculation that a Rogue Earth tossed out of the solar system would freeze up and then would accumulate a very insulating layer of Hydrogen and Helium, and would have a pressure like the bottom of the ocean.  So much insulation that geothermal heat would be able to allow for liquid water on the surace in some cases.

Such a world as that would indeed allow mining of the surface, and perhaps some strange type of farming, either by induced lighting or by chemicals.
Of course robots would be in use.  At that end of the spectrum, however there would not be much wind to harvest energy from.

I suppose something that has an even denser atmosphere (But not as much as Neptune), might have a red hot surface, but no methane oceans, and the water would exist as vapors.  Not impossible to mine, and in that case perhaps an energy source.


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#13 2013-04-04 12:33:45

karov
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From: Bulgaria
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Re: Rogue super-Earths/mini-Neptunes...

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#14 2013-04-07 02:52:37

karov
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From: Bulgaria
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Re: Rogue super-Earths/mini-Neptunes...

Terraformer,

Pls, give some specs about your planet - mass, size, atmosphere composition... I think there is enough enthusiasm in this topic so we could play out most of the place here.

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#15 2013-04-07 06:57:51

Terraformer
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From: Logres
Registered: 2007-08-27
Posts: 3,362
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Re: Rogue super-Earths/mini-Neptunes...

Well, my initial thought was, what would planets be like if they had the mass of Terra and were accreted from the Plutoids that exist out there? I suspect they wouldn't be merely scaled up versions of Pluto - with so much heat, the Methane, Ammonia and Nitrogen those worlds have would form a very significant atmosphere, maybe so much that it would be more akin to Neptune than Venus. I don't think helium would form a major component in such worlds, due to the scarcity at such distances - compare Saturn and Uranus as an example. Hydrogen might, however, given the amount of Methane and Ammonia that could decompose. Alas, it might be predominantly Hydrogen, which poses problems for our Aerostatic colonies. It all would depend I suppose on whether the Methane settles out. If it's more like a planet with a 10 kilobar atmosphere, and temperatures of a few hundred degrees at the surface... maybe we can rely on convection to keep the atmosphere well mixed, in which case we should be able to keep our colonies afloat, especially if the atmosphere is dominated by Nitrogen and Methane (which would give our warm colonies several kilogrammes of lift per cubic metre at least, much like on Titan). If we can get some Helium - and I wouldn't be surprised if there was at least a little bit - and use a 2 bar trimix atmosphere, with an exterior air density about 11 times that of our colonies atmosphere, we'd be able to lift 10kg for each cubic meter of the colony, and about as much using hydrogen at the exterior temperature. That's enough that a small bedroom could lift itself... certainly, there'd be no worries about getting enough lift. A sphere about 3m radius would be able to lift over a tonne. Going back to the previous example of our hectare of parkland, and we're lifting 50,000 tonnes - 5 tonnes per square meter, so we can use actual soil to a decent depth. At such lifting ability, we could have massive floating islands, with ships flying and fighting between them.

If we want extra lift, we could maybe use spheres placed at a lower depth and build on top. If we could get 50kg for each cubic meter in those spheres, we could support... a lot of floating island.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#16 2013-04-07 08:53:46

StarDreamer
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From: Ontario, Canada
Registered: 2007-04-28
Posts: 92

Re: Rogue super-Earths/mini-Neptunes...

At the mass of Terra, hydrogen from decomposing gasses like ammonia and methane wouldnt stay on the planet but escape to space, over the long haul, which means by the time humans discover the planet, we wont see what it was like before it went rogue, if you know what I mean. In the abyss of Time that its origins lie, we will not be so lucky as to catch it early and see it in transition, but only in its new equilibrium. Without the tidal action of Luna or the community of planets and our Sol to act upon it, the centre of this little world should freeze out and tectonic geology cease with it.

I like the idea of planets by plutoid accretion however. By Bode's Law, the plutoids are where Neptune should be and Neptune should not be where it is at all. Which makes me think that Neptune has grown and is still growing by accretion and it is gouging out the near edge of the Kuiper Belt the way a river scoops land from one bank and deposits it on the other, to make a bend, a valley bluff and a floodplain. As Neptune picks up KBO's, it gains mass; as it gains mass, its orbit shifts outwards; as its orbit migrates out, it gouges out more and more of the KBO and so grows ..... Eventually, it will have a 250yr orbit at 1:3 resonance with Uranus.

Terraformer wrote:

Well, my initial thought was, what would planets be like if they had the mass of Terra and were accreted from the Plutoids that exist out there? I suspect they wouldn't be merely scaled up versions of Pluto - with so much heat, the Methane, Ammonia and Nitrogen those worlds have would form a very significant atmosphere, maybe so much that it would be more akin to Neptune than Venus. I don't think helium would form a major component in such worlds, due to the scarcity at such distances - compare Saturn and Uranus as an example. Hydrogen might, however, given the amount of Methane and Ammonia that could decompose. Alas, it might be predominantly Hydrogen, which poses problems for our Aerostatic colonies. It all would depend I suppose on whether the Methane settles out. If it's more like a planet with a 10 kilobar atmosphere, and temperatures of a few hundred degrees at the surface... maybe we can rely on convection to keep the atmosphere well mixed, in which case we should be able to keep our colonies afloat, especially if the atmosphere is dominated by Nitrogen and Methane (which would give our warm colonies several kilogrammes of lift per cubic metre at least, much like on Titan). If we can get some Helium - and I wouldn't be surprised if there was at least a little bit - and use a 2 bar trimix atmosphere, with an exterior air density about 11 times that of our colonies atmosphere, we'd be able to lift 10kg for each cubic meter of the colony, and about as much using hydrogen at the exterior temperature. That's enough that a small bedroom could lift itself... certainly, there'd be no worries about getting enough lift. A sphere about 3m radius would be able to lift over a tonne. Going back to the previous example of our hectare of parkland, and we're lifting 50,000 tonnes - 5 tonnes per square meter, so we can use actual soil to a decent depth. At such lifting ability, we could have massive floating islands, with ships flying and fighting between them.

If we want extra lift, we could maybe use spheres placed at a lower depth and build on top. If we could get 50kg for each cubic meter in those spheres, we could support... a lot of floating island.


~~Bryan

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#17 2013-04-07 09:51:47

Terraformer
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From: Logres
Registered: 2007-08-27
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Re: Rogue super-Earths/mini-Neptunes...

At those distances, the exospheres wouldn't be much higher than the background temperature. At those temperatures, the hydrogen is going to stay on the planet.

I suppose a lot of it depends on what sort of temperatures the "surface" of the planet is at. If it's only a few hundred degrees, then the methane and ammonia are unlikely to decompose, especially given the high pressure.

The internal structure would have a lot of influence on what the planet would be like. Would there be a metal inner core, a rocky outer core, and a mantle of high pressure ice? How would the ice affect heat transfer from the core to the atmosphere?


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#18 2013-04-07 13:45:52

Void
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Registered: 2011-12-29
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Re: Rogue super-Earths/mini-Neptunes...

Here is a website:

http://en.wikipedia.org/wiki/Mini-Neptune

My opinion is that if it were a true Mini-Neptune, your first thoughts that the moons of it would present opportunities does make the best sense.

However, I would not rule out a robotic system within the Mini-Neptunes atmosphere, where some Methane is available, then some construction materials can be hydrocarbons.  I would suggest perhap a well insulated cylinder filled with heated hydrogen gas, with one end up and one end down, and long enough that it can be pushed in opposing dirrections by wind currents of different directions.  Then some method to harvest energy from the wind power striking the surface.  The Hydrogen would be lighter than a Helium/Hydrogen mixed atmosphere (With a bit of Methane in it).  A warm interior would help flotation also as has been mentioned by others.  Such robots would then have to be able to "Beem" the presumed excess energy to a location in orbit, for it to be of use.

A tall order, but perhaps someday.

As for a Earth or Super Earth that does not have so much atmosphere that the rocky core is covered by an ocean of some fluid, then it would perhaps not be that good of an energy source, but a source for robotic mining.  Also very futuristic.

But the Oort cloud is an interesting topic, since if humans were ever able to make a living there, then it would not be that fantastic to imagine them moving to the Oort cloud of another ajacent solar system, and such a solar system would not initially have to have a habitable planet to make it attractive to Oort cloud dwellers.


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#19 2013-04-07 13:57:30

StarDreamer
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From: Ontario, Canada
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Posts: 92

Re: Rogue super-Earths/mini-Neptunes...

mmm -- Oort Cloud or Kuiper Belt? There is a considerable difference.
At the distance of the Oort Cloud, you'd have a tough time identifying your host star.....


~~Bryan

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#20 2013-04-07 21:13:31

Void
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Posts: 3,884

Re: Rogue super-Earths/mini-Neptunes...

That would be the human race all grown up.  (If ever).

Of course I understand how remote that achievement is relative to what I will actually experience, but while practicle matters have to rule our real actions, it makes it all much better to see that a pathway does exist.  I really don't like the "Just give up, because it's just not possible people"  they likely are among those who see the human race as existing to gratify their needs.

I prefer possibility, but seek justification to say it is possible.


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#21 2014-03-27 13:58:12

Terraformer
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From: Logres
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Re: Rogue super-Earths/mini-Neptunes...

Aaand... there's suggestions that there's a 5-10 Earth Mass planet out in the Oort cloud, based on the orbit of a recently discovered dwarf planet. That sounds fun. Even better if it turns out there are abundant Mars-Luna mass worlds out there.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#22 2014-03-27 16:26:57

SpaceNut
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Re: Rogue super-Earths/mini-Neptunes...

Saw the article of planetoid unofficial named Biden...which suggests a large planet that is influcencing the orbits which seems to be 4000 years plus when we start looking so far away.

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#23 2014-03-29 05:58:32

Tom Kalbfus
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Re: Rogue super-Earths/mini-Neptunes...

Void wrote:

Well I found this, which is related.

http://news.nationalpost.com/2012/11/14 … smic-void/

http://www.cfa.harvard.edu/news/2012/pr201212.html

http://www.livescience.com/19753-rogue- … stars.html

But you are talking about mini-Neptunes formed from the parent stars materials, and I am just killing time.

Otherwise I would suggest gravitational flexiing and magnetic fields as a source of energy.

Other than that, if you like space elevators that only reach down into the upper atmposphere, and want to link more than one in a ring around the planet, there is the possibility of gathering electrostatic energy (lightning), from the weather systems of your mini-Neptunes.

I have a big problem in such a system seeing how you get metals to work with.  They are burried under deep ice.

The exception could be a moon like Enceladus of Saturn.  A case like that might permit access to core materials.  Or perhaps a ancient collision might have laid a core somewhat exposed by blowing the ice shell off of a moon.

Alternately a Venus ejected to the outer solar system might collect Hydrogen, cool off, develop plates and have volcanism, and mid-ocean ridges where a canyon between ice sheets might reach to rock, maybe warm/hot rock.

Venus has volcanism now, its CO2 atmosphere would be a layer of dry ice, and would sublimate when certain events such as volcanoes erupting under them happen. A terrestrial planet such as Earth or Venus have warm interiors that are unrelated to their distance from a star. If you can find a rogue terrestrial planet the size of Earth, and it was about the same age, it would still have a warm interior that can be tapped for energy purposes. A frozen Earth would not be frozen to its core. the Earth warms up the deeper you drill. If you don't have a fusion reactor, a geothermal source of energy would do just fine for a surface settlement.

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#24 2014-06-26 05:43:09

agent009
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From: a galaxy far away
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Re: Rogue super-Earths/mini-Neptunes...

Terraformer wrote:

Which is good news for anyone looking to colonise the planet itself, since it means there's both more carbon available for building and more buoyancy can be obtained from our hydrogen aerostats (and more lift from our wings).

Yes, it would be very difficult (see impossible) to build an aerostat based on "light gas balloon" principle that would work in hydrogen atmosphere, though "hot air balloon" and "low pressure balloon" principles would work just as fine in hydrogen, given sufficiently high pressure and sufficiently low temperature. But true, "light gas balloon" principle is usually the most efficient, since lighter-then-air lifting gas maintains its volume and density by itself under ambient atmospheric conditions and neither any power input (as with "hot air balloon") nor any particularly strong and heavy support structure (as with "low pressure balloon") is required in order to achieve buoyancy buoyancy. 

Lifting efficiency of a wing, on the other hand, has no immediate relation to the mole mass of the environment - it is mainly a function of the global value of density, airspeed, wing area, wing aspect ratio, angle of attack and gravity (since g has no effect on the lifting force, but lifting force needs to offset weight which is a function of g). The global value of density for gases is pressure * mole mass / (R * temperature) but, as far as dynamic lift is concerned, you do not particularly care how much of that density comes from pressure, mole mass or temperature as long as your density/g ratio is high enough. Efficiency of winged aircraft is also directly related to the speed of sound (which is a function of temperature, mole mass and specific heat capacity) - the higher this value is, the more airspeed (and thus lift) you can achieve without hitting the critical Mach and being subjected to negative effects of reduced lift, wave drag and amplified skin friction (this is also a limiting factor or propeller/rotor blade lengths and RPM rates). In this respect low mole mass is a good thing since it usually results in higher speed of sound under the same temperature (e.g. the sound travels about 4 times faster in hydrogen then in nitrogen at 20 °C). 

Terraformer wrote:

karov, I'm not sure the pressure difference between a trimix atmosphere at 293K will give that much lift in a Helium/Hydrogen/Methane atmosphere, even when said atmosphere is at 50K (would there even be much methane at such a temperature?).

Relying on extreme pressure difference between inside and outside environment might not be a very viable concept, as that would either entail super-strong (and thus super-heavy support structure) or reliance on some dynamic forces (which could lead to a disaster in case of power failure). We are talking more about temperature difference here, which has nothing to do with pressure. Neutral buoyancy equation for gaseous environments looks like this:

m₀ = p[env]·M[env]·V[disp]/R·T[env] - p[L]·M[L]·V[L]/R·T[L]

where m₀ is the mass of your aerostat (without mass of the lifting gas), p[env], T[env] and M[env] are pressure, temperature and mole mass of the environment (atmosphere), p[L], T[L], M[L], V[L] are pressure, temperature, mole mass and volume of the lifting gas, V[disp] is total volume of the environment gas displaced by the aerostat and R is the ideal gas constant (8.3144621 J/mol·K). This comes from the fact that, under constant gravity, total mass of your aerostat must be equal to the total mass of the displaced environment gas and terms on the right side of the equation are merely masses of the displaced environment gas and the lifting gas expressed in terms of their volumes and densities using the ideal gas law.

For all practical purposes, you can consider that the any useful volume of displaced environment gas the same as the volume of your lifting gas, since you do not normally get much useful buoyancy from the volume of gas displaced by solid/liquid substances. Which gives you this:

m₀ = (p[env]·M[env]/T[env] - p[L]·M[L]/T[L])·V[L]/R

Basically, in order to maximize the useful mass of your aerostat (m₀) you would want the mole mass of the lifting gas to be as low as possible and its temperature as high as possible. You do not really want to mess with pressure too much, however - although decreasing pressure of the lifting gas could potentially increase your buoyancy, in practice, it would also oblige you to reinforce the structure and, usually, lead to increase of mass by greater magnitude (and thus nullify any theoretical buoyancy advantage you might have gained).

Now, if your environment gas is mostly H₂/He you haven't got much room to play with mole mass of the lifting gas, though you'd still want to keep it as low as possible - so, you would likely want to use heliox, hydrox or hydreliox as breathing gas inside your habitat. Humans can safely breath heliox under pressures up to about 14 ATA - above that pressure, helium causes HPNS which is likely not healthy in the long-term. Under high pressure, you can also add some hydrogen into the mixture (hydrox or hydreliox) but, for obvious reasons, you do not normally want to mix H₂ with O₂ at ratios greater then 4:96. Hydrox (H₂/O₂) with 1-4% oxygen mole fractions is perfectly safe and breathable and for pressures between 5 and 8 ATA (lower pressure would force you to increase O₂ content above safe ratio and higher partial pressure of H₂ could cause narcotic effect).

Trimix (He/N₂/O₂) would have no real merit for your floating habitats - humans do not really need any nitrogen and it will only unnecessarily increase mole mass of your "lifting gas" (they use trimix on deep dives because N₂ narcosis somewhat compensates for HPNS effects of He, but that's probably not very good health in the long run). If you want pressure above 14 ATA, H₂ and Ne would be much better options for diluting heliox.

Since hydrox atmosphere inside the habitat would give you best aerostatic efficiency, and pressure amplifies lifting effects you get out of mole mass or temperature difference, 8 ATA outside pressure would probably be ideal. So, let's see how heavy you can make hemi-sphere habitat of 1 km diameter filled with 1/99 hydox at 20 °C (293.15 K) in 20/80 helium-hydrogen atmosphere at 8 ATA (≈ 8×10⁵ Pa) pressure and 50 K temperature.

Volume of the lifting gas would be approximately equal to 2/3·π·500³ m³. Mole mass of 1/99 hydrox is 0.00232 kg/mol while mole mass of 20/80 He/H₂ mixture is 0.00241 kg/mol. Thus:

m₀ = (0.00241 kg/mol / 50 K - 0.00232 kg/mol / 293.15 K) × 2/3π × 500³ m³ × 8×10⁵ Pa / 8.3144621 J/mol·K = 1014794730 kg

That's about 1 million metric ton - not bad!

Now let's see how well 10/90 heliox (M = 0.00681 kg/mol) at 2 ATA pressure would perform (we cannot drop the partial pressure of O₂ much below 0.16 ATA if we want humans to live there and even 8/92 hydrox would be already be a major fire hazard). 

m₀ = (0.00241 kg/mol / 50 K - 0.00681 kg/mol / 293.15 K) × 2/3π × 500³ m³ × 2×10⁵ Pa / 8.3144621 J/mol·K = 157244535 kg

About 150,000 metric ton. Still impressive, but would that be enough for a structure of 1 km diameter?

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#25 2014-06-26 09:55:23

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
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Re: Rogue super-Earths/mini-Neptunes...

Welcome to newmars, agent009!

Interesting post.  I'd always written gas giant aerostats off as infeasible, myself, but I guess I was wrong to do so

I always wonder about the possibility of hydrogen oceans on rogue planets.  According to some back of the envelope calculations, the radioactive decay energy produced in the earth's core is enough to heat a planet to an average temperature of about fifteen kelvins.  Under a helium atmosphere of appropriate pressure I see no reason why hydrogen oceans wouldn't form.


-Josh

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