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I think that one of the biggest stumbling blocks with idea of colonising Mars as an individual project, is that there simply isn't a viable reason for any nation/individual to go to so much effort or cost.

Gerard O'Neill's plans for colonising near Earth space offered the appearance of being viable for the simple reason that his plan was centred upon something economically useful that could be sold to nations on Earth: Solar Power Satellites.  This provided a tangible economic benefit with the potential to justify the initial cost.  Hence, space colonisation would take place as a consequence of economic development of a useful resource.  This is why the US was rapidly colonised on Earth, despite being a generally harsh and hostile place: Britain, Spain and France were able to sink massive amounts of capital into their colonies on the American continent in anticipation that the colonies would be able to provide a return.  They did, in the form of timbre, tobacco, gold and sugar.

The fatal flaw of Mars colonisation plans is that they cannot offer any similar return that could not be more easily provided by other, more easily accessible locations.  Why bother shipping volatiles and metals out of the (relatively) heavy and distant Martian gravity well, when the same things can be provided much more cheaply from the moon and asteroids?

Plans for colonising Mars generally rely upon vague analogies between Mars and the Wild West and the simple idea that it is somehow our colonial destiny to go there and turn it into a space age version of the United States.  But the analogy is a poor one, for the simple reason that the embyronic US had lots of accessible resources and advantages that Mars simply does not have.  Those that believe that colonising Mars is in some way analogous to colonising the US, must therefore justify why it is likley to be cheaper to produce food, manufactured goods and other comodities on Mars and ship them to near earth space, when compared to producing the same thing in Near Earth space from lunar/asteroid resources.  I have never seen a convincing analysis that shows this to be the case.

A floating solar sail would rapidly lose orbit due to sunlight pressure.  Another solution is to manufacture trillions of tiny aluminium balloons filled with a lifting gas (hydrogen , helium, nitrogen, etc) and inject them into the upper atmosphere.  Eventually, these could be manufactured using native Venusian resources.  As the atmosphere cools, CO2 will begin to liquify into seas on the surface.  It could gradually be bottled and buried.

We have been through the 'terraforming small bodies' concept before.  It would work and Ceres could hold onto an atmosphere without being bagged, for geological timescales.  The column density of the atmosphere would need to be 40 times higher than Earth for the same surface pressure, so terraforming may not be economic.  But it could be made to work in principle.

The physiological problems of living in such greatly reduced gravity problem have no easy answer and would no doubt create additional health problems for anyone attempting colonise small worlds.  Whether this is a show stopper or not, I am not qualified to say.

You could equally argue that all of the materials needed to build an o'neill habitat are already there.  If you are happy with the location, very little extra energy is needed to lift the materials out of the asteroids limited gravity well and construct whatever habitat you want.  Solar power can provide the heat needed to melt the metal and glass that you need.

The problem with a spinning gas-bagged asteroid as far as I can see is that little useful surface area is available and to create it one must carve out caves and ledges from solid stainless steel.  If we take this idea to its logical limit and maximimise the internal surface area through extensive digging, then we have a hollowed out asteroid that looks very much like ........an O'neill habitat.

So the question is, do we mine the asteroid and use the materials to build a habitat, or do we try to achieve the same thing by carving out a natural stainless steel asteroid and encasing it in a bubble?

Build a superconducting ring around the equator and trap a comet in high orbit around ceres.  The sun will vaporise the comet and ionise the comet gases into H20, O2, H2, dust and various other minor gases.  The ions will spiral down the field lines of the magnetic field onto the poles, gradually building up an atmosphere.  The sun does all of the hard work of creating the atmosphere, so all you need do is provide the comet and the superconducting magnet.  The H2 will escape into space and the water will freeze out onto the surface, giving you a thich, almost pure oxygen atmosphere with a little nitrogen and a trace of various other gases.  Make the atmosphere thick enough and the greenhouse effect will keep the surface warm.  I'm not so sure about growing crops though.  Insolation at Ceres' orbit is about 1/9th that at Earth, making Ceres a very dark, cold and miserable little planet.

'Geothermal' might be an effective energy source for a Mercury base.  The day side of Mercury is +400C, the night side -170C.  That gives an average rock temperature (just a few metres beneath the surface) of 230C.  The only difficulty as far as I can see is that a substantial heat exchanger would be needed for heat rejection, given the vacuum conditions on Mercury's surfcae.

A similar Seleno-thermal heat engine could be set up on the moon to provide power during the long lunar night, given that ambient surface night-time temperature are -100C and average subsurface rock temperatures approximately -20C.  The downside again, is the sheer size of the heat rejection radiator that would be needed at such a low heat rejection temperature

The issue of nitrogen crops up again and again in Mars terraforming discussions.

The question is, how much nitrogen is actually needed for nitrogen fixing bacteria to work within soil?  Without those, we would be forced to either import masses of N2 into the Martian atmosphere or artificially produce and propogate nitrates across the biosphere.

It may turn out that a 200mb atmosphere which is 85% oxygen, 14% water vapour and 1% nitrogen is perfectly sufficient to sustain an active biosphere.

The alternative to natural terraforming is paraterraforming, ie, covering the surface in giant greenhouses.  The biosphere would consist of thousands of small glass cells, interlinked by sub-lunar tunnels.  In the event that a meteorite strike takes out an individual cell, the tunnels connecting to the cell will be sealed until it can be repaired and repressurised.

Paraterraforming the moon offers a number of advantages:

1) The amount of gas needed is smaller and altogether more achievable;
2) The process can take place incrementally - small investments gradually building up to a fully terraformed world, but habitable from day one;
3) The glass needed can be produced from abundant lunar anorthite and the oxygen making up 90% of the atmosphere and mass of water, can be produced from lunar ilmenite.
4) Paraterraforming would leave unterraformed areas of the moon in vacuum, which allows mass drivers to continue operating.  This is likley to be the moon's primary source of export revenue.

Most of the mass of the atmosphere could come from the moon, as could 90% of the mass of the water needed to produce an active hydrosphere.

A lunar atmosphere consisting of 90% pure oxygen and perhaps 10% water vapour at 1/3rd bar, would produce the same O2 concentration in human blood as Earth's atmosphere.  Only trace amounts of nitrogen would actually be needed.

Getting enough water/hydrogen to the moon to fill the lunar mare may present a significant problem, given that we would probably need about a million cubic kilometres to fill the mare to a depth of 80m.  Thats an iceteroid of diametre 120km - a very sizable TNO.

Soil minerals, phosphorus and nitrogen would be needed in relatively small quantities (~10s millions of tonnes) and would presumably be available in the asteroid belt, in addition to what already exists in lunar regolith.

Cryogenic heat engines have been proposed as alternative transportation power sources here on Earth.  There are a number of problems:

1) Low power density ~ LN2, the most common assumed propellant, has energy density 1-2% that of gasolene;
2) low thermodynamic efficiency of the heat engine (<10% for the entire cycle)
3) the power of the vehicle is limited by the rate of heat transfer from the surroundings.

Electric power is generally used to compress (and in the case of H2, manufacture) the working fluid.  Which raises the question, why not simply use a battery electric vehicle, which is far more efficient and has a similar power to weight ratio?

Generally speaking, H2 is a very poor fuel in any situation, due to its low boiling point, low density, its ability to soak through most metals and its very low ignition activation energy.

Zubrin's methane/oxygen rover concept is about the most practical suggestion for an extraplanetary rover that I have seen so far.  You generally have to go a long way to beat a bog standard IC engine in terms of energy density and performance.  It would be even better if the methane could be replaced by something denser or with a higher boiling point, like ethylene, (m)ethanol, acetylene or even synthetic gasolene.  All can be manufactured on Mars from air and water.

On Mars, the Tharsis region provides the balancing effect that is provided by the moon on Earth.  The Tharsis bulge always aligns itself on the equator, effectively stabilising the planet.

Coal may not be as bad as most people think.  When well-head and pipeline losses are taken into consideration, the greenhouse gas potential of natural gas is just as high if not greater than coal.

What's more, the dirtiest of coal plants relesae large amounts of sulphur dioxide and dust into the atmosphere, which actively reduce global heating.

Also, I would like to point out that global warming is not the only disaster with the potential to wreck human civilisation.  The global peaking of oil and gas production could return us to the stone age in a much shorter timescale (~decades).