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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"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

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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Rovers forever! - ToSeek

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.

The only real disadvantage I can see to a rotating habitat is their vunerability to attack. A high powered laser could punch a hole in a habitat cylinder. converting it into an airpowered rocket; the internal gravity would be affected as the cylinder begins to tumble, making evacuation difficult.

A similar laser attack on a planet could take out a few city blocks at most.

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The only real disadvantage I can see to a planet habitat is their vunerability to attack. A highly virulent virus could punch a spread throughout the biosphere converting the planet into a morgue; the gravity well and risks of spreading infection making evacuation difficult.

A similar attack on a habitat would have difficulty penetrating and would take out one habitat at most.

This depends on the way the habitat is designed. For example, the design I describe above is highly compartmentalized. If the outer hull is breached, only the outermost ring of air in the local sector is lost. Still, it's a vulnerability.

The best lasers for long range attack are X-ray lasers. These can't even reach a planetary habitat, if a planet has pretty much any sort of atmosphere at all (even one as thin as Mars).

But this does depend on the scale of the attack. When we scale up to interstellar war and planet-killing attacks, things start looking bad for planets. Planets are so big and heavy that it's not very practical to dodge an interstellar laser. At the same time, they're also so big that they're easy targets for an interstellar laser.

In contrast, orbital habitats are smaller and are essentially mobile--orbital habitats will tend to be in loose clusters (like around Lagrange points). Being in such general proximity with many other objects of similar mass, their orbits will be impossible to predict precisely over the timescale of many years. They aren't as easy to attack with interstellar lasers.

In this day and age, possible terrorist attacks are a quite legitimate concern. This is even more urgent with such a vulnerable shells that could house 5-digit figures or more people.

Perhaps some combination of the following will work to delay the laser's destructive power (or particle beam's for that matter)

*Compartmentalize the habitats, especially for ones designed for human habitation: This the simplest countermeasure. If an enemy scores a hit, only a relatively small percentage of the hull takes on water especially one with a Matrioshka Doll type design. I think Naval vessels divide their interiors into compartments to make them less vulnerable to torpedo or surface simming missiles. (incidentally, the interior shell can "triple" as an imitation sky AND serve as a center of low-gravity manufacturing or medical facilities).

Unfortunately, I'm still having trouble posting (something's very tempramental). That is not the complete post.

Random manuvering could make a habitat a hard target to hit. Although a habitat could not change direction rapidly without disturbing the people inside, if the laser was being fired from a great distance such as a light hour away slow movement could be enough for it to miss and presumably detected. Since reaction mass would probably be a problem habitats could be moved by connecting them with string and tugging on it. You'd just have to be careful they don't bang into each other.

I'll note that the ultimate in compartmentalization is if every family had its own "RV" habitat unit. The idea is to basically have your own personal apartment, so if you want to move to another habitat or go on a vacation, you just unhook from the current habitat and latch onto a transport liner. This concept works best if everyone has decided they like living in zero-gee.

Wrong wrong wrong. We could have begun several decades ago, which by now would have made asteroid mining and building orbital habitats into mature industries. Even starting now, we'll gain valuable knowledge of what needs to be done to develop long-term self-sustaining habitats. The longer we put it off, the longer it goes undone. Waiting for cheaper launches (which also could and should have been developed decades ago, but that's a separate rant*) is just procrastination. The infrastructure needs to be built in space for large-scale construction, and nothing we do on the ground will hasten that. Let's just get our hands dirty, go up, and start DOING it! Time's a-wasting! Chop chop!

* It's actually a closely related rant. But even with only currently existing and near-term launch technology, we can start building the needed framework, especially in the areas of orbital construction and in-situ asteroid and/or Lunar mining and processing. The basic engineering and math behind the concepts has been sitting around for three decades now. And the only way to really develop them is in space.

ADDED: It's not the technology that keeps manned launch costs so high, it's availability. The only manned orbital spacecraft currently are in the hands of national governments who spent millions or billions to build them, and price their tickets accordingly. That's no way to promote colonization.

__________________
"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

Kinetic weapons are also dangerous to habitats- in space this includes any reasonable sized spacecraft. If you have a large, single parkland-type interior
like this Bernal sphere
http://64.40.104.21/spacecolonies/sm...76-0628.sm.jpg

all the atmosphere will leak out, as there wouldn't be any bulkheads.

I have had the idea of making the living accommodation separate from the cylinder, and attached to the outside using seal-able ports; if a catastrophe occurs, the living quarters could split off and become separate lifeboats, possibly with independent propulsion (although they will be going pretty fast if they split off from a rotating habitat).

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I completely disagree. I think space launch technology has actually been progressing about as fast as could be expected, given the level of demand and the inherent costs and risks involved in technologically challenging R&D. If you look at the progress from decade to decade, it's actually not that bad, considering.

No, it's pretty much the technology. There is simply no shortage of private enterprises working to develop cheap access to space launchers--but the problem is difficult to solve! There's also the problem of limited demand. The fact is, there are only a handful of applications which satellites can handle better than terrestrial hardware. Hopes and dreams of industrial space applications have proven so far to simply not be there. And it's not for lack of trying--the hundreds of space shuttle experiments investigating possible zero-gee industrial applications don't make headlines.

When it comes to research and development on the bleeding edge, there's often no alternative to waiting. It takes years to conduct basic science research experiments, and one experiment leads to another, and so on. You can't know beforehand what direction of research will be most important.

For example, look at the most promising laser technology--efficient high power solid state lasers pumping fiber lasers tipped with extremely efficient dielectric mirror gratings. This combines three technologies which didn't even exist three decades ago and were in there infancy a decade ago. Solid state lasers are only economical because of the personal computer revolution. Fiber lasers are only conceivable because of the fiber optic communications revolution. Dielectric mirrors were only developed in the 1990's out of the blue (contrast with the dubious 1980's SDI concepts involving actively cooled metal mirrors).

Note that while pictures of the Bernal sphere and O'Neil cylinder colonies show the expansive inner area, they don't show the layers of "underground" living floors underneath. In case of a catastrophic penetration causing the inner area to lose its air, the population would evacuate locally underground.

Yeppers, nine-eleven -- nothing more need be said

They'll need to have independent propulsion!

(a)An orbital spinning apart --> greater area for search and rescue operations -->more time to find any one habitat --> run out of oxygen, water and food

(b) It's practically guaranteed some of those habitats would spin right into a planetary body...or the sun!

Agreed. It's an oft-quoted maxim that it's more expensive to launch into space today than it was forty years ago in today's dollars.

If someone wanted to cross the Atlantic ocean, using a caravel designed for the relatively calm Mediterranean waters would be a risky and expensive way to go. But Columbus didn't delay his exploration plans to wait for someone to invent the 747 first. Later, the opportunities incurred by nations separated by a wide ocean was the driver that spurred the technological development of inter-continental airlines, such that today someone can cross the Atlantic in a few hours for a few hundred dollars.

Point is, we can get to Mars today, using the same kinds of rockets we developed in the sixties. Sure, using lasers may get us to Mars more quickly, but we can't do it today. But if someone could save umpety-ump dollars on a Mars resupply mission, then the drivers will be in place to develop better, faster, and cheaper propulsion methods. But until there's a true need for such, any progress in alternative technologies will remain experimental.

Which sounds pretty darn ridiculous. Today, you can even get discount rates if you go russian. They need the money. Can you even imagine a billionaire space tourist forty years ago?

We already had our expensive but non-sustainable adventure in space--Apollo was great. Now that we've splurged and had our fun, we should buck up and spend responsibly.

I agree with Issac, we don't need to do everything quickly. For that matter, we can launch a probe to Alpha Centauri or Gliese 581c - but it doesn't make any sense to do so because its plausible that another, future, probe will get there even faster, be able to transmit better, etc.

Enthusiasm is great, but enthusiasm without proper planning will just make the Space Programs a White Elephant. Like Issac said, we had our kicks and sense of adventure and excitement - now, we need to come off the high and get down to the serious sustainable business of building efficient space infrastructure.