What's the best non-Earth/moon object to have a large extra-terrestial city on?

Some factors I can think of:

1) Gravity of the planet or moon, as higher gravity requires more energy to take off from the planet, thus making industry/trade with Earth more expensive and making the planet less desirable for colonization.
2) The presence of substances useful for life, like water.
3) The ease of transporting people and goods to and from Earth (spacecraft delta-v and launch time window)
4) Size of the object, some are just too small. (would asteroids of a few km in radius be suitable for this?)
5) Stable, non-extreme climate (low eccentricity and temperature around 0C, the colder, the more energy is needed to maintain warmth)

Aside from being practical, would it be likely that the first significant humn settlement would be on that object, given the choice?

The Earth's moon is an obvious choice but lets say that they want to stay out of the Earth countries' military and political reach as much as possible? Anyhow, they won't want to be on the moon for the purposes of this question.

One of the easiest to colonize might be Mars because of its proximity and relatively nice temperature, but it requires a lot of energy to launch objects from Mars, making trade with Earth significantly more difficult. Maybe one of it's (tiny) moons Phobos or Deimos, but they are small, yet far larger than most Earth cities and comparable to it's largest ones.

Next are the moons of the gas giants, some of which have more useful resources than any of the inner planets, having large amounts of water ice, and in the case of Titan, large amounts of a substance that could be used as fuel in a similar way that oil is used on Earth. But these moons are also of the larger variety, with a good amount of gravity. So perhaps a tiny moon is more practical, with the larger moons only used for resource extraction, where it is practical. Launching cargo from resource extraction into space using a mass driver might not be difficult, but mass drivers seem to create too many g's for humans to handle without getting crushed. Thus, a tiny moon would be more practical.

Another option is Ceres, a small object but not tiny, maybe just the right size, it has plenty of space for human societies (2 million square km). Also it is in the asteroid belt, possibly the cheapest place to mine for valuable resources, and it could attract an "asteroid rush", like a "gold rush". The asteroids have many extremely valuable resources that are worth many trillions, the rare-earth materials and metals like gold. There are also comets in that belt, giving the colonists of Ceres water, if it isn't on the dwarf planet itself, and it is heavily speculated that there is. If it isn't subject to an asteroid rush, then it is a good strategic object for an Earth government to claim and establish a sizable presence, because of the surrounding material wealth. Ceres also does not have the radiation issues of the gas giant moons and it is closer to Earth.

Another possibility is a large space station, but that would probably be a lot more expensive to construct.

Looking forward to reading everyone's thoughts.

 

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The expense in constructing a space station is in transporting materials.
So, choose a city-sized asteroid, and turn it into a space station.

But in the long run, the construction costs will be outweighed by the ongoing transport costs, so the question then relies on exactly what the city is for.
Is it a holiday retreat? (LEO space station)
A backup for if/when Earth meets killer asteroid? (Moon, or Earth-Moon lagrange point)
Mining? (Defined by the resource location)
Base for interplanetary travels? (Probably Earth-Moon lagrange point, maybe an asteroid)
Other???

 

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given how difficult it is to leave earth I would say right in earth's orbit still protected by the magnetosphere. It's mayor function would be a central gathering place with facilities to receive scientists and tourists and if not provide a means of producing food have a decend recycling system other functions being the centre of scientific and logistical operation somewhat like the McMurdo station on antartica. (link)

the first true export product could be a space based solar plant that beams it's energy to earth. (link)

Perhaps that would make it profitable

afther that eventually anything is possible, while mars has some decent greavity and day night cyclus other objects have other reasons why they might be colonised, Mercury has the sun and it's minerals wealth I could imagen eventualy they will eventualy produce anti matter here and transport it over the solar system; venus has it's gravity (and terraform possibilities); the moon is closeby; mars is afther earth the most hospitable world; the asteroid belts have it's minerals and low escape velocity; Jupiter's moons have the largest concentration of surface area in relativistic short distances (from any moon you could teleoperate on any other moon) and if you would tidal lock on of jupiters smaller potato shaped moons so it's always in sunlight it's the furthest from the sun where photosynthesis could be aplied on a grand scale).
Saturn with it's moon Titan gives us a great testing ground for xenobiology; uranus has the lowest delta v for a gas giant meaning it will be the cheapest to mine hydrogen and helium; Neptune has it's frozen trojans with the raw minerals (nitrogen ice) to terraform the inner planets 10 times over.

 

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Leaving Earth orbit is easy, because you can use efficient but low-thrust propulsion over a sustained period, or solar-powered rotating tethers. Getting there is the hard part, because it requires high-thrust propulsion for a few minutes, and for the forseeable future that means chemical or fission rockets.

This is also why leaving Earth to colonise another body with significant gravity is not the best idea ever. Since you already have to create an entire ecosystem to sustain you, you'll be better off building a complete habitat in free space.

 

The OP asks for an individually maintained system of human life support systems and infrastructure allowing self-sustainance without help of Earth. In this case Mars is obvious choice as it has been studied thoroughly and has the necessary materials for processing required for life, mainly presence of water (breathing, fuel, food). The presence of water either as ice or perhaps as pressurized water dictates presence of the colonies in space. The current locations are moons of other planets and asteroids known to have water (as ice) outside the Earth and Mars: Ceres 1, Europa, Enceladus, Triton, Titan. The escape velocities for these bodies play a big role in the economy of the human infrastructure. The presence of water in main planets like Jupiter, Neptune, and Uranus...does not help us in any way, as the escape velocity + extreme pressure required to access such water will be beyond human technology for many centuries to come. The asteroids are great place for future colonizations...however they present a challenge to the safety, as I am sure there are other micro-asteroids located in the vicinity. In my opinion Titan would be the best candidate for the post-Mars colonization as it has the atmosphere, the ready liquid hydrocarbons that could be used for manufacturing of various of human items, and water as ice rocks.

 

perhaps I didn't make myself clear but yes I agree here.

This is a tall order and ultimatly not realistic Why would anybody on earth pay for something that is self sustaining and therefore doesn't need trade/Earth?

I've suggested a space station in earths orbit because this is where Earth could most easily benefit from it. because of 2 reasons.

Part 1 Services that make space travel cheaper.
A advanced recycling system could decrease the need for imports like food and water, whilst a advanced power source could decrease the need for space voyage (by transfering cargo from low earth orbit to a vehicle with a lower thrust but a greater ISP), these services would mean that getting to space becomes a whole lot cheaper.

Part 2 Making money.
link Solar power is In these days and space based power cells are more efficient then earth base power cells because there is no atmosphere solar cells can recieve up to 144% the energy that earth based do and as long as you stay outside earths shadow you can collect for 24 hours in stead of the day period. This energy ones collected could be send as microwaves to earth and power our cities making them as marketable as any other power plant.

Arguable you could wonder why not Mars, but producing fuel and products on Mars will only make the mission somewhat cheaper and wouldn't really benefit anybody perhaps maybe the comfort of the visitors on Mars

 

My top three:
1. Europa - decently close, tons of water (cheap, abundant liquid rocket fuel), small chance of native life. Issue = getting through ice crust to ocean
2. Mars(duh) - ice caps, workable gravity, useful metals. Dust storms might be an issue
3. Ceres - only because it's close (asteroid belt) and it's surface wouldn't destroy you in one second

 

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Buddha12 nice picture, thought I'd find some other examples.

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