Although the Moon is much closer and "right there" for us, it's subject to numerous meteoric incidents, some of which are disastrous enough to wipe out any settlement we set up there.
Moreover, only half of the Moon sees the light of the Sun. This fact alone reduces available surface for settlements to only half of the Moon.
While the Moon could be a temporary backup plan, in the long-term, you're going to need to terraform a planet and make a suitable atmosphere, both of which are not possible to do to the Moon.
Edit: Thanks to the commentators below who mentioned that while the Moon is tidally locked to the Earth, most of its surface does indeed receive light from the Sun.
Only one question though: If that is true, then why do they call it "the dark side of the Moon"? (Honest question)
> Only one question though: If that is true, then why do they call it "the dark side of the Moon"?
This doesn't refer to sunlight, it refers to radio communications. The back of the Moon, relative to Earth cannot receive any kind of signal we can send without some kind of artificial relay. Because the moon it tidally locked with the Earth the same side always faces us.
So the back side is always dark to our radio even though it has about a 28 earth long "day". Any (non-polar) spot on the Moon gets about 14 Earth days of light and then another 14 of darkness, corresponding with the phases of the moon we see here.
Yeah, it does. The real concern is that the moon-day is a full month long and there is no atmosphere transferring heat; any permanent structures on the surface will get very very cold during those two weeks in the dark. If we don't solve this with suitable solar-charged batteries we would need multiple outposts -- say 4 of them dotted around the Moon -- with colonists rotating through the cities to stay in the Sun. But that's also kind of a non-starter because the distance between those cities is something akin to the distance between Denver and New York City -- it's not impossible but it's also a nontrivial drive to be doing every week just to remain in the sunshine.
I'm glad you asked! So there's an important physics reason why the equator is warm and the North and South poles are cold on this planet, and it has to do with how you point a surface. If you ever find yourself in a dark office with a bright desk lamp (or maybe your home office or kid's bedroom might be suitable) you might even try this experiment: take a flat surface, a notepad or steno pad, and hold it up facing the light source, then slowly tilt it away and look at the color of the surface; you'll notice that it doesn't sharply transition from illuminated at 0 degrees to dark at 90 degrees -- instead it smoothly varies like the cosine of that angle.
This mathematical effect is incredibly important, it means that sunlight, during the day, averages out to being half as strong as its maximum over the whole of the Earth's surface. The calculation isn't even particularly difficult: the surface area of a sphere is well-known to be 4πr², half of that or 2πr² is illuminated at any one time, but the actual irradiation that we receive is proportional to the cross-section area, which is just the area of the circle: πr². So if the Sun-directly-overhead light were to be illuminating that entire half of the world, we would get k·2πr² light for some k, but instead we only get k·πr² light for that same k, so it works out to be 1/2 when averaged over the whole surface of the Earth.
While I was at university a fellow student asked me to guess the coldest place in the Solar System. I guessed "the middle of the dark side of Mercury." I guessed this for a couple reasons: (1) I knew I needed a rock without an atmosphere since atmospheres sustain convection currents that transmit energy, and (2) I figured since Mercury is so close to the Sun it's probably tidally locked to the Sun and therefore this part probably has not seen a speck of sunlight in millions of years.
It turns out that general relativity makes point #2 wrong and my friend gleefully informed me that he was looking at an article (there were many, so let's take [1] as representative) suggesting that it might be in a crater on the South pole of the Moon. This has basically the same reasoning of (1) and (2) above, except substituting the shadow of a crater for the shadow of tidal locking. But Mercury is still in the running -- the only issue is that we might not be looking at the dark side of it, but rather, again, at its poles.
Mercury won't be under any serious consideration for habitation for a long time. It is down at the bottom of the biggest gravity well within a few light years. Too much delta-V to come or go. It is the least explored inner planet for a reason.
I think you are drastically overstating the difficulty of heating a lunar polar base. You are surrounded by vacuum, one of the best insulators around. It can be used.
Of course a lunar base would probably be underground (for more radiation protection) and the lunar soil would quickly wick heat away. Simple to deal with. Excavate a small cavern and suspend the base inside the cavern. Aluminize the cavern walls if possible. Now the colony has its own thermos bottle and staying warm is easy.
Sunlight is just as intense at the lunar poles as the equator. The ground receives less solar energy, but that doesn't mean that less energy is present. In fact there is more solar energy available at some sites since the panels could receive light continuously instead of in 28 day cycles. (Might require putting the panels up on a tower to help reduce libration shadows?)
A solar panel tower at the North Pole of the Moon might be a really good idea for solving these problems, yes.
Vacuum is only one of the best insulators around if you're near other warm things, I'm afraid. Out in space the dominant form of heat exchange is the Stefan-Boltzmann law, which says that if you have a half-acre of total surface area on your moon-base, and it's at 300 kelvin, then you need to supply about a megawatt to keep it warm.
It's not an insurmountable problem, of course -- Tesla just announced a battery for Australia at 129 MW-hrs, which would sustain the thing for 5 days, which is not enough but it's only a half-order of magnitude off. (I'm also not considering something like nuclear heating; it's not uncommon to have a 1000-megawatt nuclear reactor in the US and that's just the power output, not the heat --
so it's not a terribly bad way to go if you can shield the rest of the base from it. The only hazard with that is one recently discussed by John Oliver on his show, firing nuclear things into space is heavily, heavily complicated by the nowhere-near-close-enough-to-zero failure rate of launch rockets and the insanely-scary-cost of distributing a bunch of reactor-grade nuclear fuel into the atmosphere.)
I still think you are overstating things. Stefan-Boltzmann is just a fancy way to
say radiation. That is more or less a solved problem with nested shells. Each layer of shell cuts the losses in half. But let's ignore that for now, since more than one layer of shell complicates construction.
Historical aside: before we had good solar panels, people tried crazy stuff like the Phaeton satellite that used a generator spun by a heat engine powered by liquid mercury heated by parabolic reflectors. Let's bring that idea back.
1MW only requires a 30 meter by 30 meter thermal solar collector. Pump that 1MW of heat into the hab and run a heat engine, using the shell of the hab as the thermal sink. Now that constant 1MW drain is a required feature.
Actually looking into it a bit more to answer your question, I'm entirely confusing two different things. Mercury is tidally locked 3:2, it rotates 3 times for each 2 times it orbits -- but that is a result of history and not general-relativistic corrections, which are necessary to explain a different value (the precession of its orbit).
In fact our best understanding of why Mercury has the 3:2 relationship appears to be (if I'm reading these papers right) "because of historical reasons." That is, now that it's in that state, that state is stable and unlikely to decay to the 2:1 or 1:1 state, but getting into that state is much less clear and potentially requires that in the past, the other planets of the Solar system had nudged Mercury into a more eccentric orbit than it has now. The claim is that the eccentricity of that orbit caused the 3:2 resonance, then in more recent history these perturbations from other planets averaged out more to make Mercury's orbit less eccentric, but it still holds on to the resonance from its past.
>If that is true, then why do they call it "the dark side of the Moon"?
It's the side we can't see from Earth. It's dark as in unreachable by line of sight or light, eg radio communication, from Earth. "Far side of the moon" is the preferred term but "dark side" is popular and persists.
> Moreover, only half of the Moon sees the light of the Sun.
What do you mean by this? The moon is tidally locked to the Earth, but the entire surface sees sunlight over the course of ~a month with the exception of a handful of polar craters where small portions are perpetually in shadow.
> Moreover, only half of the Moon sees the light of the Sun
That's not true. There is a side of the Moon that's always facing away from us on Earth but each side of the Moon actually gets about 2 weeks of sunlight followed by 2 weeks of darkness.
I used to fantasize about exploiting the temperature gradient between hemispheres for generating electricity on worlds with long days like Mercury and the Moon when I was a kid.
> then why do they call it "the dark side of the Moon"?
Because four billion wordwide population - all living - have a Computer God Containment Policy Brain Bank Brain, a real brain, in the Brain Bank Cities on the far side of the moon we never see.
Moreover, only half of the Moon sees the light of the Sun. This fact alone reduces available surface for settlements to only half of the Moon.
While the Moon could be a temporary backup plan, in the long-term, you're going to need to terraform a planet and make a suitable atmosphere, both of which are not possible to do to the Moon.
Edit: Thanks to the commentators below who mentioned that while the Moon is tidally locked to the Earth, most of its surface does indeed receive light from the Sun.
Only one question though: If that is true, then why do they call it "the dark side of the Moon"? (Honest question)