Terraforming small icy moons and comets

Antius · 2014-05-18 05:38:44

A little while back, I decided to investigate the limits of the shell world concept.

http://www.space.com/23082-shell-worlds … aphic.html

Specifically, how thick would a shell need to be for its weight within its own gravitational field to be sufficient to balance internal pressure? (i.e. if no central body existed). So I solved Newton's law of universal gravitation.

http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation

The force of gravity acting on all elements within the shell pulls them towards the centre of mass of the shell. The total mass of the shell is proportional to the square of radius, assuming that the thickness of the shell is constant. However, the force of gravity declines inversely with the square of radius, so the radius cancels out.

On this basis, for any given internal pressure, the required thickness is independent of shell radius. A shell a thousand kilometres wide would need the same thickness as a shell one hundred kilometres wide for the same pressure. The equation for required shell thickness is then:

t^2 = Patm/(4π.G.ρ^2)

Where ρ is the density of the shell material, a G is universal gravitational constant. For a 1 bar internal pressure, a shell made from pure ice would need a thickness of 11km. For a 0.25bar pressure, the required thickness drops to 5.5km. For 0.1bar, the thickness is 3.5km.

This sets the limiting dimensions for a shell world. For an ice shell providing 1 bar internal pressure, a shell world must be at least 22km in diameter. If pressure drops to 0.25bar, the minimum diameter drops to 11km. For 0.1bar, minimum diameter is 7km.

There are in fact dozens of icy outer solar system moons and millions of icy comets in the Kuiper belt and Oort cloud with diameters greater than these dimensions. Terraforming such bodies would be relatively easy. Once colonists had found a 20km diameter comet, they would drill down to the centre and use nuclear waste heat to excavate a small initial cavity for a habitat. Waste heat generated by processes within the habitat would gradually melt the surrounding ice and the cavity would gradually grow larger. Dissolved ammonia within the ice would serve as a fertiliser for plant life within the inner cavity and would gradually be converted into nitrogen gas. Excess water would be pumped onto the surface of the comet, where it would freeze, strengthening the shell. After a time frame of perhaps 1000 years, the internal cavity will have grown to 10km in diameter. At this point, a liner will be put in place and the inner surface of the ice shell will be cooled to prevent any more melting.

The inner volume of the cavity would be large – some 500 cubic kilometres. This would be sufficient for a large low-gravity ecosystem to develop. Human beings would be housed within large rotating buildings, perhaps 100m in diameter. If each habitat is home to 1000 people and the cavity is able to house 10,000 such habitats, then a 20km diameter comet could be home to 10million people.

Void · 2014-05-18 08:30:13

I do like the idea of pressurized shell worlds without a core.

I think they could be sensible for a future reality where stony/metalic materials could be obtained from the inner solar system or from comets or dwarf planets where an impact has stripped most of the ice off.

Here are some links about "Dwarf Planets".

http://en.wikipedia.org/wiki/Dwarf_planet

http://www.cnn.com/2014/03/26/tech/inno … ar-system/

I put an interstellar twist on this as well. While if the human type or some derivative of the human type may persist into the future and may find ways to travel to the nearest stars, I really do understand that that such a enormous task as to cause pause and unbelief in the notion.

However modification of the human species, connected with better propulsion systems, and leapfrogging across tiny worlds might suggest the chance that some derivative of the human race may become a star spanning humanoid type.

We can hope that small rocky asteroids have been displaced into the Kepler belt, and the Oort Cloud. If so, then with an energy source, (Fusion, Vast focused solar concentrator, dark matter interface, dark energy????), I would suggest the preference would be to create shells without a core around dwarf planets. cause them to be moons of such worlds. For Pluto, it might be possible to mess with Charon. Melt the interior, push a bubble of air into it and cause cryovolcanos on the surface, which will freeze. Repeat? Then strip mine Pluto to make more shell moons. In this case, it might be possible to have an economy where humans desiring housing, offer their savings, to obtain such, and that money could pay to obtain materials that those dwarf planets could not supply. Eventually, however the ice layer would be removed, and a rocky/metalic core exposed. Then a different economic reality.

Void · 2014-05-18 08:40:33

I will not stop there, but offer a helper, a double vacuum shell world.

I have considered that a metalic/silicate double shell world could offer an environment where a vehicle could have wheel contact with both an outer shell and a nearby inner shell, a non-pressurized enviornment, but one which could facilitate the retention and re-collection of some lost evaporates. The vehicle being mobile within a slightly more than 2 dimensional sphere bounded by an inner shell and an outer shell may house humans. If small, then no synthetic gravitation. If larger the gap between shells, then synthetic gravitation.

The void inside of the inner shell would be a place to store materials and machine parts and such, a dump/warehouse more or less.

The mobile habitats which exist between shells could travel the domain between shells, and so interact with each other, to perform tasks. Robotic systems being used to manipulate objects in a partial vacuum, (Nearly complete) between shells.

Last edited by Void (2014-05-18 08:42:33)

Antius · 2014-05-18 11:02:24

Void wrote:

We can hope that small rocky asteroids have been displaced into the Kepler belt, and the Oort Cloud. If so, then with an energy source, (Fusion, Vast focused solar concentrator, dark matter interface, dark energy????), I would suggest the preference would be to create shells without a core around dwarf planets. cause them to be moons of such worlds. For Pluto, it might be possible to mess with Charon. Melt the interior, push a bubble of air into it and cause cryovolcanos on the surface, which will freeze. Repeat? Then strip mine Pluto to make more shell moons. In this case, it might be possible to have an economy where humans desiring housing, offer their savings, to obtain such, and that money could pay to obtain materials that those dwarf planets could not supply. Eventually, however the ice layer would be removed, and a rocky/metalic core exposed. Then a different economic reality.

In many ways it might be easiest to use truly tiny worlds where there has been no gravitational differentiation. A 500km diameter dwarf planet may in fact be too large. Since cometary nuclei are about 50-50 rock & ice by weight, as the central cavity gradually expanded by melting, it would liberate all of the metals and silicates that a civilisation might need as grit within the melt water. This would allow the colony & ecosystem to start small with perhaps just a few hundred people, relatively little money and equipment and gradually expand into a society of millions over a period of centuries.

I realised that I made an error earlier. For small diameter shells which are relatively thick compared to their radius, my simple equation tends to overestimate the minimum required shell thickness. This is because shell elements become increasingly wedge shaped as they get thicker relative to radius.

Hence, a 10km diameter comet may be able to support a large internal cavity if pressure is kept beneath ~0.25bar. Ultimately as Void alluded to, these colonies would need some means of active waste heat rejection into space, in order to prevent the shell from melting entirely. However, for the first few centuries of its existence, the waste heat could simply be used to melt out and expand the inner cavity, with waste water pumped onto the surface.

Void · 2014-05-18 11:08:11

I did not want to step on what you are proposing. That is fine, I just wanted to augment it with the potential of Dwarf Planets, rocky asteroids displaced into the outer solar system, and interstellar travel. I do agree with your point that a certain sized comet might be more ideal.

Antius · 2014-05-19 05:58:05

Void wrote:

I did not want to step on what you are proposing. That is fine, I just wanted to augment it with the potential of Dwarf Planets, rocky asteroids displaced into the outer solar system, and interstellar travel. I do agree with your point that a certain sized comet might be more ideal.

The idea using a shell as an interstellar arc idea is interesting.

A couple of years back there was a discussion on this site around the possibility of nuclear fission powered interstellar transports. The baseline concept was a fission fragment rocket, achieving a final velocity of ~2% c, allowing a transit time to a-Centauri of 250 years when acceleration was factored in. The general consensus was reached that a slower but larger ship would be preferable. Since the journey inevitably takes several human life-times, why not instead take it slower and build a world-sized ship?

A shell world would appear to fit this criterion quite well. Seed a medium sized KBO (100km diameter) with a small habitable internal cavity (100m) and perhaps a thousand people and set it onto a highly elliptical orbit around the sun, with a close approach of perhaps 1million km. At closest approach, release a large diameter solar sail and accelerate the shell to 300km/s.

Flight time to a-Centauri would be 4000 years. The interior volume and resources of the KBO would be sufficient to grow a society of hundreds of millions of people, capable of settling the star system by the time of arrival. Hence, the colony grows in transit, starting with modest resources and investment. A shell world therefore allows interstellar colonisation at low cost, using the natural human reproductive ability and economic growth as leverage.

Void · 2014-05-19 08:43:43

I guess, I. Use such similar notions to dispute those who would say interstellar travel is impossible. I am thinking oort cloud to oort cloud. Thousands of years in the future.

Last edited by Void (2014-05-19 08:47:43)

Void · 2014-11-11 14:59:08

This reference "MAY" show that there could be rocky objects in the Oort Cloud.

http://phys.org/news/2014-11-surfaces-oort-cloud.html

If so, that would make things much better since you would not have to remove so much icy material overburden to get to the metals and such.

Void · 2014-12-09 10:02:16

I guess terraformation being the most liberal of sections I will post this here.

http://www.skyandtelescope.com/astronom … 111220142/

From my point of view it seems that it could be important to move humans to several alternate locations in the solar system.

Why? Because although it is not proven, the life form may have potential (Or may not).

The plan to scatter humans, hoping that one blind pig will find an acorn.

In the solar system, unfortunately they will still find means to murder each other for stone age and I suppose logical reasons.

The notion of asteroids in the Oort cloud (4% of the objects?), offers the chance of a greater expansion. I guess I don't care if it is by humans, or their robots who are programed to foster humans (Maybe they will obey ).

So, a bridge to the stars, where 4% rocky materials, and 96% mixed materials (Icy and Dusty/Rocky).

For my part, I think that Ceres is the big target. Crossing an "Ocean of Stars". OK! Well getting from one Oort Cloud to another, finding a small world closer to a star that offers a spectrum of materials, that then in my opinion is the most important. Finding another Earth not so important. To adapt to the Oort cloud, then Earths might seem horrible places that seldom match what we have, and always have a nasty gravity penalty. Ceres protoplanets the most desired. And for me there is the chance that most lately formed solar systems, if they do not have an "Earth" might very well have a Ceres.

Last edited by Void (2014-12-09 10:05:01)

Terraformer · 2014-12-09 11:15:36

I don't think we'll find that many habitable planets. I do, however, consider it likely that easily terraformable (think - habitable temperatures and pressures, non-toxic but non-breathable atmospheres) planets will be very common, maybe even one per star.

If we're lucky, we might even find worlds like Jupiter, orbiting at Mars like insolations, with moons like Titan. Just move right in and set up dome cities.

Void · 2014-12-09 15:09:39

I guess we are fairly close on our ideas.

I think that Mars to Earth sized worlds in the outer habitable zone or even to 2,000 Lumens (20% of solar for Earth) might be best bets. They can be warmed up if they have a Nitrogen dominated atmosphere in particular. I also like Dwarf Planets as well however, because they have shallower gravity wells.

Antius · 2014-12-10 06:12:48

One important thing I have learned from this forum is human beings are capable of adapting to a huge range of environments. So long as the raw materials are there, ie, we have access to all the required elements in the periodic table, then we can find a way to live there, wherever it is.

In a sense, Earth replica terraforming is an aesthetic exercise. We can build pressurised brick buildings on Mars simply by building them a few metres underground. On Pluto, we would be building ice castles, with internal insulation and rotating interiors, but the basic engineering is similarly uncomplicated. We can adapt to practically anywhere.

Void · 2014-12-10 11:24:41

Good point.

I would like to know if solar systems where planets have not been detected, might in some cases contain a very large number of smaller worlds.

Where we have gas giants, many solar systems with planets do not. So, is it possible to have a solar system where the largest orbital object would be a Dwarf Planet, and could a large number of Dwarf Planets be stable in such a solar system.

Tom Kalbfus · 2014-12-12 01:22:57

the first step in terraforming a comet is to spin it up for gravity.

Terraformer · 2014-12-12 11:48:18

All that does it get you lots of ice dust...

If you do the original proposal with a dwarf planet, I imagine you'd end up with a Virga. Several thousand kilometres of air, warmed by fusion suns and inhabited by people living on rings.

Void · 2014-12-14 22:14:15

I guess I have greater confidence that a human like population could migrate over time from one star system to another, since apparently 4% of the objects in the Oort cloud of our solar system should be composed of rock. Also it appears that the distinction between asteroids and comets is blurry, where many comets have a fair content of non-ice materials.

So, I am wondering if since a minority of solar systems have gas giants, and perhaps also rocky planets, and more only have rocky planets, if those where planets are not detected, in some cases may have many smaller objects which we currently cannot detect.

Perhaps various factors determine what the condensation process is in a solar system. Presuming one has materials in a disc, then it has to create gas giants within a certain period of time, or it looses the ability to create them since the Hydrogen gets blown out of the system. It also appears that water ice, and perhaps other ices are important to glue the original small objects together, and then other forces can build on those components, perhaps creating dwarf planets.

I think that there might also be factors of chaos that appear random to us, where if a chance combining of two significant bodies does not happen in time, then there are reduced nucleation points. If those random factors start a planet early, then gas giants. A little later, then rocky planets. Is it possible that an original planet forming early can promote a disturbance in the orbits of other objects that would cause other planets to form? If a "random" situation happened where the first planet formation was late, and some of the ices had been blown away, and so less glue for the initial process, could there be a system with small planets, and many of them?

If so, then some would still have ice shells like Ceres, if they were far enough out, but of course if they were inside the snow line they would be dry.

If this were so, then we are very fortunate, because although there will not be many Earths, there might be some variations on Mars, Ceres, and Titan. You could use them like a color pallet, and imagine planets or even moons (Much less likely) where those worlds might combine aspects of each of those three worlds.

An intergalactic species would I presume consider those to be prime targets, supposing they were basically like us, but more advanced.

Another game of the imagination might involve disturbing such a solar system so that objects would combine to make significant planets. But that is just a mind game. I am not sure an interstellar species would see an advantage with that since they would have adapted to places with small worlds anyway. After all Titan can hold an atmosphere, and could be warmed up.

A rocky Titan at the orbit of Saturn (Radiation wise) could also be warmed up and would have a rather shallow gravity well. An adapted species might find that ideal.

Void · 2014-12-15 09:15:15

Returning to the original topic "Terraforming small icy moons and comets":

I would think that a layering of devices should make sense. A rotating metallic device in the center, of significant size with synthetic gravity. That encased in a sphere of metal. Surrounding that additional shells. Over that layers of ices.

Using slow means of propulsion perhaps solar driven, perhaps fusion driven, moving to a next new object, and then as desired, either expanding the world, or cloning it.

Using this then after a very long time entering another star system, where worlds similar to Mars, Ceres, and Titan could be prime targets.

Void · 2014-12-16 11:09:17

Looking at such a situation, a comet altered in that manner might resemble a single cell organism.

http://en.wikipedia.org/wiki/Unicellular_organism

So perhaps it would be good to study single celled organisms to see how they are structured, to get some ideas how to build a comet habitat.

Last edited by Void (2014-12-16 12:28:55)

Antius · 2014-12-18 06:30:10

Void wrote:

Returning to the original topic "Terraforming small icy moons and comets":
I would think that a layering of devices should make sense. A rotating metallic device in the center, of significant size with synthetic gravity. That encased in a sphere of metal. Surrounding that additional shells. Over that layers of ices.
Using slow means of propulsion perhaps solar driven, perhaps fusion driven, moving to a next new object, and then as desired, either expanding the world, or cloning it.
Using this then after a very long time entering another star system, where worlds similar to Mars, Ceres, and Titan could be prime targets.

At Oort cloud distances, orbital velocities will be <<100m/s. A modest velocity change is all that is needed to send such a body falling into the inner solar system, where gravity assist from jupiter and saturn could be used to fling it out of the solar system at 10km/s. At that speed it might take 100,000 years to reach the nearest star. In that time, an initial tiny group of people would have time to construct a society of many millions, complete with cities and all of the equipment neccesary to settle the new star system. Since it inevitably takes more than 1 human lifetime to reach the stars, maybe it doesn't matter much to the individual whether the journey takes 10 or 1000 generations.

Void wrote:

So, I am wondering if since a minority of solar systems have gas giants, and perhaps also rocky planets, and more only have rocky planets, if those where planets are not detected, in some cases may have many smaller objects which we currently cannot detect.

Maybe larger worlds further out, with orbits too slow to produce rapid, detectable oscilations in star movement. Present detection techniques bias towards detection of large worlds close in.

Void · 2014-12-18 14:56:56

Sounds reasonable. I would suppose they might employ magnetic breaking to navigate into the star system at the end of the journey.

Antius · 2014-12-19 10:45:30

For very minor course corrections perhaps. But the sheer size and mass of comets, makes them of questionable value as interstellar vehicles if we need to use large propulsion systems to subject them to velocity changes.

For example, a 10,000km diameter magsail would weigh 400,000 tonnes and develop only 700KN of thrust at the Earth-Sun distance. A 10km diameter comet would weigh 500billion tonnes. The acceleration produced by this planet sized magsail at Earth-sun distances would be 1.3x10-9m/s^2. Assuming we could maintain Earth-sun distance acceleration levels for a year, the velocity change would still be <1m/s. By this time, the comet’s gravitational velocity (i.e. the speed it picks up from falling through the sun’s gravity well) would have carried it a long way from the sun.

The best option would seem to be to select a large Oort cloud object (perhaps 100-200km in diameter) subject it to a 20m/s velocity change using thermo nuclear propulsion (over perhaps a century or so) and let it fall towards the inner solar system. Time it just right to allow multiple gravity assists with all of the outer planets and pick up perhaps 20km/s of delta-v. At that speed, you have a 60,000 year cruising time to Proxima Centauri. That is a long time to wait, but you are essentially taking a whole world with you, with plenty of resources to grow along the journey.

Although the cruising velocity is small, it only works so long as you get the delta-v free from nature by gravity assist. Consider that at 20km/s, the kinetic energy of a 200km diameter cometary body would be 10^27 joules. That’s as much energy as human civilisation generates in 2 million years. Maybe not beyond the capabilities of a whole-system human civilisation a thousand years hence, but beyond the capabilities of the small band of interstellar travellers we are considering.

Using cometary bodies as vessels clearly only makes sense if you are willing to travel very slowly. For the same reason, it is likely that the travellers would either consume the body along route, or abandon it in smaller vessels shortly before arrival. It simply doesn’t make economic sense to produce a propulsion system capable of breaking that much deadweight.

Last edited by Antius (2014-12-19 10:47:30)

Terraformer · 2014-12-19 17:27:08

Well, a planet sized relativistic starship *would* make a cool interstellar cycler for immortal transhumans...

Tom Kalbfus · 2014-12-22 09:44:32

Void wrote:

Returning to the original topic "Terraforming small icy moons and comets":
I would think that a layering of devices should make sense. A rotating metallic device in the center, of significant size with synthetic gravity. That encased in a sphere of metal. Surrounding that additional shells. Over that layers of ices.
Using slow means of propulsion perhaps solar driven, perhaps fusion driven, moving to a next new object, and then as desired, either expanding the world, or cloning it.
Using this then after a very long time entering another star system, where worlds similar to Mars, Ceres, and Titan could be prime targets.

Tell me does Pluto count as a comet? Could we terraform that? How about the Dwarf planets further out?
Sedna is way out there, about 518.57 AU from the Sun with an orbital period of 11,400 years a mass estimated to be on the order of 1 * 10^21 kg About 1.4% of the mass of the Moon, the average orbital speed is 1.04 km/sec, the escape velocity at that distance is 1.83972 km/sec.
http://www.calctool.org/CALC/phys/astro … e_velocity
So the question is, could we accelerate Sedna to the Solar Escape Velocity from this orbit? The speed of light is 300,000 km/s so to start out at 1 km/sec, we need to accelerate to 2.83972 km/sec this will put Sedna on a departure velocity of 1 km/sec after it escapes the Sun, it will take 300,000 seconds to cross the same distance as a photon does in 1 second. If we can get it up to 300 km/sec it will take 4,400 years to reach Alpha Centauri. So basically what we have to do is make Sedna habitable for 4,400 years with the mass left over from accelerating it to 300 km/sec, as for slowing down, we don't have to slow down Sedna. Sedna can pass through the star system at 300 km/sec while the colonists in smaller spaceships will slow down to match the target system's velocity.

Void · 2014-12-22 11:31:39

I have just been on this topic, to test the idea that human like populations could be involved in interstellar travel. Antius added a more active concept, then just slow migration.

I guess I would not consider trying to move a large object (Dwarf), unless some completely unknown powering method was discovered. I don't think it likely.

The only modification I could add to what Antius has specified, is that when you were done with the Comet, you might have choices. You could split into pieces, and use one piece to impact a target planet. For instance a cold world with a Nitrogen atmosphere, to give it a temporary warm up.

Depart from the comet with your ships, slow them down, possibly have been able to target the said planet with the comet remnant. Land you ships on a temporarily warmed up planet. Release super greenhouse gasses.

Tom Kalbfus · 2014-12-23 07:07:32

Void wrote:

I have just been on this topic, to test the idea that human like populations could be involved in interstellar travel. Antius added a more active concept, then just slow migration.
I guess I would not consider trying to move a large object (Dwarf), unless some completely unknown powering method was discovered. I don't think it likely.
The only modification I could add to what Antius has specified, is that when you were done with the Comet, you might have choices. You could split into pieces, and use one piece to impact a target planet. For instance a cold world with a Nitrogen atmosphere, to give it a temporary warm up.
Depart from the comet with your ships, slow them down, possibly have been able to target the said planet with the comet remnant. Land you ships on a temporarily warmed up planet. Release super greenhouse gasses.

I think if you want to terraform Sedna, you are going to need a massive power source to warm it up anyway, it is 518.5 AU out from the Sun. Do you know what sort of mirror you would need to concentrate sunlight on it to warm it up? You will need a mirror that was about 518,500 in diameter! I think building a fusion reactor to do the same job would be a lot easier than building the mirror. Sedna has a lot of hydrogen in the form of compounds, water ice, methane etc. Much of the dwarf is fusion fuel by weight, you will need to get rid of it anyway. I think fusing in is a fusion rocket would be enough to put the rest of Sedna on an interstellar trajectory. There probably will be enough rocky material left to build a decent sized space colony.

Last edited by Tom Kalbfus (2014-12-23 07:08:56)

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Terraforming small icy moons and comets

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#8 2014-11-11 14:59:08

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#10 2014-12-09 11:15:36

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#11 2014-12-09 15:09:39

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#13 2014-12-10 11:24:41

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#14 2014-12-12 01:22:57

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#15 2014-12-12 11:48:18

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#16 2014-12-14 22:14:15

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#20 2014-12-18 14:56:56

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#21 2014-12-19 10:45:30

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#23 2014-12-22 09:44:32

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