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One of the great weaknesses of many terraforming proposals is the requirement for mega engineering efforts that are unlikely to be financially viable in any conceivable economy. One only need look at the Earth itself and the failure of mankind to place any limit on its CO2 emissions, to realise how difficult it will be to commit resources to environmental objectives that are several orders of magnitude more expensive with return horizons centuries or millennia into the future. People are interested in local benefits over timescales that are relevant to them.
With this in mind, the most plausible planetary engineering projects are those for which one or both of the following conditions are met:
1. Return windows are short, rate of return is high and the overall cost is within the resources of existing inhabitants. Very difficult for most terraforming projects but possible in some limited cases (i.e. paraterraforming a small asteroid or limited area of a larger body);
2. The terraforming effect occurs as a by-product of something that the colonists are doing anyway, for their own short term benefit.
For icy bodies such as outer solar system moons, both factors can be brought into play, making terraforming a plausible option.
One of the great limitations to building any settlement in space is the need to dump waste heat. On Earth, we are afforded excellent heat sinks in the form of large water bodies and our dense atmosphere. This makes it possible to cool by mass transfer (convection) which is far more efficient than radiative cooling at reasonable temperatures. In space, all cooling must be radiative. If one wishes to generate power from a nuclear reactor, this requires either extremely hot core temperatures and heat rejection temperatures or huge radiators, or some compromise between the two. For living spaces the problem is more acute, as they are limited to dumping heat at a temperature of 300 Kelvin. Radiator areas must therefore be comparable to inhabited area – a very severe economic limitation on space based colonies.
Off world colonies on most bodies (with the exception of Titan) and in free space, must be pressurised, and this makes them extremely massive and expensive. The mass of a pressure vessel is proportional to its volume, so it makes sense to utilise the volume of the structure as efficiently as possible. This immediately creates a problem with dumping waste heat, as by packing more people and functions into a finite volume; we are effectively increasing power density. The problem is solved quite well if the colony is built on an icy body, because the mass of the body itself provides a highly effective heat sink. In fact, a colony built on ice, dumping waste heat into the ice, would rapidly find itself floating on a lake. At this point, dumping heat is even easier as we can simply pump the lake water through heat exchanger tubes.
On an airless outer solar system world, the lake would gradually evaporate, although the evaporation would be slowed by a floating layer of surface ice. At low evaporation rates on smaller bodies, the resulting water vapour atmosphere would escape rapidly. However, on larger bodies such as the Galileans, the atmospheric life time would be substantial and water would gradually dissociate into hydrogen and oxygen. The hydrogen would escape rapidly; the oxygen would persist for a longer period. As time passes, normal human population growth and economic growth will result in more and more of the surface being covered by floating habitats and increasing evaporation of water vapour into the atmosphere. Eventually, photo dissociation would build up a tenuous oxygen atmosphere on the larger Galileans. Algae would metabolise ammonia in the melt water, releasing nitrogen. As pressure gradually rose towards 1-5% of Earth sea level, the evaporation rate would slow. Oxygen would dissolve into the lakes supporting algae and marine life. The thin atmosphere would attenuate cosmic radiation and provide the basis for an ozone layer. At this point, it may be possible to grow specialised crops in non-pressurised greenhouses on the Galileans. More significantly, it will be possible to pressurise surface habitats using atmospheric air, removing the need for closed life support systems. A surface environment suit would need a compressor but would not require any complex or heavy gas storage bottles or air processing.
The surface of the body will remain too cold for a conventional ecosystem, as cold or colder than Antarctica. In the areas surrounding human habitation, a marine ecosystem would develop.
For smaller bodies without sufficient mass to hold an atmosphere, the ecosystem relies upon oxygen dissolved in the water, directly released by photosynthesis, without the step of passing into the atmosphere and then dissolving. This would rely upon sunlight penetrating the floating ice layer and being metabolised by algae. Beyond the orbit of Jupiter, sunlight would likely be too weak to support photosynthesis without an additional light source.
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I like your direction.
You do indicate that after Jupiter, without solar concentrators, sunlight will not be sufficient to drive biological activity.
I suggest making a shell world, but not around a planet or moon.
Your choice of materials, presumably using what is locally available.
For worlds like Jupiter, Saturn, Uranus, Neptune.
Example, for Jupiter-Callisto.
Make a shell world. It can likely be made using metals and silicates. Make it thick enough to block radiation. Paraffin wax and water are possible radiation blocking materials.
Inflate an interior atmosphere with water vapor and Air. The component of water vapor will be dependent on the interior temperatures. (Presuming water ice or water are present in the interior of the shell).
If you don't want to use tanks and conduits, then you can have water sticking to, condensing on the shells inner surface. An extractor would return liquid water to your hot process, and evaporative cooling could be utilized, venting the vapors to the interior of the shell.
Or at higher pressures, you could have a completely liquid cooling process.
However, I think low pressures, and evaporative cooling.
Spinning habitats inside of this structure.
For electrical power, even at Pluto, you have the solar wind.
However, I mentioned an outer planet. Inside it's magnetic field, the shell having embedded conductors to cut magnetic lines of force. Of course this being a source of energy.
The shell/habitat assembly being locked by gravitation to Callisto, in one of it's "L" locations. Eventually dragging Callisto deeper, so that it gets into resonance with Ganymede.
Io, Europa, Ganymede, and eventually Callisto (Maybe).
The energizing forced would at first be Callisto's own orbital energy, and later on if locked in resonance, then the spin of Jupiter itself which pushes Io>Europa>Ganymede> and maybe Callisto.
If there were an large undiscovered planet in our outer outer solar system, and it had moon(s), then this might be a good way to "Inhabit" it's location.
Done.
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I will provide a more detailed response later on.
I like the idea of a free floating shell. Although very large, it would actually be quite easy to build. Find an icy outer solar system body anywhere between 10-100km in diameter. Surround it with a micron thick layer of aluminium or aluminised polymer sheet. Build a base on or in the body and start generating waste heat using the nuclear reactor. The sheet will inflate into a sphere (you would only need about a microbar of pressure to do this). The sphere would fill with water vapour which will gradually condense on the inner surface. Reinforce the new layers of the shell periodically with polypropylene fibres.
As the shell grows thicker, the internal pressure can be allowed to increase, as the shell self-gravity would balance the pressure within. Most cometary bodies include CO2 and ammonia as the dominant trace gases. If you provide a source of energy, plants and algae will gradually convert these into O2 and N2. As more and more of the original body evaporates, the residue will be dominated by silicates and metals which you can build with.
I like the idea of using Callisto kinetic energy as a power source. As current is induced in any cable system, it will tend to self inflate under its own electric field. It would also tend to hang towards the dominant gravitational body, in this case Jupiter. The rate of power generation will depend upon swept area and magnetic field strength, which is weaker at callisto. But once you have paid for the cable, it is free energy and the orbital energy of Callisto is enormous.
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Venus would make a great asteroid catcher! No one lives on Venus, its atmosphere can be used to slow down asteroids coming from the asteroid belt, the materials can then be processed using abundant solar energy making Venus an industrial part. Aerobraking maneuvers will tend to introduce needed volitiles into the Venusian atmosphere, especially for asteroids that come from the outer belt. The outer belt asteroids will be easier to sent towards Venus as their orbital velocity is slower, they simply fall towards the Sun, have them intercept the atmosphere aerobrake and some orbital tugs will pick them up after they skip out of the atmosphere.
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Well that is a choice Tom, if the asteroid is not a rubble pile.
One interesting variation would be to have the asteroid crash into the supercritical part of the surface of Venus. Just possibly this would process some of the asteroid, extracting minerals, and then condensing them elsewhere.
Antius, Yes, I see your method. Saturn might be a good place. Lots of small icy moons, but you would also perhaps want some Metal/Stone asteroids. Use Titan to tow the shell though the magnetic field, and as a source of Plastics & Nitrogen.
Of course if Fusion power ever arrives, then you have a ticket to move all the way out, even to the Oort cloud and beyond.
Done.
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