After thinking about this topic for a while, I decided that we humans need to concentrate on developing space habitats at least as much as dreaming of ways to colonize a planet. Don’t get me wrong, I am all for colonizing large natural bodies (minor planets and larger), as have numerous advantages. However, even assuming a “quick terraformability” scenario for the planets (especially Mars), they would still suffer from multiple problems based on two primary shortcomings: gravity and radiation:

*Deep Gravity Well (even if not quite to the extent as Earth’s) - increased transportation expense for goods and services. Space piers, space fountains, and space elevators can help reduce the expense, but the fact remains that escaping a space habitat’s gravity well is much easier than escaping even Ceres’ well.

*Gravity Substantially Weaker than Earth: Advantageous for construction of surface structures, but quite perilous for humans (assuming people want the option to visit Earth in the flesh). This is especially true of the Moon and Ceres. I do admit that this point becomes less relevant if (a) we develop drugs to strengthen human bones, muscles, and organs when visiting Earth, (b) gene-enginnering so that space colonists can adapt very quickly to their present gravity-environment, (c) gene-engineer local Martian (or other body) foods so that the foods contain vitamins and minerals that rapidly replenish the bones, muscles, and organs (this is my favorite long-term solution, if it’s achievable)

*Radiation Hazzards - these can be easily overcome by (a) filling a dome with ozone AND creating a coating that makes the dome largely opaque to UV. An electrically-generated magnetic field can also screen out much of the magnetism in each local settlement (we can presume they’d have multiple backup generators should a mag-field generator fail). Even so, time and again, nature proves more reliable than even the most advanced human technology, and probably even foreseeable human technology as well.


For the above reasons, I now believe space habitats / orbital habitats are the optimum solution, at least for the next several centuries. They have several advantages over colonizing a planet.

* Negligible Gravity Well: means less energy (and expense) when transporting goods and people to and from the habitat.

* Easily Adjustible Rotation Speed: This translates into adjustable centrifugal pseudo-gravity, meaning that inhabitants won’t have to worry about low-gee-induced bone, muscle, and organ deterioration. This reduces the need for any future drugs/treatments to maintain the human body to earth-gee tolerance standards.

*Can Predetermine Their Orbits (including their eccentricity): Useful for the following purposes.
-- Solar energy available is already known, meaning you can set the orbit so that on-board equipment can function at it’s optimum solar energy level (i.e. Venus’s solar energy is about 200% that of earth’s, Mars is about 44%, so there’s a wide range of choices) . This ties into the next point.

---Easily adjustable magnetic field shield strength - the closer you are to the sun, the more concentrated the solar energy is. This translates into more power for the habitat’s magnetic field. Combined with thick habitat shells, this can easily keep cosmic radiation to tolerant levels.

-- Highly Taylorable Natural Environment - Climate Control is much easier. Indeed, it might be possible to develop climate-oriented habitats for differing human tastes, perhaps even including those not found on earth to a significant extent. Just on temperature alone, you have your choice of the following for any one habitat

---Tropical - Always Warm or Hot
---Subtropical - Hot Summers, Cool to Chilly Winters
---Temperate Continental - Warm to Hot Summers, Chilly to Cold Winters
---Temperate Oceanic - Mild to Warm Summers, Cool to Chilly Winters
---Subarctic - Warm Summers, Cold to Frigid Winters
---Arctic - Cool to Chilly Summers, Long Frigid Winters

Moreover, each type of climate listed can have its humid, sub-humid, semi-arid, and desert variants (some of which might not naturally exist on earth!). Engineering moisture reliably is likely to be complicated, at least if we wish to maintain a natural looking habitat. In fact, orbital habitats can be an excellent, if expensive way, to preserve endangered ecosystems and cultures (for example, making a polar arctic habitat with all its trimmings for polar bears and their main food source, seals. Also in this scenario, it’s theoretically possible for such places to house indigenous cultures - although there’s the question about the authenticity of such a native culture in an orbital vs. the original one on Earth, but we can presume that at least some members of these cultures will jump at the opportunity to recreate their culture in the habitat.

I freely admit that preserving/recreating entire ecosystems is well beyond our capabilities on at least two counts: (a) such orbital would probably need an interior space of at least 50,000 square km or so, possibly more, (b) we still have little knowledge about how any particular ecology operates even regarding macroscopic life - don’t even ask about the microbes at the bottom of the food chain! Even so, I think we can partially overcome (b) by using microbes already present in the original Earth environment.

Last but certainly not least -- Assuming we’re still around for the next few billion years, we can adjust the orbit or our habitats to the new “Earth Solar Constant Line” (the point from the sun where solar energy = 1,368 Watts per sq meter per second). Because it’s much easier to move an orbital than it is to move a planet, we have more assurance of our survival.

Tom Mckendree of NASA looked into the prospects for very large habitats; in order of increasing size, steel, pure diamond and carbon nanotube. The largest possible habitats were those with an outer skin of carbon nanotube, which could be a thousand kilometers in radius and perhaps ten thousand kilometers long.

There's a lot of carbon in the atmosphere of Venus to use; but carbon from asteroids may be considerably easier to get hold of.

Such habitats would need to be illuminated artificially, as there is no way to put windows in them to let the light in.

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A lot of us are fans of orbital colonies and have a rather dim opinion of planetary body colonization. Besides the advantages you mention, there's one I find particularly compelling--it's simply more efficient. On a major planetary body like a large moon, the overwhelming majority of matter is unavailable for human use. We can only use a thin layer on the surface, while the crushing depths of the core can neither be inhabited nor even reached (much less exploited).

In contrast, smaller moons, asteroids, and comets can be fully utilized--converted from compact rocky bodies into spacious colony "bubbles".

"There's a lot of carbon in the atmosphere of Venus to use; but carbon from asteroids may be considerably easier to get hold of."

One idea I like for exploiting Venus is "aeroscoop" satellites in elliptical orbits that graze the atmosphere. These may use solar electric engines or laser thermal rockets to compensate for drag, as they gather carbon dioxide from the upper atmosphere. Not only do you have a practically unlimited supply of carbon and oxygen for Venusian orbital habitats, but if you can keep it up you might even eventually scoop away enough atmosphere to cool the surface of Venus to exploitable levels. (Still not a fan of living on planetary bodies, but that doesn't mean they can't be mined for stuff.)

Time to do some math (correct any errors I make, please)

1000km X 2 X pi = 6283.18 km circumference

TIMES 10,000 km length equals

~ 62.8e million sq km (or about 1/8 the the size of Earth's surface area - including oceans).

In present earth population density terms, that comes out to about 812 million inhabitants - or about the same population as India in the 1980s. Of course, if we think our population density is too large, we can either adjust the water area of the habitat downward to a degree - or we simply assume something akin to present day USA population density (still rather low in global terms despite having 300 million people).

Assuming 20 people km^2 for land area, and 50% water cover for habitat, that's still 628 million people - about the same size of Europe (including ALL of Russia, and let's throw in ALL of Turkey as well)

True, but the asteroid belt still is a low density place. It may be easier to mine the carbon directly from an asteroid. However, on a "transportation energy cost" basis (if that makes sense to you), you have to expend a lot more Watts to transport the carbon from the asteroid to a place that wants it. At least with Venus, you'd have all the carbon in one spot? Seems to me that'd reduce Venus's transportation energy per kg of carbon
obtained from the atmosphere. I can't prove it, but that's something worth looking in to.

There's no way to lace the shell interior with fiber optics? I'd think a curly-Q-shaped fiber optic cable configuration would allow us to get sunlight without much risk from UV (especially if we can put a UV opaque filter at the outside end of the cable). Maybe that wouldn't work, but that's just an idea.

Good points, Issac. As for sattelites mining venus's atmosphere, would a scramjet do just as well?

Besides, with Venus colonization, at least "terraforming" (such as it is) would have an economically sustainable basis - carbon mining for nanotubes. In fact, one can plausibly argue that atmospheric mining will yield quicker short-term economic benefits than Mars colonization. That by itself just might be enough to deplete Venus's atmosphere. Still won't do anything about the horrendously slow rotation rate, though (the polar regions MIGHT be a good place for human settlement under that scenario, though that's not absolutely certain).

Anyway, Venus and Earth, at least, can easily engage in trade. Carbon from Venus to Earth and the habitats, heavy duty high tech mfg'd goods from Earth to the off-world locations, low-bulk high tech goods from the orbitals (including any orbitals orbiting Venus)...ahh...triangle trade all over again (lets hope this will be a more humane one)

Why make the largest possible when for less cost you can get more usable living area out of a bunch of smaller habitats? The latter is more resilient to disasters also.

While carbon nanotube may be stronger than amorphous diamond, diamond is plenty strong enough and makes for a wonderful window material. With vacuum deposition, amorphous diamond may be an economical material to build with (depending on location). You can even incorporate dielectric mirror layers to reflect away out undesired UV and IR light. Being in Venus orbit, you're probably going to want the windows to filter out everything except for visible light. The outer surface of most of the colony will look green except for the windows; the greenhouse windows will be reflective at all wavelengths except for the photosynthesis bands (which ironically don't include green). For privacy, opaque interior carbon walls can incorporate diamond dust for white or graphite for black.

Steel is something we can do right now, today. I'm of the opinion that we should start doing what we know is possible as soon as possible, since we can't know if --or when-- some unforeseen circumstance is going to come along and distract or disable our current civilization, making space colonization untenable.

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Fibre optics are a good way to illuminate the habitat; one way would be to create upward pointing fibre-optic lamps pointing at the opposite side of the cylinder. Such lamps could have substantial lampshades to prevent people on the ground next to them getting too much glare. I am not a great fan of illumination strips running down the axis of the cylinder, but they are also possible with fibre optics too. But the light pathways would be much longer.

Cylinders in Venus orbit would need to reject much of the light falling on them for thermal equilibrium purposes; they would get too hot. I'm a little wary of throwing away usable energy like that...
Rotating habitats would also work in Earth's orbit, and Mars' orbit, and even in the asteroid belt if you get the thermal balance right, and less energy would need to be reflected away uselessly.

Further out you can always add mirrors or illuminate and heat the inside with fusion if that ever becomes feasible.

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Sorry, but I'm having a great deal of difficulty posting to the forum. I'll have to get back to you later about this (maybe the BAUT staff can help)

I disagree with the "as soon as possible" part of that. I mean, we could maybe haphazardly implement a surface launch Orion heavy lift vehicle to get a crude space colony up in orbit within a few decades. Heck, it might even only cost an Iraq war budget or ten. But it wouldn't be able to survive on its own, so it doesn't add any extra homo sapiens survivability.

IMHO, we should wait for space propulsion technology to catch up to our desires. Personally, I have high hopes that high power laser research will lead to cheap access to space in the form of laser thermal rockets. After this technology or some other technology dramatically reduces the cost per kg of space launch, then we can seriously talk about human expansion into space.

The temperature zones are important. The radio array colony where I was born was somewhat similar. However, it is always prudent to colonize the planets around one's mother star.

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If you can figure out a way to power the scramjet, sure. But the most straightforward solution I could come up with was either a solar powered electric rocket or a laser powered thermal rocket. The latter is more practical, if you've got the suitable laser technology. It lets you use a huge solar collector array which doesn't have to dip into a thin atmosphere at hypervelocities.

BTW, before we get carried away about this, I'd arguably be dishonest to a degree if I didn't mention the advantage of planets over habitats.

*On an mass-per-asteroid basis, it's much harder to destroy a planet's ecosystem than a habitats. - A 100 meter asteroid hitting a habitat vs. one hitting Mars, or even Ceres. Need I say more?

*Less, if any, need to replenish life-sustaining materials - This is especially the case of a large rocky body vs a small habitat, even if the habitat has 99% recycling efficiency.

*Less mass needed to transport fertile material from Earth - On Mars, all you basically need is to concentrate the organic material in a few square meter spot in order to farm, however tenuous the true soil may be. Arguably, all you'd need to do is to pick a good spot on Mars, extract what "thin" organic material there is from the regolith, then mix that organic material into your pre-determined garden spot. With a habitat, you'd need to transport MUCH more mass (i.e. soil or lunar or asteroidal regolith) into the habitat - at least if we want a habitat interior even approaching a pseudo-natural appearance.

*Longer construction time for a habitat with anything approaching a meaningful ecosystem than for a domed planetary habitat. - Planetary habitat domes are much easier to construct because they require much less mass than building a whole "360 degree" dome. Consider two shells of the same material: one covering 10 sq km of a planet vs a habitat shell with an interior area of 10 sq km. Conversely, consider how much habitable area you can get for a dome of X kg mass on a planet vs the living area available in a orbital for X kg mass of that same material.

*(assuming eburacum's right and I'm wrong about the lighting issue) Natural sunlight is available on a planet, and not in a habitat. - I'm not conceding to eburacum yet on this issue, but just in the event that I'm wrong, it's the responsible thing to keep in mind. As for Natural vs. Artificial lighting, natural sunlight is more psychologically comforting in the long run than artificial lighting. Psychological well-being is NOT a trivial point when considering colonizing a strange place.

*Minerals available on a planet, and not necessarily on a habitat. If for some reason a habitat's society breaks down, then as time passes, they're likely to be as ignorant about their surroundings as Stone Age humans were about the nature of the earth. Given all we've written, that's practically assured to spell death for a habitat's population in the long run. At least on a terraformed planet, the inhabitants still have several hundreds of thousands of years to rebuild civilization again and leave the cradle on Earth or Mars once again. That requires mega-resources! Planets have them, orbitals likely will NOT.

Overall, I'm a big fan of space orbitals, but we have to recognize their limitations as well as their strengths.

I am not saying you are wrong; I am just saying I don't like O'Neill's concept of putting windows in a rotating habitat. If you put windows in, the sun will cause moving shadows inside the habitat all day long; this might be disorienting. Large windows reduce the usable surface area of the cylinder too.
Fibre optics, or a system of mirrors along the axis would reduce the surface area loss , and reduce the disorienting effects of a moving light source.

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I personally don't feel there are significant advantages to planets.

I'd say things look a lot worse for the planetary colonies. A 100 meter asteroid hitting Mars will throw up a lot of secondary debris, which can damage multiple Martian colonies. A 100 meter asteroid which hits Ceres will produce shockwaves which will extensively damage all Ceres colonies. In contrast, a 100 meter asteroid hitting an orbital habitat will likely only damage one habitat.

Oh wait--you mean a terraformed planet, right? Well, Mars I could see...but Ceres's gravity is just too feeble to hold an atmosphere. If a 100 meter asteroid hits Earth or a terraformed Mars, the death and damage could be a lot worse than if it hits an orbital habitat. It depends on where it hits, of course, but tsunamis or fires can be devastating.

If you're talking about a terraformed Mars, then there's a huge need to replenish atmosphere. Mars's gravity is weaker than Earth's, so a breathable atmosphere would simply leak away on the order of thousands of years. Advocates of Mars terraformation note that it's possible to continuously keep replenishing atmosphere through the same method that was used to supply it in the first place (like myriad cometary impacts). But of course, that's still a huge budget.

I think this is mostly a red herring. There aren't any good spots on Mars, today. If there are good spots on Mars someday, it's because a heck of a lot of effort went into a massive and ongoing terraforming effort.

But really, I don't "get" the appeal of a "pseudo-natural appearance". See below for my dream habitat...more Manhattan than Montana, to say the least...

Planetary habitat domes are roofs--unused for living area because gravity is pointing the wrong way. But orbital habitat shell material is almost all floor--useful for living area because gravity points outward. There's no roof required.

Planetary domes are also somewhat annoying, structurally, because there's a combination of tension and compression forces going on. You have to worry about buckling as well as simple tensile strength. With orbital habitats, the main structures are all in pure tension.

Eburacum is just anti-window. There's no technical reason you can't put windows on an orbital habitat. Eburacum just doesn't like them, for whatever reason.

Personally, I like putting windows underneath main avenues of traffic. I'm inspired by a library I visited which had clear floors (glass or plastic). You could look down or up, and it was a bit like walking on air between tall stacks of books. I like to think of orbital habitat architecture as a dense city of skyscrapers, but with continuous clear floors extending all the way between them. Some of the clear floors may be for pure foot traffic. Others may be for electric cars and buses. The visual effect would be something like your stereotypical sci-fi city with floating car traffic, but with floating pedestrians also.

Imagine an orbital habitat as a stack of inhabited "skyscraper" discs. In between the discs are concentric transparent ring "floors". Sunlight enters from the side. Depending on where you're standing, the Sun might look like it's above or below you, or off to the side. The "skyscraper" discs don't go all the way to the central axis. This leaves a large central "rooftop" zone for people who want a break from the claustrophobic main living areas.

I just don't buy the society break down argument. It seems to me that a terraformed planet is more of a risk for total society breakdown. Individual orbital habitats are more insulated from each other.