Why build space habitats on/within moons?

Question

Consider yourself a decision maker for a fledgling space infrastructure project. You've been tasked with establishing the first few extraterrestrial colonies for your species.

What aspects of the Solar System's moons make them more (or less) attractive than other bodies (like comets, asteroids, and/or airless planets)?

Are any of these clinchers (reasons that make it obvious to establish or avoid colonies on moons)?

09/30/2015 edit/addition: I can imagine several reasons and/or constraints for a space colony:

1. As the first piece of essential space infrastructure
2. As a transportation hub
3. To become a self-sufficient colony
4. As life insurance for the human species
5. As a profitable venture

As many of you have already realized some of these overlap. A successful colony would possess multiple aspects of these to some extend.

But let me elaborate on some consideration.

Location

Space travel is more tightly constrained by the propellant budget (delta V) than the time scale. Other than finding safety from major solar events, one place is rather like another in space. Locations very distant from Earth (Deimos, Earth crossing asteroids, etc.) might be "very close" from a delta V budget perspective yet be very distant from Earth.

Distance isn't the constraint, delta-V is the constraint

Resources

All colonies will require extensive resupply from Earth in the beginning. However, colonies located at sites with essential resources have a hope of becoming self-sufficient for bulk necessities (propellant, water, oxygen, plus construction materials) in fairly short order.

Furthermore, a permanent settlement in most locations of the Solar System will require radiation shielding and radiation shielding requires mass. So a man-made settlement in mid-Earth orbit probably can not compete with other ideas unless an Earth crossing asteroid is brought into orbit and used for materials.

Some resources are more valuable to the colony than others. A new colony will want large quantities of water, oxygen, nitrogen, and hydrogen to begin their closed cycle ecologies.

A self-sufficient colony requires resources in bulk.

Size

Obviously, humanity's initial colony attempts will tend to be smaller so we can learn without endangering the lives of millions. But the number of people that we intend to settle makes a huge difference on which bodies would be viable sites. For instance, if the goal is to resettle a large portion of the Earth's population, Mars would have to be the main colony site. No where else in the Solar System has as much "hospitable" real estate as Mars.

Energy

If the colony is established outside of Mars orbit, then it will necessarily use a fission reactor (unless we assume fusion is available by then). From a mass perspective, it will likely cost more launch mass to power the colony using solar power than fission (unless something like a solar concentrator method is used).

Communication

The further away the colony is from Earth, the slower communication will be (both latency and bandwidth).

Export

Ultimately a colony trying to become a viable commercial venture will need to produce something for Earth and do so more economically than it can be done on Earth. The reality is, it competes with terrestrial markets, it is going to fail. The only location your colony has an advantage / barrier to competition is in space.

So my thought is the colony will initially build and service satellites for Earth. Along with this, a colony with access to lots of volatiles (e.g. water) can be contracted to clear all the junk from terrestrial orbit. Incidentally, it'll provide the colony with a bunch of valuable already refined materials.

As a side business it will do many other things (e.g. build fantastic astronomical facilities, perform probe flybys, serve as a tourist destination, and perform sample return missions for Earth) and these might be highly profitable but I don't think of them as high volume business.

Unless there's a compelling reason why it makes more sense to do it in space than on Earth, initially the colony won't be in the manufacturing business. I understand that some process work better in space (it is possible to make better ball bearings in space - perhaps for very expensive high precision machinery? I've heard it is easier to control crystal growth too) but the colony's first goals will be to provide its own bulk materials. The low mass stuff requiring a lot of infrastructure will come later.

Selection Criteria

What I want is a location that provides:

• Room for a colony of small size (50-500 people)
• A convenient (from a delta V perspective) destination for Earth and other points of interest
• Access to in situ resources - especially water, oxygen, hydrogen, carbon, nitrogen, and basic industrial materials/metals
• Adequate radiation shielding
• Adequate power
• Ability to expand (from a room and resource perspective)
• Not a necessity but bonus points for, providing a reason for tourist travel (view, uniqueness to the surrounding, etc.)
• Not a necessity but bonus points for, providing the ability to
  establish or expand a manufacturing base (drugs, crystals,
  electronics, ball bearings, extra pure silicon for chips, and other items made best in zero gravity, vacuum, or both)

FYI, my favorite McGuffites (reasons for going to space) are room temperature electrical superconductors, pure vacuum required for making very long (km+ length) carbon nanotubes, and alien artifacts.

Answer 1

It depends on your colonization goals. Note that this answer is specific to Earth's moon (Luna). I don't see much point in colonizing other moons.

I'm going to mostly contrast with this link from NASA that Frostfyre posted. I would favor the moon for the following reasons:

• I don't care about going back. As long as my human colonists are healthy in 1/6th of normal gravity - which is by no means a certainty - I'm ok with that.
• I don't care about tourism, so the view, or lack thereof, isn't a major factor.
• It provides free gravity, even if reduced. Spinning habitats complicates engineering. Building or digging in low gravity is presumably easier.
• You can get materials and resources from the body you're on. It may be more efficient to go elsewhere, but if something goes wrong, I think a moon colony is more likely to survive and "rescue itself" vs an asteroid or orbital habitat.
• It's in a sweet spot, location wise. It's always close to Earth, which means you have access to most of humanity and their resources if you really need it. On the other hand, it's far enough away that you can more easily defend yourself. If you get in a fight with a ground-based country, an orbital habitat is extremely vulnerable. Building on Mars makes it easier to defend, but harder to build up and the variable orbits make transportation a real pain.

To sum it up (TLDR):

Orbital habitats are too vulnerable to attack and will likely never be self-sufficient.

The Moon is close enough to Earth to build a colony efficiently, but has the resources and is far enough away that you can eventually become independent.

Answer 2

Factors favoring moons over planets:

• Moons are smaller in size so it is easier to terraform them.

• Most planets we have discovered so far are much bigger than Earth. This means their gravity is too much for us (earthly organisms) to bear with. Moons, being smaller do not pose this problem.

• Airless moons are easier to fill with an atmosphere because they are smaller and need lesser amount of atmosphere to form the same amount of atmospheric pressure.

• Similarly, moons need lesser volumes of water to form a pan-lunar water body (ocean) to regulate temperatures.

• Moons have lesser terrain variation than planets which makes habilitation easier.

• Planets are larger than moons and hence have much greater gravitational pull. This means that most incoming asteroids get deflected towards planets, sparing moons.

Factors disfavoring moons:

• Most moons have too weak gravity so movement is much harder than on planets.

• Most planets have a strong magnetic field while their moons have next to nothing (compared to planets) so that solar wind devastates moon while having little impact on a planet.

• While a planet's high gravity pulls most asteroids towards itself, the impact damage of an asteroid hitting a moon would be dozens of time greater than if that asteroid had hit the planet.

• Vortexes and typhoons are much more severe on moons than planets due to the greater temperature difference and lesser surface area of the moons.

• Moons rarely ever have high enough elemental variety to support complex life. While sodium, potassium, oxygen, nitrogen, carbon, sulphur, calcium, phosphorous and chlorine are absolutely vital for earthly complex life, smaller amounts of vanadium, titanium, aluminum and magnesium etc are also necessary for health. Such a high variety of elements is extremely rare to expect on a moon but quite common for planets.

• The tidal effect of the nearby planet would create huge tides in the oceans and might also gradually strip a moon of its atmosphere.

Answer 3

"Once you get to earth orbit, you're halfway to anywhere in the solar system." ~Robert A. Heinlein

Early on in colonization, orbital mechanics and fuel sourcing will be the deciding factor in how your colonies spread. Although Earth has unimaginable quantities of resources, the atmosphere and gravity prevent those resources from getting places. In particular, it is impractical to source rocket fuel from the Earth's surface for long-term colonization projects because it takes so much fuel to lift the fuel, and the fuel needed to lift the fuel needs more fuel to lift it, and so on. It's known as the tyranny of the rocket equation.

One of the reasons why NASA looks at the Moon as a stepping stone to the universe is because there is ice at its poles, which can be hydrolyzed into oxygen and hydrogen, providing reaction mass that can easily be transported off of the Moon's surface.

Ideally you want a solid celestial body that has material that can be used as reaction mass for rocket engines, has little or no atmosphere to impede rocket launches, but has lots of gravity so spacecraft can achieve low, fast orbits. The reason you want low, fast orbits is for efficiency. The energy stored in rocket fuel is much more efficiently converted into useful velocity change (delta v) when a rocket is travelling at high speeds. When performing interplanetary transfers, the rocket needs to escape the gravitational sphere of influence of the near body AND it needs to have excess velocity to make the journey to the distant body. Having a high orbital velocity to start with makes it easier to achieve these excess velocities.

"Why not just take off from Mars?" you might ask. Mars is actually a pretty good candidate, because of its thin atmosphere. But Mars is a planet, and planets are missing something: speed. The advantage of moons is that spacecraft taking off from a moon is already going around the planet at the speed of that moon. For Earth's moon, Luna, that amounts to an average of about 1 km/s (http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html) of velocity for free.

"Why not use a comet?" Comets are crazy. The defining characteristic of a comet is that it has a highly eccentric orbit around the sun. This makes comets exceedingly hard to get to and from because the changes you have to make to your spacecraft orbit require huge changes in velocity. Once you get to the comet, you're pretty much done because there's not much material there anyway.

"Asteroids?" Asteroid belts are dense by cosmic standards but surprisingly sparse in practical terms. Individual asteroids are pretty small and they are millions and millions of kilometers from each other. It's still not too bad if you have fuel and time to move between them. But again, getting to and from asteroids is tough because they don't have much gravity to capture an incoming craft.

So moons are pretty good places to start because they are big enough to capture incoming vessels, big enough to make big orbital changes efficiently (fast orbits), are small enough to have very little atmosphere, small enough that it doesn't take a huge amount of fuel to get off the surface, and big enough to colonize and manufacture fuel for a long time.

Tycho crater near the south pole of Luna has been theorized as a good place to start space colonization because it's near the ice cap, can receive sunlight all year round, yet has available shade because of the crater edge, and has all of the orbital advantages I just mentioned (except for being out of the Earth-Moon orbital plane, which increases fuel costs, slightly).

So if low Earth orbit is halfway to anywhere, being perched on the Moon is about three quarters.

Answer 4

People might pay $200,000 a night for a hotel room to see an EarthRise under a star lit glass bio-dome while passionately enjoying 1/6 Earth gravity with the one they love.

Answer 5

The best place to build a colony isn't on the surface of, or within, a celestial body, but rather in orbit. NASA has a good briefing on Space Colonization Basics. In a short summary from the article:

Because orbit is far superior to the Moon and Mars for colonization, and other planets and moons are too hot, too far away, and/or have no solid surface.

While we could build facilities on other worlds, I suspect the first step to colonization will be in orbit around a body, rather than immediately on the body. There's a whole host of issues pertaining to building a colony on a body: difficulty of getting to or from space, local weather, local atmosphere (or lack thereof), local geography, local geology, etc. Building in space has fewer variables.

Note: This is a very extensive topic and not really suitable for the Q&A nature of SE. For a more in-depth examination, please read the linked briefing.