For the coming century or two, far below the surface of Mars is our best bet for colonization. We may even find that most of the tunnels and cavities for a million humans already exist below the surface of Mars. Assuming Mars has a center temperature of 500 degrees c = 932 f, pleasent temperatures are likely common 100 kilometers below the surface. We should practice now by using female prisoners to live a mile below Earth's surface. I think we would have little trouble getting volunteers.
We don't need to send but two female humans to Mars, with a sperm and embyo bank to have thousands of offspring in about a century. Two or three pairs at different Mars locations will get the population quicker and surer. Neil

I think with all the people who have said they'd love to go to Mars, they'll take care of population growth by themselves .

Mars is one possibility, sure. Space habitats built using materials from Near Earth Objects can also work-- plenty of raw materials, including water and other volatiles and carbon compounds in the carbonaceous asteroids, plenty of building materials and radiation shielding mass. Abundant 24-7 solar power and spin gravity to any level you desire (gravity is Mars' one potential downfall as a colony site. We don't know yet how that'll work out.) And less of a commute for the builders.

As for using prisoners to test it-- why prisoners? If they're criminals, they've already proven they can't be trusted. And most convicts are undereducated, so they couldn't give accurate reports of their condition.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

That's possible, but first we need to colonize the moon. At least for practice and testing stuff. Maybe I'm wrong, but I don't see Mars getting colonized before the moon. The underground thing seems difficult at first and I never heard of Mars having miles of underground tunnels. I think more likely are self-contained atmospheres in bio-domes. As for the gravity obstacle, we will have to somehow make/simulate gravity to match that of earths (or maybe some other value that is ideal for human conditions).

I would volunteer to go to Mars if they sent female prisoners over there

Testing designs and colonizing are two very different things. If Mars' low gravity may be a problem, Lunar gravity will be a much worse problem. We need to first determine where we can live, before we actually try to live there full time.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

I also like near Earth asteroids, but it is a harder sell as it does not have the glamour of Mars. If we use prisoners for inital testing a mile below Earth and for the first two or three Mars and asteroid colonies, fatalities aren't much loss.
If we have lots of volunteers we can likely choose well educated, prisoners who will report accurately, and not complain much as it will be a better life than in prison. One way trips are much cheaper than round trips for the next twenty years or so. I think we should think one way, with the promise of assistance, if the colonists devise a way to go elsewhere. That would provide lots of motivation if the colony is unpleasent. If we start soon we may have a dozen tiny colonies (no two alike) thoughout the solar system by 2060. Neil

Then again a lot of SF authors have argued that putting yourself deep inside another planetary gravity well is not a good thing.

There's lots and lots of water, minerals, raw materials and small planetoids that would make great colonies for billions of people in the solar system (and a stepping stone across interplanetary space to other star systems....)

Only if we use death-row convicts, who I don't think would be the ideal candidates. Sending our least wanted also totally fails to build public confidence in space as a place to live. Kinda destroys the "glamour".

Why a mile below Earth? Only a few meters of rock or dirt are necessary to stop dangerous levels of radiation. No need to work harder than we have to.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Yes and no. We need to test colonization itself. Problems will come up over the course of a year that can't be reproduced in a lab on earth. We need to actually see what happens when we build our bio-dome and live in it.

Unfortunately, Mars is no where near as close to us as the moon. So even though the gravity situation is worse on the moon, it still serves as a better testbed than any other body in the solar system. I have faith that the human race can overcome the obstacles needed to colonize the moon and simulate gravity on the moon. If we can do it on the moon, than simulating gravity should be easier to do when we get to Mars since, as you insisted, is less of a problem.

Colnizing implies permanence. A year-- or decade-- long settlement isn't a generations-long commitment. How can we simulate gravity? The only way consistent with the laws of physics is spin, and for that, an orbital habitat is the only real solution. They can be built anywhere, even right in Earth orbit. They can even be moved if the need should come up. No gravity well, no dust storms, any g desired, which means we can test the effects of living in Lunar and Martian gravity longterm without going all the way there. An hour from Earth versus three days or a year.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

For some reason I am now thinking of the movie Doctor Stranglove.

Hmm, who needs rockets when we can dig our way to Mars.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

Unfortunately, keeping living space open 100 kilometers below the surface of mars would be something like trying to keep a party balloon inflated at the bottom of the ocean. Any structure would be under an awful lot of pressure. I think it would be a lot easier to live nearer the surface as there are plenty of ways to generate heat but not many to successfully resist 100,000 tons of pressure per square meter.

I have some serious moral problems with using prisoners as test subjects, either on Earth, or for one way trips to Mars. Sure, this has been done before on Earth, but so have lots of immoral things.

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At night the stars put on a show for free (Carole King)

One Earth, One Sky - IYA 2009
All moderation in purple

Biodomes can be built here on Earth, thousands of times less expensive, far less dangerous, and involving faster reaction times and therefore much faster development.

I strongly disagree. The Moon is not a good “testbed” for anywhere else we might want to go. Except for the gravity, every other possible parameter can be tested here on Earth far far cheaper, far far safer, and much faster.

Simulate gravity? Sorry, but that is merely a sci-fi movie gimmick to make filming a lot less complicated. Creating artificial gravity in a rotating space station may be practical, but “simulating” a gravity other than that available locally is science fiction.

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Reality: What a concept!……………………..><Ç(((Ç°>

That would help against meteoroids and other space debris, too.

How far underground would we have to go for reasonable protection against
radiation and impacts?

I don't think we will ever run out of volunteers for space-related ventures.
No need to get entangled in morally questionable experiments.

Three meters should be plenty to protect against radiation. Mars doesn't have a micrometeorite problem because of its thin atmosphere. For bigger impacts mars colonsists should take about twice as much precaution as we do on earth. That is, none. Of course, in the future even that small risk may be deemed worth protecting against.

Three meters? Is that because of cosmic ray cascades? One meter should be sufficient for gamma rays.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

O'Neil said three meters of lunar regolith was enough to protect against radiation for space colonies, so I said three meters would be plenty, especially considering you have the bulk of mars protecting from below and to the sides. But one meter of packed dirt might be enough, and should stop over 99.9% of gamma rays. I'm not quite sure how much protection is needed against cosmic rays.

One way or another, it appears that humans require a certain level of gravity to remain healthy. The free gravity, plus the availability of resources at hand, seems a good reason to inhabit a planet.

Are you suggesting we colonize small planetoids such as asteroids and small moons? Remember that on Phobos an average man would weigh about 2 grams IIRC. On some asteroids he would weigh even less. Since they cannot be spun up for artificial gravity, I don’t see how they could be colonized.

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Reality: What a concept!……………………..><Ç(((Ç°>

Olympus Mons might be a good place to set up a colony. Since it is so tall, the side could be used for an electric rail launch platform. Since it is volcanic, it may well have hundreds of miles of lava tubes that could be used for habitation.

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Reality: What a concept!……………………..><Ç(((Ç°>

Build a centrifuge inside and spin it up for people to spend time in. Or change humans so weightlessness doesn't bother them.

Colonizing Phobos Deimos should be cheaper than colonizing mars. Mars's atmosphere can be used for areobraking, but instead of bringing an expensive lander, colonists will instead bring parts for a centrifuge and colonize one of the moons instead.

But the moons of mars and other asteroids are probably low on volatiles and organic chemicals. They would be useful as substrates for stations and have minable resources, but you would not be able to live off of the land, so to speak. The hope is that Martian regolith can be adapted for agriculture, if not by terraforming then by use within controlled structures. At least, I don't think people were expecting to import earth dirt.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

Well, no. The moons of mars should be richer in volitiles and organic chemicals than a typical piece of mars. They appear to be captured carbonaceous asteroids, which contain large quantities of water and organic compounds. This type appears to be very common in the asteroid belt.

For a short term visit, that might be acceptable. But for actually living there permanently, That would be totally unworkable. Spending perhaps ¼ of your life in a centrifuge just to stay healthy would not be living at all. Let’s see, you said billions of people…………………

If that could be done, which is highly unlikely, they really would not be humans any longer. Although it is possible that there are a few people that would be willing to be so radically modified (or allow their children/grandchildren etc. to be so dramatically different), it is unlikely that any significant number of people would want that.

The problems with micro gravity and lack of resources would make colonizing the Martian moons impractical. Mars would be much much cheaper and easier to colonize, for many reasons, all of which have been posted here repeatedly.

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Reality: What a concept!……………………..><Ç(((Ç°>

Hmmm………. based on the information we currently have on Phobos and Deimos, that is an unsupported guess.

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Reality: What a concept!……………………..><Ç(((Ç°>

It's not an unsupported guess. We have charbonaceous chondrites that have fallen to earth and we know their typical composition, density and spectral characteristics. We know the density and spectral characteristics of Phobos and Deimos. They appear to be carbonaceous. I suppose they could be somehow faking it, but it seems unlikely.

I don't recall that. Could you be so kind as to quickly summarize why mars would be cheaper and easier to colonize than it's moons? Since it's not really possible to accurately estimate dollar costs, perhaps we should consider tons of payload that would need to be sent from earth.

Depending on the soil, you'd want to go down to at least 3 meters or more. The depth is more than enough for radiation coverage- you use the extra mass to keep from blowing your roof off due to pressure difference between the surface atmosphere and the livable habitat beneath. Of course, you don't have to dig all that out- you can dig part of it and then pile the soil back on top/around when you've built your internal structure.

Mike

On Mars, you can extract Oxygen and Carbon from the atmosphere right from the start using rather simple and robust chemical processes, lessening the tonnage required for supplies. (For rocket fuel alone, over 90% of the mass comes from Carbon and Oxygen, which means you can replace a LOT of your fuel tonnage from Earth with other stuff). If you can find a reasonably available source of water, Hydrogen becomes readily available as well. These would drastically lessen the supplies necessary for sustainment of a colony (air, water, and rocket fuel) which would otherwise have to be supplied from somewhere else. While they may well be available on Deimos or Phobos, they would have to be mined or otherwise extracted, while Carbon and Oxygen are easily available on Mars right from the atmosphere. Once a station/settlement/colony gets larger, other elements will almost certainly be available to be extracted from the soil for various amounts of effort.

As to other advantages of the surface, habitat temperature control, micrometeorite and radiation protection are aided greatly by the atmosphere. The gravity of 3/8 Earth will certainly aid in reducing or even eliminating the health effects of microgravity, and it will certainly make the indoor "shirtsleeve" environment more endurable for the inhabitants, as it will be more closely matched to the traditional, Earth-based environment (with up and down being physically distinct, for instance).

The surface of Mars also offers a vastly larger area with many possible landing sites to choose from (selecting for availability of local resources, proximity to areas of interest for scientific research, etc.).

Mike