Where in the solar system is the most viable place to put my colonists, after Mars and Luna?

Question

Mars and Earth's moon already feature in a lot of fiction about space colonization. After these two planets, and assuming that other solar systems are too far away, what is the best place within this solar system to colonize?

Answer 1

The Asteroid belt.

• It has similar requirements for life-support systems as the moon. With the additional need perhaps for some areas with simulated gravity.

• It is next closest in terms of energy required to get to and from the orbits.

• It is still close enough to the sun that collecting solar energy is workable (the farthest out we have used solar panels is Jupiter - on the Juno mission).

• The cost of landing and launching from an asteroid are reduced thanks to the low gravity. This makes them attractive as way stations to inner and outer locations, as well as making it easier to deliver supplies or to collect produce.

• It is thought that some asteroids will be worth mining for rare elements.

• Yet other asteroids contain water ice or other volatile compounds, potentially useful fuel or raw materials for production or organic feedstock.

• Collisions are much rarer than you might believe from science-fiction staple of spaceships dodging rocks in a dogfight. Typically you would only see one asteroid at a time, maybe a few others could be seen as sunlight-reflecting dots many thousands of kilometres away.

Answer 2

My choices: Ganymede and Callisto (and maybe Titan)

This is perhaps a bit of a buzzkill, but I honestly would not recommend colonizing much in the solar system besides the Moon and Mars. Here's why I would take a lot of bodies off the table:

• Mercury - Too hot on its sunny side for colonization (up to 700K) and too cold on its dark side (down to 100K). Solar power on its sunny side would be tricky as parts of the solar array could melt. Also not too special resources-wise.
• Venus - Generally a hellish place to be. High atmospheric pressure means spacecraft can (and have been) completely crushed. A runaway greenhouse effect means temperatures are extremely high (737 K), and clouds of sulfuric acid make rainy days pretty bad.
• Asteroid Belt - A lot of small bodies. Hard to track smaller asteroids because of the concentration. Not a lot of collisions, but there's always the possibility. Low-gravity means that you won't have a substitute for terra firma anywhere.
• Jupiter - obviously not a good place to land a spacecraft. Fairly far away, which means that "Houston, we have [had] a problem" will go unheard for a couple minutes. The one thing in its favor is that many of its moons are fairly large. From here on out, solar power becomes less feasible.
• Saturn - Same issues as Jupiter. This would actually be, in my opinion, the best of the bunch, because you could find raw materials in its rings. It, too, has a lot of moons, but they really aren't big enough to support a proper colony.
• Uranus - same issues as Jupiter. Also, we the voyage here would take a long time.
• Neptune - same issues as Uranus.
• Beyond - You have the Kuiper Belt and the Oort Cloud as you get beyond Neptune. There are lot of small, rocky and icy bodies here, such as comets, and a few dwarf planets. Again, travel time and communication time are issues, and there's not a lot of gravity to make a large-scale colony. You could get some water here, but it would be in the form of ice. There could also be minerals, but, once again, it's pretty far away. Its cousin, the asteroid belt, is a better option.

Here's why the Moon and Mars are much better targets:

• The Moon - Very close to home. Easy to get to and easy to send messages to. We've already been there, and have proven we can land there and even drive around.
• Mars - Not too far away. We've sent a lot of landers there. Good gravity, so it would be easy to adapt to. Terraforming is also an option.

The Moons

You can thank durron597 and cHao for this little section. I did not address the moons of the solar system in my answer - in fact, I may have unfairly written them off - and so I'll add a little bit about them here.

• Moons of Mars - Mars has two moons, Deimos and Phobos. They're fairly small, and are most likely former asteroids. I hope we can all agree that they're not fantastic targets for colonization - in fact, Phobos is expected to enter Mars' Roche limit at some time in the far future, and be broken into a small ring. This of course doesn't stop near-term colonization. Their size is an issue, though.
• Moons of Jupiter - Jupiter has at least 67 moons, the most of any planet in the solar system. The biggest four are the Galilean moons - Io, Europa, Ganymede, and Callisto. They rival (or surpass) Mercury in size. Europa is famous for its icy oceans, and has been considered (by others in this question) as a good place to colonize. I would put that destination on hold until we know more about it, but it has merit. Io has many volcanoes, which could prove detrimental to any intrepid Jovian lunar explorers. Ganymede is larger than Mercury (good) but is much less massive (bad). This might be the most promising of all the Jovian moons, especially since it has a thin atmosphere of oxygen. It also has a magnetosphere. It would be one of my two picks to colonize. That said, Callisto also has some promise. It has a thin atmosphere and is about the size of Mercury - though much less massive. It has also been well-explored, and has water on the form of ice. Therefore, I would nominate it, alongside Ganymede, for a target of human colonization.
• Moons of Saturn - Saturn has 62 moons, the most famous of which is Titan. Like Ganymede and Callisto, it is about the size of Mercury, but, like them, is much less massive. It has hydrocarbon lakes and a nitrogen atmosphere. It does have some water ice but also rains of methane. It could supported life in the past (or could now), but any such creatures would have different metabolisms than us, and be based on different chemicals. I wouldn't rule out Titan as a target, but its methane lakes worry me a little. The nitrogen atmosphere is really good, though. Saturn's other moons are less prominent, although not to be ignored. The ring system could also be a decent target. I'd rank Titan as my third choice, after Ganymede and Callisto.
• Moons of Uranus - Uranus has a measly 27 moons, most of which are named after Shakespearean characters. Most are much smaller than Earth's moon. Many may have water in the form of ice buried inside them. The downside to colonizing Uranian moons is the travel time - it would take years to get to Uranus. But again, the ring system could provide some good materials.
• Moons of Neptune - Neptune's 14 moons are, like Neptune, fairly far away. Once again, we have the travel/communication problem. Triton, the largest, most likely has a lot of water - again, in the form of ice. It also has nitrogen, although its atmosphere is much less dense than Earth.

Answer 3

Venus but not the surface. In some ways Venus is much better than Mars for a colonization target. It has better gravity for humans (.9G vs .38G for Mars). It is closer than Mars(40 million Km vs 55 million Km). At 50 km above the surface the temp is 0-50 C and the atmospheric pressure is the same as Earth's(no need for pressurized suits, no explosive decompression). The atmosphere is largely made of carbon dioxide so it would be possible to produce oxygen. Being closer to the sun means solar power would be quite usable. The biggest draw back would be the clouds made of sulfuric acid.

Incidentally this Wikipedia page covers pretty much every where in the solar system we might try to colonize.

Answer 4

Europa

http://en.wikipedia.org/wiki/Europa_(moon)

Pros:

• Lots of water, and all the advantages that brings
  • Narrow range of temperature (even though it's still -171 °C)
  • Relatively easy power source (fusion, assuming we have effective fusion by the time we're colonizing Europa)
  • Raw materials we can use to build things like hydroponics bays
• Lots of oxygen in the atmosphere
• Gravity only slightly less than Luna
• Close to Jupiter, for when we start mining Jupiter's atmosphere for fuel

Cons:

• Really really cold
• Low gravity still a problem, but by the time we're colonizing Europa, we should have solved this problem on Luna.
• Jupiter's radiation (thanks Michael Kjörling in the comments)

Answer 5

Why so little love for Ganymede?

I feel that if we had already colonized the Moon due to proximity, Mars due to being close and similar enough (and heavily studied), we'd probably be ready to take on other worlds. However, picking a planet like Venus seems like what you'd do in game for challenge (see Eve) - Venus is hell; it's hot, it's thick and probably has raging storms all the time. It would be dangerous to try to poke through the atmosphere. Mercury on the other hand is way too close to the Sun - while it has a strong magnetic field, getting to it would require a lot of fuel. It just doesn't seem cost-effective to try colonizing these two, not even for scientific research which can be done through probes.

Jupiter's moons however are a lot calmer. While they may have issues with heat cycles, day-night cycles and radiation, if we've colonized the Moon and Mars, we'd already have solved these issues - the radiation on the either would be too high for humans and we'd need some very effective systems to prevent problems. The Moon is also tidally locked, so we'd have solved this already.

Jupiter's moons have the advantage of being both closer than the other gas giants and being part of a system that resembles the inner solar system's terrestrial planet group. As you can tell, I've got my eye on Ganymede. Here's why it's awesome:

• Has a differentiated inner structure, so we get a magnetic field (it's the only moon known to have one, so if Mercury is out, this is the best place to go to find one)
• Seems to have oceans, which is great for terraforming
• Isn't too far or too close to Jupiter (it'll feel like home, with Io and Europa oribiting closer and Callisto behind it).
• Has no atmosphere, but we've already dealt with that on the Moon, right?

Callisto is too barren and cold and Europa is little more than an ice moon. If I was asked, I'd recommend Ganymede.

Answer 6

I'm not sure about after Mars and the Moon, but there's a place I'd take before them: Earth orbit. Low Earth orbit is pretty compelling: easy access to all existing human civilization, radiation protection, fairly easy access to space resources and a good launching point for other locations.

High Earth orbit or Lunar orbit are both also very good. They are preferable to the Moon's surface because we can make custom gravity and day lengths via rotation and shutters, taking off from them is a lot easier than a surface launch, there's more sunlight available, and they are just physically closer to Earth, where everybody is going to want to be coming and going for the near future, among other things.

I'd prefer orbit to a planet's surface even if space colonies were very well established and proximity/ease of access to Earth wasn't important anymore!

Read more about them here:

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

http://space.mike-combs.com/spacsetl.htm

Answer 7

Mercury, but with a major and rather obvious caveat - not in the direct sunlight.

Possibly subsurface at the poles or within some of the deep craters that are always in shadow. There would be almost limitless energy provided from the sun. The extremely low axial tilt means that the areas of shade would remain relatively consistent during the Mercurian year.

Low gravity, very similar to the value for Mars (see link above) and no appreciable atmosphere making transport easier.

Would provide a very stable base for solar observation and power generation.

Answer 8

As many have suggested, the Asteroid Belt would be an ideal location for colonization and resource-collecting, but with so many asteroids, which one do you choose? I nominate...

Ceres

Ceres is an incredibly ideal choice for colonization. For an asteroid, it has an incredible abundance of water, possibly even oceanic water, and plenty of clay material for construction. It's the largest object in the asteroid belt, so it'll have more natural gravity than any other asteroids (though stilly very little). It's even relatively warm compared to other asteroids, so heating costs won't be as high.

There is of course the problem of being located in the asteroid belt, and the possiblity of asteroid collisions - though this is much rarer than media would have you believe. Any colony on Ceres will have to account for this possibility, though realistically the risk of collision isn't significantly higher than anywhere else in the solar system.

Answer 9

Habitability

For a cosmic body to be habitable by humans, it needs to have a few things (or substitutes for those things):

• Oxygen
• Water
• Correct temperature
• Food source

Some of those we can bring with us (like some food and some water) but we won't be able to send enough supply ships to sustain a colony. Ideally, there should be some there already.

Temperature is a little different. We can affect it, but it's not easy. The easiest thing to do would be to find a planet in the Goldilocks (habitable) zone, which, for Sol, looks like this:

It seems that no other planet is in Sol's habitable zone. However, since we can affect the temperature of a planet, we can use those on either side of the zone.

Candidate 1: Venus

Is not that great for colonizing at the moment. Its atmosphere is 96% CO₂; the temperature at the surface is 735K; and the atmospheric pressure is 92 times Earth's. However, it could be made habitable.

• Remove CO₂.
  This both reduces atmospheric pressure since there isn't so much gas in the atmosphere, and reduces temperature by removing greenhouse gas that keeps it hot. This is by far the biggest job: you want to remove CO₂ until the atmospheric pressure is about 0.79atm.
• Add oxygen.
  This makes it livable for us, as well as adding some more cooling effect.

For added bonus, you could remove the sulfur dioxide clouds in the atmosphere, which would lower acid rain levels once you add water. Don't forget the water.

Candidate 2: Asteroid Belt

This could be viable. The optimum here would be to make lots of smaller colonies on separate asteroids and develop asteroid-hopping ships to share resources. The asteroids have:

• Minerals
• Water (mostly ice)
• Reasonable temperatures

They do not have:

• Atmospheres
• Gravity
• Oxygen

Your colonists would need to build airtight, pressurised, oxygenated domes to live in, and they might well want to add some kind of artificial gravity. Other than that, this is a good target.

Candidate 3: Europa

The Wikipedia summary image captures why Europa is a good target quite well:

Europa's trailing hemisphere in approximate natural color. The prominent crater in the lower right is Pwyll and the darker regions are areas where Europa's primarily water [and] ice surface has a higher mineral content. Imaged on 7 September 1996 by Galileo spacecraft.

From that we can straight away tell that Europa has:

• Minerals
• Water (partially ice)
• Low (or no) atmosphere (implied by the impact crater - bodies with atmospheres are less likely to be struck)

This is already good. All your colonists need now is artificial gravity and a sealed oxygenated dome to live in. As others have said, it is still possible to collect Solar power on Europa.

Choice

Out of the three candidates, I pick this order:

1. Europa
2. Asteroid belt
3. Venus

Europa wins because it takes the least amount of effort to make habitable and has most of the resources we need there already. The asteroid belt is not far behind, given that it has quite a lot of resources, but it takes more effort to set up and there is a higher risk of collision. Venus is a lot of effort to get to a livable state, so I would only send colonists there after a lot of work.

Answer 10

As already mentioned the Asteroid Belt is a solid choice.

The other obvious choice is the moons, rings and general orbits of the gas giants, you have huge amounts of raw materials and energy available and the option of both gravity wells and free fall for people and industry to take place in.

Answer 11

For a space colonization except the cases the planet is located in the midst of habitable zone, it is usually desirable the planet/planetoid had no atmosphere. Vacuum works as a perfect termo-insulator, thus allowing to colonize planets located too close or too far from the Sun.

As such, in Solar system we have

• Mercury - having no atmosphere is perfectly suitable for colonization. Lots of solar energy, there is some water ice in the permanently shaded areas on the poles. Unfortunately it rotates which makes the poles the most suitable areas for colonization, although populating other areas is also possible (but one would need to import water and erect sun-shields built from light-reflecting material). A better case would be if Mercury were tidally-locked, then the most habitable zone would be along the terminator line rather than only the poles. Surface area of Mercury is 75 million sq. km (compared to 144 for Mars and 460 for Earth).

• Venus - has thick atmosphere and high temperature, totally unsuitable for surface colonization. Floating habitats are possible though, but why?

• Ceres - has no atmosphere and a lot of water, perfectly suitable for colonization. This is most likely the third destination after Luna and Mars. Surface area is 2.8 million sq km.

Jupiter's system:

• Io, Europa, Ganimede - all have no atmosphere and a lot of water, but also deadly radiation due to Jupiter's radiation belt. Seem to be unsuitable for colonization.

• Callisto. Has no atmosphere and no deadly radiation. Also has a lot of water. Good place for colonization after Ceres and Mercury. Surface area is 73 million sq km, just like that of Mercury.

Saturn's system:

• Titan while the largest sattelite, has a thick atmosphere at very low temperature. Also the atmosphere (containing cyanides) is deadly poitionous to humans even in small concentrations. Seems to be unsuitable.

• Tethys, Dione, Rhea, Iapetus. All seem quite OK for colonization, having no atmosphere. Combined area is around 18 million sq. km.

Uranus' system

• Ariel, Umbriel, Titania, Oberon. All seem to be quite suitable. Combined area is 23.6 million sq. km.

Neptune's system

• Triton. Suitable for colonization, quite large with area at 23 million sq. km.

Trans-neptunian objects

• All Trans-Neptunian objects seem to be well suitable for colonization if enough energy is provided. This includes Pluto, Charon (22.6 million sq km combined), Eris (18 million sq km), Haumea, Makemake, Orcus, Quaoar, Sedna and some others.

Answer 12

If you have a viable technological basis to move around and survive long term in space, the true answer is anywhere you can extract water and collect energy. In principle you can do this virtually everywhere in the Solar System, so long as you arrange a "pipeline" of regular transports to deliver any missing ingredients to your chosen location (water ice to Mercury, for example), but I suspect you are looking for the best and most self contained location, where imports and exports would be at a minimum.

The best place would be the Jovian system. With the 4 large Galilean moons and 60+ smaller bodies orbiting Jupiter, you have plenty of open space to settle. This is actually very important, as I will explain later.

There are vast resources of water and minerals, but most importantly, there is a huge amount of energy available locally from the vast magnetosphere surrounding Jupiter. As Io passes through the magnetosphere, it acts as the armature of a generator,creating a huge "flux tube" running from Io to Jupiter and back, channeling millions of amps of electrical current. Man made satellites or electrodynamic tethers can be placed in orbit around Jupiter to do the same thing, but with much more control of the current, which can then be tapped and beamed via microwave or laser to anywhere in the system. Jovian colonists will have access to as much energy as they want to support their industry and lifestyles, a thriving space transportation system (spaceships using external "beam power" can be smaller and cheaper than self contained spacecraft) and whatever else they desire.

Having an open frontier is also important. People may settle in cities burrowed under the ice of the Galilean moons for protection from radiation and to mine the ice, but without an outlet, their civilization may eventually stagnate. IF young and adventurous people have an easily accessible opportunity to pack up and strike out on their own to inexpensive "land" and start anew then there is always a positive outlet for their energy and yearnings. People might also want to experiment with other social, religious or economic systems without interference, and this is an ideal setup for them to do so and share their findings without imposing on everyone else. Failed experiments can also be easily recovered, since the main settlements are relatively close in astronomical terms. The end result is the Jovian civilization will be much more vibrant and outward looking than virtually any other part of the Solar System, and the Jovians may eventually emerge as the political, economic and social leaders of the Solar System.

The other gas giant planets may have many similar attributes, but are not easily self contained with a rich energy source like Jupiter. Their colonists may be able to mine the atmospheres of their gas giant planets for 3He and use fusion energy for power, but this will be a more difficult and expensive option than the Jovians enjoy.

Answer 13

Earth orbit and Lagrange points.

We start with the Moon because it is a convenient source of raw materials which can be exported to build orbital habitats. The moon has no atmosphere, so you can launch stuff using a linear motor mass driver. No big rockets needed.It's also easy to obtain shelter against solar storms and meteorites by going underground. And it is close enough to Earth that self-sufficiency in small light hi-tech items can be postponed (indefinitely?) and emergency evacuation would be possible (mass-driver, one way "lifeboat" capsule).

Mars? I doubt it is worth the effort. That thin unbreathable atmosphere is an utter pain to get in to and out of.. Vacuum is easier.

Some exploration of the Asteroid belt is likely. It may prove to be a better source of some elements than accessible Lunar rock. If this is the case then there may be semi-permanent settlemens on asteroids, like mining towns on Earth.

You need to think big if it is ever going to happen. Shielding against Solar storms requires walls several feet thick. Substituting gravity by rotation likewise implies large. Build habitats a lot like John Varley's titans, but without the insane goddess.

The goal? Unlimited real estate. The Earth is getting full.

Outside bet? Mercury's poles. There are craters which offer permanent shade, yet nearby (or up a bit) is all the energy one might need. Shame about the Solar gravity well.

Answer 14

Venus is a prime candidate, it is actually really easy to get to Venus (assuming aerobraking), and as stonemetal points out its gravity is great for humans; unlike the moon and Mars where we don't even know what the long term effect would be of living on those places would be. However, you couldn't hit Venus's surface as it is too hot, too much pressure, and too acidic. It would have to be in the clouds as per this purposed mission http://www.csmonitor.com/Science/2014/1222/How-a-manned-mission-to-Venus-could-actually-make-sense-video Unless there was some serious terraforming effort, but that would take centuries or longer to figure out. The materials to build the cloud cities would likely have to come from else where, though gasses could be had from the atmosphere. Just saw this recent update, actually not much in the way of new info, but a new article on sending people to Venus: http://www.space.com/29140-venus-airship-cloud-cities-incredible-technology.html

Near-Earth Asteroids - Asteroids are actually ideal for a lot of reasons, they don't have huge gravity wells, there aren't as many concerns about potentially destroying pre-existing life as with Mars or some other options, and in the case of Near-Earth Asteroids, their orbits are such that they are easy to get too. They can also be mined in to for radiation shielding, and depending on the size and stability be spun up to create artificial gravity (though that may be more iffy).

The Asteroid Belt- Same as with the Near Earth Asteroids but further out, there are bigger ones such as Ceres, Vesta, etc, the large ones actually make up the majority of the mass of the asteroid belt.

Jupiter has its four big moons, and then a ton of smaller ones. The big ones are dwarf planet size. Callisto http://en.wikipedia.org/wiki/Callisto_%28moon%29 is perhaps the best one to head to because unlike Io, there aren't a ton of Volcanos, Unlike Europa there isn't a likely watery ocean beneath it that could have life, and it has a smaller gravity well than Ganymede, which could also have life. NASA actually identified it as one of the most likely candidate for human colonization: http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf The radiation on the other moons is quite high.

The thing to realize about going past Jupiter is that Saturn is roughly the same distance from Jupiter's orbit as Jupiter is from the Sun, and it only gets worse from there. Issues of heating and powering a colony become a truly serious problem. If Fusion power is harnessed though, then they become more attractive. Uranus in particular has the lowest gravity, and at one atmosphere of pressure is 90% earth gravity as well as being rich in Helium-3. Also attractive are Titan and some moons that appear to have geothermal activity.

Freeman Dyson thought that comets were the future of mankind, not planets http://en.wikipedia.org/wiki/Colonization_of_trans-Neptunian_objects which would mean that colonization could continue forever and to the stars but society would be fragmented as the distances become too large to travel in any sort of reasonable time period, even communication becomes quite costly.

Answer 15

Let's look at the different bodies, their pros and cons, and see where that leads:

• the Moon Always a favourite of Sci Fi authors.
  Advantages: it's close to home, supply ships, passenger vessels, evacuation/rescue missions are only days away.
  Disadvantages: It's got no atmosphere, the gravity is very low (which might be an advantage as well, but many people will get space sickness and suffer osteoperosis, making return to high G earth difficult after spending years there).
• Mercury
  Advantages: none, unless you want to build a solar powerplant there, and then you'd best make it fully automated.
  Disadvantages: searing heat during the day, extreme cold at night. No atmosphere, no radiation shielding. And no, it's not tidally locked (which might leave a narrow band around the terminator that could be used).
• Venus
  Advantages: none, despite being a favourite of classical SciFi (from before space exploration found the truth about the planet)
  Disadvantages: HOT, very high pressure, toxic, acidic atmosphere, high radiation environment
• Mars
  Another favourite for colonisation.
  Advantages: it's BIG, lots of space there. There might be ice and most other things needed to sustain life in its soil. It even has a thin atmosphere that MIGHT be able to sustain some plant life (but definitely not humans).
  Disadvantages: It's far away, making it hard to get to. It's COLD, very very cold. The dust storms make it a bad place to be caught out of shelter.
• gas giants (dumping them all together, they're similar enough)
  Advantages: lots of gas, might be possible to mine it somehow, giving a nice economic incentive.
  Disadvantages: high radiation environment, no solid surface to build on (always a problem), atmospheres very violent and probably toxic, plus high pressures. Very low or high temperatures (depending on the planet and how deep you go).
  That said, among their many moons there might be some places that could provide interesting places for colonisation. But they'd share the problems of high radiation, low temperatures, and low gravity that we've seen so far.
  
• Pluto
  Advantages: none, really. Might work for an underground prison colony but little else.
  Disadvantages: extremely far away, no atmosphere, extremely cold, highly excentric orbit makes it very hard to get to or communicate with reliably.
  
• Asteroids
  Advantages: easy to get to, tons of materials for building space habitats out of (and enough left over to export to earth at a tidy profit)
  Disadvantages: nothing there to build on, you'd have to build your habitat in deep space, and provide it with defenses against impacting rocks as well as radiation shielding. Same disadvantages as any space station, except it will be much bigger...

Answer 16

While I think the asteroids would be the next target I'm going to add another Mercury option--because user1987s answer missed a really big factor:

As he correctly stated the poles are cold, the equator is hot. What he missed is that this translates to a Goldilocks zone between them. Dig deep enough that you have a constant temperature (this doesn't need to be all that deep) and you have a band around each pole where you do not need heating or cooling other than for load balancing.

Also, others have rejected Mercury because of the high delta-v requirements. For exploration this is a big deal. Once you have enough traffic, though, it ceases to be a problem and Mercury becomes easier to reach than the asteroids. The key is big linear motors. Once you have enough space traffic you build one on the Moon. It's wrapped around the equator. It can put a manned capsule on a transfer orbit to any point in the solar system. Once Mercury base is big enough you do the same thing there--not only is it a launch system but it's built to grapple a passing spaceship (you'll have good navigation aids, guiding a spaceship on a grazing orbit to get within grapple range certainly could be done), the whole trip is done with only course-correction fuel. The transfer orbit to Mercury is faster than to the asteroids and the launch windows are more frequent--both of those things say "closer" to me.

Any large enough, airless body could mount such a system. I haven't worked the accelerations you'll see from a system on Ceres but you certainly don't have access to the whole solar system from there. Even if it can toss stuff back and forth from the Moon you'll still have to use rockets to get to the other asteroids--and note that while the delta-v requirements for such trips are very low the travel time is long.

Answer 17

Besides already mentioned Venus (at 50 km above the surface), Callisto (probably the best of Jovian moons), Europa (if you don't mind some digging) and Ganymede (who needs it when we have Callisto?), I'd bet on Titan. Besides its popularity among sci-fi writers, I'd note some other advantages as well:

• Much less radiated than Jupiter moons;
• Has some gravity (in contrary to Ceres and other asteroids);
• Shallow gravity well;
• More fuel than on Earth (it rains hydrocarbons!);
• Lots of water (and, consequently, oxygen from it);
• Can be used as a base for Saturn mining (again, better than Jupiter, although further);
• Nitrogen-rich atmosphere (can be used as a buffer gas);
• Methane, ammonia and nitrogen can be used to produce fertilizers;
• Most important one: atmospheric pressure is 1.5 of Earth's. Which gives us:
• No need for pressurized habitats and suits (a huge engineering advantage);
• Radiation is blocked out;
• Aeronautics is easy to lift-off;
• We can strap on wings and fly! What else could we possibly ask for?

Now for some disadvantages:

• Further away than many other targets;
• It is cold (in contrary to space vacuum, it is an "active" cold because of atmospheric pressure);
• Atmosphere is toxic (as well as on other bodies with an atmosphere);
• Gravity is low (again, as almost everywhere).

Overall, Titan is often considered as one of the top-5 places for human colonization. It has its challenges, but a good thermal insulation (parka?) and some oxygen tubes are much easier to engineer and use than conventional spacesuits. Same goes for habitats which don't need to be pressurized and radiation-shielded.