‘Island-Hopping’ to the Stars

by Paul Gilster on February 13, 2012

We tend to think of interstellar journeys as leaps into the void, leaving the security of one solar system to travel non-stop to another. But a number of alternatives exist, a fact that becomes clear when we ponder that our own cloud of comets — the Oort Cloud — is thought to extend a light year out and perhaps a good deal further. There may be ways, in other words, to take advantage of resources like comets and other icy objects for a good part of an interstellar trip. That scenario is not as dramatic as a starship journey, but it opens up possibilities.

Let’s say, for example, that we only manage to get up to about 1 percent of lightspeed (3000 kilometers per second) before we run into technical challenges that are at least temporarily insurmountable. Speeds like that take well over 400 years to get a payload to Centauri A and B, but they make movement between planets and out into the Kuiper Belt and Oort Cloud a straightforward proposition. A civilization content to create way-stations and take its time could establish habitats all along the way, its distant descendants reaching the next solar system.

The idea takes me back to the island-hopping of Polynesian cultures as they pushed ever deeper into the Pacific, which is sometimes invoked to describe a civilization expanding from star to star. But the ‘island-hopping’ may actually involve small, dark objects exploited step by step all the way across to the target star, a process that could take millennia. A space-faring culture at home in the dark outer regions emerges. All of this depends, of course, upon the resources available, but the Oort Cloud is thought to be vast, comprising perhaps trillions of icy and rocky objects, a supply of raw materials on which such a culture could thrive.

Nomads Between the Stars

Adam Crowl recently passed along a new paper that takes this idea to another level. Louis Strigari (Stanford University) and colleagues have been looking at unbound objects, free-floating planets formed either directly in the collapse of a molecular cloud or ejected due to gravitational interactions in a solar system. Right now we know little about such rogue planets — Strigari and team call them ‘nomads’ — but they are quite interesting from the interstellar expansion standpoint as they, too, could provide even more stepping stones to distant destinations. Moreover, they cannot be ruled out as worthwhile targets on their own, as the paper suggests:

The name “nomad” is invoked to include that allusion that there may be an accompanying “flock,” either in the form of a system of moons (Debes & Sigurdsson 2007) or in its own ecosystem. Though an interstellar object might seem an especially inhospitable habitat, if one allows for internal radioactive or tectonic heating and the development of a thick atmosphere effective at trapping infrared heat (Stevenson 1999; Abbot & Switzer 2011), and recognizes that most life on Earth is bacterial and highly adaptive, then the idea that interstellar (and, given the prevalence of debris from major galaxy mergers, intergalactic) space is a vast ecosystem, exchanging mass through chips from rare direct collisions, is intriguing with obvious implications for the instigation of life on earth.

It’s a dizzying thought when you couple this with the paper’s estimates on the number of free-floating planetary objects. The authors estimate there may be up to 105 compact objects per main sequence star in the galaxy that are greater than the mass of Pluto. The mass function of the lowest-mass nomads is modeled from what we see in the Kuiper Belt and the distribution of diameters in KBOs, while at the higher end (corresponding to masses several times that of Jupiter), evidence exists that nomads in open clusters follow a smooth continuation of the brown dwarf mass function. Drawing in evidence from microlensing as well as direct imaging, the paper goes on to suggest a galaxy in which the space between the stars is well populated with objects of planetary mass, most relatively small but some larger than Jupiter.

The authors acknowledge that much uncertainty exists about the mass function as we move from larger to smaller nomads, which makes space-based observations critical for refining these estimates. One way to move forward is through a survey of the inner galaxy (the proposed Wide-Field Infrared Survey Telescope, or WFIRST, could be significant here), while large scale galaxy surveys like the Gaia mission and the Large Synoptic Survey Telescope (LSST) should be sensitive to nomads greater than Jupiter mass. Even Kepler may come into play, as any anomalous microlensing events it encounters could imply a high value for the number of nomads between the stars. From the paper:

…we note that an additional outcome of the observational approach discussed above, especially regarding the detection of short timescale microlensing events, is that upper limits may be set on the density of nomads. This could set very interesting constraints on the population of planetesimals in nascent planetary systems.

Indeed. If resources like these are available in quantity between the stars, then a pattern of slow expansion would make interstellar migration almost inevitable if humans (or their machine surrogates) can adapt to life in the outer Solar System and beyond. Propulsion is always a huge issue, but in this scenario we also focus on the ability to build and maintain habitats on distant objects, exploiting their raw materials and preparing for the next leap outwards. Long-haul technologies would surely arise from a culture capable of these things, but the possibility exists that interstellar travel will mean slow and steady outpost building before the target is reached.

The paper is Strigari et al., “Nomads of the Galaxy” (preprint).

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jkittle February 13, 2012 at 12:16

This is really great study though I would like to point out it does not discuss the possible implications for interstellar colonization or propose any type of probes to these objects( manned or otherwise). It does stay with present technology options of observing events from survey telescopes, a very sensible option given these ‘scopes are in the planning stages and will provide the data needed to expand our understanding of these ( possibly) ubiquitous objects. For me the strongest case for exploration remains the small worlds starting with Ceres. If 10,000 years form now is space faring human “shepherds” tend this nomadic flock, reach distant star systems, – then they will likely settle in the outer reaches of these systems because of the density of objects. The inner worlds may then be explored by robotic probes but why battle the gravity of a distant mars or earth when evolution has bred the ability to live ( or even stand) on a 1-g world out of our descendents’ genomes?
Exciting times. Soon we will have more data on local brown dwarf populations (within 30 Light years or so) and also have a better idea of the abundance of objects in the kuiper belt, and just beyond the kuiper cliff, out to the inner oort cloud.
For valentines day:
“I wish all those future trans-Polynesian explorers well and send them a message of encouragement down the dusty paths of time and space; may they prosper among the dark reaches of the outer night, and bring light to the nomadic paths that weave beyond the stars we all watch each evening “

Eric February 13, 2012 at 12:22

Hopping from star-system to star-system in small incremental jumps sounds plausible. It would also tend to create populations of cold-adapted critters (probably post-biological) that like to quietly munch on comets thousands of AU from any star.

I wonder if they’d be detectable in the infrared, as some waste heat leaks out into space. If trillions of such bodies extend beyond most star-systems, and if an appreciable fraction of these see “colonization”, they may look like a slightly warmed cloud. Instead of Dyson swarms, maybe we need to look in the places between stars for clouds of slightly too warm nomadic bodies.

Mike February 13, 2012 at 14:37

I’ve often wondered if such objects, if as numerous as asserted, would prove to be a hazard to navigation. But as well they could also prove to be a valuable resource and valuable realestate. But not numerous enough to account for galactic missing mass AKA dark matter. Not nearly enough or the microlensing results would be blindingly obvious already.

ljk February 13, 2012 at 15:06

Any good fan of Star Trek should be rather wary of any interstellar vessel named Nomad:

http://en.memory-alpha.org/wiki/Nomad

Greg February 13, 2012 at 15:23

My question about ‘island’ hopping or building habitats between the stars is where do we get the energy to survive? Although it would make for amazing night sky observations it may appear to be quit cold, dark and lonely out there.

mike February 13, 2012 at 15:36

For a fictional treatment of this see Karl Schroeders Permenance from 2001 – see http://www.kschroeder.com/my-books/permanence

Adam February 13, 2012 at 15:57

There was a paper on colonising the Oort Cloud which was published in “Analog” in popularised form. The author suggested starlight harvesting for power, which makes immense sense if long-term habitation is desired. The integrated flux of starlight will be near constant for gigayears.

Joy February 13, 2012 at 17:17

Actually, I have always liked this concept. Considering the age of the universe, and even the age of our species, diffusion migrations requiring thousands of years between stars seem quite reasonable. The devil is in the thermodynamics.

Even the inner rim of the Kuiper belt only receives 0.1% of Earth’s solar flux. One can (just barely) imagine gigantic thin film mirrors focused on Stirling generators or photovoltaics, but these would succumb to wear and radiation bombardment long before they produced enough energy to fabricate their replacements. The Oort cloud is essentially dark and well beyond the wildest possible extension of solar powered dreams.

So we are talking about a nuclear powered civilization. But what are the nuclear resources in the Oort cloud?
“On average the abundance of uranium in meteorites is about 0.008 parts per million (gram/tonne) … The continental crust, on the other hand, is relatively enriched in uranium at some 1.4 ppm.” ref:
world-nuclear.org/info/inf78.html
Okaaaay, random dirt and rocks from your backyard are 175 fold enriched in uranium compared to dry solar material! (We won’t even mention your granite kitchen countertop which at a U content of 20 ppm is a couple of thousand fold enriched in uranium compared to whatever regolith you would find on Sedna) But it would be worse than that in the Oort cloud where bodies would have a large ice content. Cryogenically frozen ice is as tough as steel, and not cheap to melt in an energy poor environment.
So U235 has an energy content of 79.5 million MJ/kg, but is less than 1% of natural uranium. Let’s run some numbers:
A tonne of material from an icy dirtball in the Oort cloud might have 4 mg of U and 40 mcg of U235 for an fission energy content of 3 MJ. Well, the enthalpy of fusion for a kg of ice (energy required to melt a litre of ice which has already been warmed to 0 degrees C) is 0.334 MJ. The U235 energy content in a tonne of Oort material could only melt 10 kg of (warm) ice! There are no extractable fission energy resources in the outer solar system, FULL STOP.

Basically, the three options for living in the Oort cloud are fusion, fusion, and fusion. Certainly D is available. Whether it is possible to produce fusion reactors in the Oort cloud is purely speculative at this point as we have not been able to produce a self sustaining fusion reaction on Earth.

Rob Henry February 13, 2012 at 17:30

Inspired by Nick, I am seeing that once the massive step of moving industry off Earth has been achieved, these Oort cloud communities will likely trade much among themselves. I am now seeing that isolating off and self-containing the economy of one such outpost that happens to be on a nomad world is a non-trivial task. The effort involved makes me think that colonising another system would be unlikely to be achieved without that specific motive.

Inspired by Joy, I realise that despite the vast accessible volume of any 1000km plus nomad world, the limitations of resources are bound to come into conflict with our tendency towards exponential growth. Here a trip to the stars would take tens of thousands of years (we are not using the spaceship velocity of 1% c but nomad velocity). Superficially, such a problems might seem just as sever for the whole Sol system, but smaller communities are less flexible, especially in psychological outlook.

Lastly, I must say governance must be very difficult in such a populated cloud, and I begin to feel that the wild West feel, and widely different (but still strangely human) cultural settings as given to the galaxy by Star Trek would be a reality in a populated ort cloud with a low maximum velocity of human transport.

Frank February 13, 2012 at 17:38

Could this be the reason we’re not seeing any evidence of Von Neuman probes?

They could be programed to avoid using material from the inner portions of a star system for their replication.

Bob Steinke February 13, 2012 at 18:18

Deuterium (D-D) fusion is probably the only realistic power source for Oort cloud communities. Perhaps they could import lithium for deuterium-tritium fusion.

Ole Burde February 13, 2012 at 19:02

Its Great to hear someboddy admitting that 1% lightspeed might be the limit.
In this very realistic scenario , it is the quality , the perfect design , production and performance of a spaceship and its crew that will make it last long enough to go at all .
Perhabs the “stepstones” can be used , perhabs not . In both cases most of the components and systems of our spaceship have to last almost indefinitely . This seems to be an impossible demand , but this might be mostly because we have gotten used to a fastmooving through-away design filosofy . There are countless examples of individual pieces of mashinery who have been happily ticking away for hundreds of years , like certain types of hand made “gold”watches , steamdriven locomotives still running in som far away places of the world . When Henry Ford “perfected” the first modern asemly line , one of the first things he did was to downgrade , and thereby making cheaper,any component that lasted longer than it was calculated to do .
To investigate and utilize theese lessons from the past , is something that it has not been worthwile to do for the last 200 years . All this ofcourse does not mean that recycling isnt an allimportant issue , but only that recycling can be complemented by another design filosofy . You dont want your recycling systems to brake down .

Joy February 13, 2012 at 19:08

By the way, in 2008 I actually island hopped across Polynesia in a wind powered trimaran, not too differently from the 13th century colonists of New Zealand. (I did have a wind generator to enjoy the benefits of electronic civilization) Those earlier journeys were possible in part because propulsion was free, and one could catch fish and collect rainwater along the way. Not only that, but the vegetated islands contained not only food, but also the materials to build more sailboats. With some imagination, one might consider the possibility of an (inner) asteroid belt civilization based on solar sailing between various small bodies to be a bit similar. One is free to postulate that the solar energy and metals resources on asteroids might be adequate to produce replacement spaceships and solar sails.

However I gag at the comparison of Oort colonization as being at all like the Polynesian experience. Here we are talking about fusion rocketry, with extremely complex engineering of diverse rare materials and high energy cost required both for acceleration and deceleration. A better analogy would be cruising the Pacific in a diesel trawler adapted to run on coconut oil. Ok, the crew could go ashore and make more diesel fuel at any coral atoll with coconut trees. So far, so good. But eventually the diesel engine would wear out, and how could you produce another diesel engine (much less multiply your fleet) with the scant resources (no metal ores) of coral atolls? Not possible.

Alex Tolley February 13, 2012 at 19:11

Joy makes a very interesting point that solar concentrators might be net energy consumers – not something O’Neill considered with his BOE calculation of space colonies at a maximum of 2.7 light days from the sun. This does seem to suggest a fusion only energy source. Assuming Icarus plans prove viable then the ships will already have fusion drives to get to the Oort, so generating power from the same/similar technology should work.

If the colony had export products, they could trade for nuclear fuel or antimatter as their power source from inner system colonies, if transit times were reasonable.

James D. Stilwell February 13, 2012 at 19:51

SEEMS TO ME WE’RE NOT GOING ANYWHERE WITHOUT FUSION POWER. PLACING A DOZEN ROBOTIC WAY STATIONS THROUGHOUT THE OORT CLOUD IS RECOMMENDED. WHAT CAPTAIN WANTS TO LEAVE JUPITER’S ORBIT AND POWER UP TO 05% LIGHT SPEED TRAVELING THROUGH THE OORT CLOUD WITHOUT THE CLOUD HAVING BEEN CHARTED FOR NAVIGATIONAL PURPOSES BEFOREHAND. MOREOVER I DOUBT A TRILLION DOLLARS WILL BE FOUND TO BUILD A CENTAURI SHIP WHOSE FINDINGS WON’T BE KNOWN FOR 200 YEARS OR MORE. ARE WE GOING FOR EXPLORATION OF AN INTRIGUING ALIEN SOLAR SYSTEM OR MAKING JUST A FLY-BY ONLY FUTURE GENERATIONS WILL SEE.
JAMES D STILWELL

Rob Henry February 13, 2012 at 20:05

Joy, I agree with the idea of performing those energy calculations, but why the fixation with enriched uranium? We already have fast breeder technology, and however you cut it thorium fission looks much easier than fusion (these two changes should give roughly, 100 and 300 times the energy availability that you describe).

I realise that the outcome might still look reasonably similar but I put it to you that this nomad would be abandoned unless a deliberate interstellar voyage was the desire. Therefore, anything as obvious as an energy shortfall would be brought in from off-world before the journey.

Nick February 13, 2012 at 20:30

The easiest nomads to detect may be moons of gas giants, from the heat (as infrared) given off by radioactivity and tidal forces. Think of how much Io, for example, would stand out in an otherwise extremely cold and dark system. Also, I wonder if the large magnetic fields of gas giants are detectable even with only the system’s own material rather than a solar wind interacting with it.

Joy, the availability of uranium and various fission alternatives (e.g. thorium) will depend in part on how much heavy core material can be found that far out. If, as the authors speculate, there is enough radioactive material to melt subterranean oceans, then must be enough for humans to be able to melt the ice as well (and in such systems it won’t start out at cryogenic temperatures). A few such worlds may have been shattered, exposing this core material near the surface, or some may like earth be geologically active planets that move material between the core and the surface.

Also, we don’t need fusion reactors to figure out how to melt ice with fusion power:

(1) Use explosives to break up the ice into hundreds of trillions of floating chunks of a few tonnes each.

(2) Surround this cloud with a big balloon. Line it on the inside with some of the ice chunks.

(3) Set off a long series of small fusion bombs at the center of this vast cloud. Each vaporizes some ice (later as the pressure rises this condenses to liquid). The iceberg cloud is large enough that its outer ice damps the explosion on the inside, preserving the integrity of the ice-reinforced balloon.

Result: a hundred-trillion-tonne ocean of liquid water surrounded by ice and sealed up by a balloon, all floating in microgravity.

One might also build similarly large power plants in which small fusion explosions are used to generate steam. In short, we already know how to harness fusion power without reactors, it just requires large scale structures, which are readily achievable in the microgravity of space.

Rob Henry: I agree that this will facilitate trade and trade will allow for division of labor and thus wealthier, more high-tech, and yet smaller population settlements. And as with the Polynesians, trade greatly increases the motivation and opportunities for colonization.

All in all, a large population of nomad planets makes the existence of ETI in our own galaxy even less likely, since it makes the spread of ETI across any galaxy filled with such nomads (and presumably most are if ours is), which we would have discovered by now but haven’t, easier and thus more likely. However, the time frames involved may be very long — hundreds of millions of years to spread through a galaxy by island-hopping. Still far shorter than the age of nearly all galaxies though.

JohnHunt February 13, 2012 at 20:45

I’m not a fan of island hopping to the stars. For starters, once traveling in space, you have to expend energy to slow down and stop. Why do something wasteful like that of you don’t have to? If the reason as Paul suggests is that we’re traveling too slow to make it all the way to the next star system, them why not just wait a few more decades until one harvests enough power to travel at the necessary speeds? During that period, why not simply build up the energy-rich areas of the inner solar system which are prime real estate?

History and especially scifi can at times be misleading. In the distant past, if you wanted to travel from Taiwan to Australia, you would island hop. Now, you would just hop a direct flight. Just because we did something in the past doesn’t mean it would make sense now or in the future.

The idea that we couldn’t survive long enough on a really long voyage to Alpha Centauri is probably based on the assumption of a ship with living, breathing crew or colonists. From an expansion of the species standpoint, this wouldn’t be the way you’d choose to do it. Rather, send frozen crew, embryos, or replicators with DNA data – very small craft for relatively little propulsion energy. Don’t just go to the next star system and then spend 500 years developing the technology to launch again. Just have one of your small craft bypass that star and continue on to the next, or next, or next.

Mark February 13, 2012 at 23:05

A new life awaits you in the Off-world colonies! A chance to begin again in a golden land of opportunity and adventure! (from Blade Runner, 1982)

David February 13, 2012 at 23:25

We have the technology to go 3-10% of light speed now. We dont have to wait till 2100 unless we want to engineer the 80% option .
But we could be at Alpha Cenauri by then…….Its called

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

Eniac February 14, 2012 at 0:01

I agree with JohnHunt. While island hopping is a great idea, and may make interstellar colonization possible for the first time, in the long run the outer envelope of expansion is going to be defined by fast, unmanned self-replicating probes sent much further than the nearest system, followed by a somewhat slower front of manned colonization ships with frozen crew and embryos.

This scenario assumes that we will be able to boil down our hi-tech economy into a transportable “seed”, which, when given suitable resources, will be able to bootstrap a production facility capable of producing the next generation of ships. This facility will also remain in the system and wait for colonists to arrive, so they can find everything in move-in condition. It sounds fantastic, but in my opinion it is no more daunting than the propulsion and energy issues, and within range of what we could achieve today if we set our minds to it.

I also believe that solar power is more convenient than nuclear, so the bulk of the action is going to take place in the “habitable zone”, most likely.

Joy February 14, 2012 at 0:05

@ Rob Henry – Yes, if one wishes to breed thorium and U238, the energy content of dry solar (chondritic) material could be in theory 300x more, but the gap between that value and even the lowest economic (energy positive) grade of fission ore (500 ppm) is another factor of 200x , fission resources in small bodies of the outer system are nil no matter how you calculate it. In the Oort, it is fusion or nothing.

@ JohnHunt

I have come to the same conclusion. There is no benefit from accelerating beyond solar escape velocity, burning more fuel to stop in the Oort cloud, repeating this cycle again and again and again unless the Oort cloud bodies contain the resources to expand your fleet at each stop. I can’t see how this could be possible given cosmo-chemistry and thermodynamics. In the inner asteroid belt, we have M-type asteroids which are believed to be exposed cores of differentiated bodies. 16 Psyche seems to be a giant hunk of metal waiting to be mined. Oort cloud objects are more likely to be composed of undifferentiated solar dust and ices, very energy intensive and uneconomic to mine.

“The idea that we couldn’t survive long enough on a really long voyage to Alpha Centauri is probably based on the assumption of a ship with living, breathing crew or colonists. From an expansion of the species standpoint, this wouldn’t be the way you’d choose to do it. Rather, send frozen crew, embryos, or replicators with DNA data – very small craft for relatively little propulsion energy. Don’t just go to the next star system and then spend 500 years developing the technology to launch again. Just have one of your small craft bypass that star and continue on to the next, or next, or next.”

100% agree. If we make it out, we will go as seed pods. Personally I favour a solar sail departure on a sundiving course, as the same means could be used at the destination if the destination star is of solar luminosity. (But there are several other credible propulsion methods) Yes the departure velocity would be less than 1% c, and the success rate of seed missions could be very low, but that would be a level of expenditure that a steady state solar civilization might be able to sustain.

Nick February 14, 2012 at 0:16

“Rather, send frozen crew, embryos, or replicators with DNA data – very small craft for relatively little propulsion energy. ”

Another grand sci-fi idea based on a whopping economic misunderstanding. What happens to the embryos when the freezer malfunctions and the necessary replacement part can’t be made? What happens when a meteor takes out navigation sensors that cannot be replaced? How is the skeleton crew, human or robot, necessary to make such repairs, over the course of thousands of years going to feed itself? Clothe itself? Take care of its heating, air conditioning, air, water, and sewage? A small crew lacks the division of labor necessary for all but the most primitive ancient island standard of living. It can’t come anywhere remotely close to being able to maintain high-tech equipment, far less manufacture replacement parts.

The “send embryos” idea doesn’t work, and it’s not even close. Making the skeleton crew (or skeleton robots) fewer just makes the problem worse.

amphiox February 14, 2012 at 2:01

My question about ‘island’ hopping or building habitats between the stars is where do we get the energy to survive? Although it would make for amazing night sky observations it may appear to be quit cold, dark and lonely out there.

Fusion would have to be an enabling technology to make this happen. Hydrogen is plentiful in the frozen ices of the kinds of Oort Cloud and Kuiper Belt objects we are talking about.

JohnHunt, I think the big advantage of this scenario is that a civilization would spread through the stars this way without ever having to directly plan any interstellar mission at all. And that means no need to overcome the political and economic barriers that such a grand mission were surely entail. All that would be required is the standard perogatives of individuals and groups to expand as numbers increase. Just like the ancient Polynesians, who had not grand plan for colonizing the Pacific – they simply went from island to island as populations increased, driving the need to find and settle new lands.

So just from standard economic pressures, you will get new colonies spontaneously being set up by individuals, not governments, and each colony will be economically motivated, and therefore be a net gain in resources, not a drain (of course you have to get to the level of technology where making that one hop from “island” to “island” is an economically viable prospective for a small group of enterprising individuals to undertake). Over time you get colonies popping up and, indeed, reproducing (as the descendents of colonists in one colony decide to move on and colonize the next “island”). Starting with orbital habitats around the home planet, moving on to the near-earth asteroids, then slowly to the asteroid belt, and gradually out through the solar system, until what you’ve basically got is a scattered halo of colonization all around the home star, and you start filling up the Oort Cloud.

Then, at that point, the outer edge of our Oort Cloud almost overlaps (or at least we think so) with the outer edge of the Oort Cloud equivalent of the next nearest star system. So, now that you have colonies throughout the Oort Cloud, each one economically viable and growing on the resources of the Oort Cloud objects, the cost to move to an Oort Cloud object actually gravitationally bound to another star is hardly more than the cost of colonizing another object within “our” Oort Cloud, and, just from natural economic forces, eventually some colonists will move over.

And then they can set about colonizing the Alpha Centauri Oort Cloud, until they fill that up, and can hop on to the next Oort Cloud over on the next nearest star, and so on. (At this point I’m not sure they will bother venturing deeper into the gravity wells of the stars to colonize any actual planets, if they exist. But this of course means that humans can colonize the Alpha Centauri system even if there are no planets there!)

The exciting thing is that we can almost get started on this project RIGHT NOW. (Well not right now, but soon). It only takes a slight advance in current technology for us to gain the capacity to build orbital colonies. There will still be an initial economic barrier to overcome (almost like a reaction energy in chemistry) in that the first such colonies will be dependent on earth resources and will need a big outlay of funding and support from the home planet. But once you get over that hump, the space-based colonies will start being to harvest resources for themselves from space, and once they become self-sufficient on space resources (and remember, all that it takes to turn a space colony into a space ship is to add an engine – assuming that it has by this point self-sustaining life-support capacity, engine speed no longer matters. One could envision such colony/ships moving slowly on impulse power from near-earth asteroid to near-earth asteroid, mining resources as they go along, and building new colonies when their populations get too big, essentially self-replicating like bacteria) we are off to the races. It basically becomes an exothermic reaction, and everything else flows almost automatically from economic imperatives.

Kenneth Harmon February 14, 2012 at 2:14

Paul,

I have always been a big fan of this approch (which I like to call the Lilly Pad approch) to limited Interstellar Travel. It really starts to look good if we can achieve .05C given all of the various constraints instead of just .o1C which seems somewhat pessimistic. However, this approach is really only viable for a trip next door to Alpha Centauri if there is anything of interest there which we should know one way or the other by 2020. Of course once we reached Alpha Centauri there is no reason why the same process could not be repeated for another Star System that is very close to it other than Sol.

In the past I have argued on this site that an Interstellar Trip to Alpha Centauri should be thought about differently and treated differently from an Interstellar Trip to any other star system since there seems to be plausible ways to get there within the next ~100 years given derivatives of known technologies. A trip to Alpha Centauri while very challenging should be within the limits of an early Type-1 Civilization since with a stepping stone approch it shares many characteristics of a long range Inter-planetary trip given the possible overlap of the extended “ORT Clouds” of the two systems. In fact, with a little luck they may even “touch” to some extent. By treating a trip to Alpha Centauri as a mere logical extension of Interplanetary Travel and Colonization it may begin to seem less far out, and more of a “nearer term” (next 100 years) end state destination that even a Conservative Organization like NASA may come to embrace over the next ~50 years. The first step is to understand exactly what is there and if there is anything of interest such as a planetary system.

Adam February 14, 2012 at 4:18

Interstellar dust is hardly erosive enough to threaten sollectors large enough to warm planets. Extrapolating from meteoritic flux near the Sun is ridiculous, which is essentially what Joy’s argument amounts to. Go out to 1,000 AU and the flux has dropped ~1,000,000 fold, if that.

Another way to look at it is comparing against the Zodiacal dust, which masses ~10^16 kg within 1 AU. The Kuiper Belt and Oort Cloud might mass 35 Earths. Grind it all up into microscopic dust and disperse it evenly through the 100,000 AU volume of the Oort. The dust is 20 millionths the density of the Zodiacal dust. More importantly its speed is much, much lower.

From extinction studies and the motion of the stars, the dark baryonic fraction of the Galaxy’s mass is inferred to be ~0.03 Solar masses per cubic parsec. 1% of that is dust. Thus there might be an additional 48 Earth masses of dust moving through the Oort Cloud. Thus about 47 millionths of the zodiacal dust density. That assumes it’s all microscopic, 100 nanometer dust grains. Every square meter encounters 0.5 grains per second at the Sun’s velocity relative to the ISM. Ultra-thin reflectors have a thickness similar to the grain size, thus each grain punches a hole equal to its cross sectional area in size. Thus to erode a square meter of area, the reflectors need to be exposed to the dust for ~6 million years.

Anthony Mugan February 14, 2012 at 6:30

An interesting addition to the range of panspermia mechanisms, although I wonder if the probabilities of transfer from impacts stack up with nomads in interstellar environments (although presumably some will eventually get captured by other systems). If life did get going on a planet that became a nomad then subsurface micro-organisms may well retain viability in such environments for a prolonged period, even if not replicating actively.
I ‘ve seen this idea before somewhere (think it was a paper by Rawn Joseph) but didn’t think much of it at the time as I didn’t realise how numerous such objects actually are, and I was somewhat sceptical.

A. A. Jackson February 14, 2012 at 7:36

‘Nomad’ is an interesting term. ‘Free range planets’ did exist in SF prose , possibly before being mentioned in astronomical literature.
Indeed the name ‘rogue’ was a name used. One nomad play a crucial role in James Blish’s Cities in Flight, a spindizzy and a nomad saved the Earth, read it!
For a mild mannered man Jim Blish had a wild imagination tempered by refined command of sophisticated story telling.

Anyway, 10^5 per main sequence star! Goodness then, there must be a gazillion objects less than 10^-8 Earth Masses nomad objects roaming the Galaxy. Which jibs with Solar System formation models, most comets were removed from the solar system during late stage formation, maybe as much a 99% (Oort cloud formation and dynamics, Dones, L.; Weissman, P. R.; Levison, H. F.; Duncan, M. J., Comets II, M. C. Festou, H. U. Keller, and H. A. Weaver (eds.), University of Arizona Press, Tucson, 745 pp., p.153-174).
The picture in article is both the inner and outer Oort cloud. The inner Oort cloud should be ‘flat’ , containing an unknown amount of mass. The outer Oort cloud , source of most, maybe all long period comets, may have as many as 10^12 comets. Yet the outer Oort cloud is subject to being stripped during close passage of Giant Molecular Clouds during the sun’s orbit about the center of the Galaxy. It gets replenished from the inner Oort cloud and possible by the capture of outer Oort cloud comets about other stars.

So lots of stuff in galactic wide open spaces. That space is BIG however.
Consider, over two hundred years of comet observing not a single long period comet has had the orbital characteristics of an interstellar comet.
Jupiter has produced some interstellar comets from swing bys , but those have been identified in the catalog.
Another empirical point, since the solar system settled down about a little less than 4 billion years ago with all it’s spacings and especially orbital resonances there has not been a ‘nomad’ stellar encounter close enough to disrupt that configuration. (On the Dynamical Stability of the Solar System,
Batygin, Konstantin; Laughlin, Gregory,The Astrophysical Journal, Volume 683, Issue 2, pp. 1207-1216, 2008.)
There may have been penetrations of the outer Oort cloud that sent in comet showers.
Future solar system stellar encounters are possible and could sent the Earth whizzing into the Great Beyond.
(Story used in When World Collide, but the astrophysics and celestial mechanics of Philip Wylie and Edwin Balmer has zero content.)

Ronald February 14, 2012 at 8:03

I agree with JohnHunt, who basically says it all, also acknowledging Joy’s comments:
Space is mostly not like the earth’s oceans, despite the tempting comparisons.
The three paramount decisive factors (parameters) are distance versus speed and survival time. Like the oceans.
But the essential difference is that the energy required is just for acceleration, coasting at an attained speed is energy free.
Decelerating, settling, building an industry under the most adverse conditions will cost enormous amounts of energy. It may be done for various reasons (?), but probably not for ‘refuelling’ or ‘restocking’, etc.

It would probably be much more efficient and effective to try to attain higher speeds and longer on-board survival time.

Interstellar travel and colonization will likely hinge much more on a combination of (fusion/antimatter/breakthrough) propulsion and suspended animation plus life-extension.

kzb February 14, 2012 at 8:49

I think it’s worth pointing out that the Nomads of the Galaxy paper is highly speculative, in particular the way they’ve estimated the 10^5 objects per star figure. The planetary system modellers say that even the 1.8 PMOs per star figure found by Sumi et al can’t be accounted for by their planetary ejection models.

Greg February 14, 2012 at 10:26

Joy expresses my concern’s eloquently, after reading joy’s island hoping it made me think of a project that was done in Hawaii, there may be another solution to the energy needs of a Oort cloud colony, a much simpler one than fusion, in such a cold environment a heat engine would be ideal. The more extreme the temperature difference the more efficient the heat engine is. The heat source can at first be nuclear fission then when enough hydrogen from electrolysis of the ice water is produced heat from hydrogen fuel cells or just burning the hydrogen would produce the heat differential we need. The temperature differences are so extreme in the Oort that I can believe that even human body temperature could produce useful energy.

kittlej February 14, 2012 at 12:38

a few observations
first I think that while we use an exponential growth model for a species in introduced into a novel environment , these populations crash. ( think E. coli inoculated into rich growth media) All exponentially accelerating populations crash as they inevitably reach the limits of their container. However, I think that we may be able to model human expansion as a polynomial. There is a linear growth term that is due to improved resource utilization without new habitat expansion, and a squared term that describes the expansion across a 2 dimensional surface ( our planet) . we also have the possibility of a cubed term IF we are expanding in a three dimensional space ( like the oort cloud or kuiper belt, or even the galaxy) however, the trick is understanding the coefficients. -By the way any short term fluctuation in growth can be exponential growth or even a collapse, but on the long haul we are limited by the out ward reach of our populations range. it is just ecology.
I wrote before that our exploration will be very different if we are limited only to fusion and chemical rockets.
There are some missing pieces to the argument. first, these dirty iceballs should have Hydrogen and oxygen as well as other elements – nitrogen from ammonia, carbon from Poly aromatic hydrocarbons but also carbon monoxide and carbon dioxide. These have all been seen in molecular clouds. There will be lithium because there is a measurable amount left over from the big bang, There will be trace iron and nickel, silicon and aluminum all evident in our won solar system. these iceballs should have formed from molecular clouds not so different than our solar system, but not have collected much Helium and molecular hydrogen as a giant planet or star. while there may not be a lot of “rock” other than ice, the ice will have abundant traces of elements such as litium magnesum and calcium, sodium and potassium, and lots of halides and sulfur compounds and phosporus. as dissolved salts, dispersed in varying amounts throughout the dwarf planet.
If you give me O, H, C , N, S and trace metals, I can build you a world . Distill off and use the water, collect and separate the salts.
Who is to say we cannot use magnesium as a building material, or even reactive metals like calcium in fantastic new alloys?
Also- the average Polynesian did not care or depend on exploration in their daily lives. they just lived loved and worked much as we do. they only traded with nearby islands, though materials would make their way across the sea passing form hands to hands. It will be the same with the nomads, every generation or so some slow moving encounter with another body within a few AU’s or so many occur. These would include even very small worlds of 100km or even less. At those times a probe or ark might be exchanges, perhaps some embryos or bits of technology. Design and building sophisticated devices would be restricted to larger worlds, not seen in generations but still in communications. The smaller worlds are the medium for exchange.
As they spread across the sparse parts of the pacific, the Polynesians only discovered and colonized new islands by accident or in desperation. ( read Jared Diamond’s “collapse”) where people would take on virtually suicidal missions into the open sea to avoid starvation. In these future nomad worlds, people will not be all that eager to colonize except when the opportunity presented itself by the infrequent passing of another worldlet. Most inhabitants would not even see themselves on a journey to the stars, except in a romantic sense. Perhaps if we are to succeed as a species we have to be prepared to live lives in warm dwellings on cold planets.

kittlej February 14, 2012 at 12:50

by the way with C,O, N and H you can make a lot of different types of plastic, often using engineered microbes or other material such as advanced carbohydrates . Plastics and composites are the preferred lighter mass materials for building ships that use reaction mass for propulsion.
We do not live in a steam age, were steel is the limiting building material. The argument that interstellar civilizations will be seriously limited by the lack of iron or other metals is probably not valid. As long as there are trace amounts to support building biological tissue, and some aluminum and silicon in small amounts for electronics. . all this is a few percent of the total mass of a human colony, nowhere like the incredible amount of steel used in a sky scraper ( i leave it as an exercise for the reader to understand why…)

ljk February 14, 2012 at 14:54

Greg said on February 14, 2012 at 10:26:

“The temperature differences are so extreme in the Oort that I can believe that even human body temperature could produce useful energy.”

Why am I suddenly thinking of The Matrix?

Adam February 14, 2012 at 15:32

kzb, such objects are more likely low end condensations in star-forming regions. Brown dwarfs seem to form in heavy, wide disks around heavier stars, and there’s no reason why smaller objects can’t form too via smaller disks around the brown dwarfs.

Doug M. February 14, 2012 at 16:08

“Basically, the three options for living in the Oort cloud are fusion, fusion, and fusion.”

I’ll add a fourth: someone in the Inner Solar System builds a solar power array, and beams power out to you.

Of course, nobody’s going to do this without some incentive. What could possibly be found or produced on a KBO that might make this worthwhile?

Doug M.

JohnHunt February 14, 2012 at 17:04

Nick February 14, 2012 at 0:16

> Another grand sci-fi idea based on a whopping economic misunderstanding.
Actually the frozen embryo idea is the most economic because the mass of the “craft” are so small it takes the minimal amount of energy to launch.

> What happens to the embryos when the freezer malfunctions…
The “freezer” never malfunctions. The freezer are the temperatures in deep space. Embryos are naturally frozen so long as they get beyond the “snow line” from the parent star. No energy is needed to keep them frozen.

> and the necessary replacement part can’t be made?
Again, since equipment generally doesn’t need to be running because generally nothing needs to be done, there won’t be the normal equipment breakdown. Temperatures in deep space are so low, the thermal movement of atoms within the equipment will be minimal.

> What happens when a meteor takes out navigation sensors that cannot be replaced?
The density of micrometeorites is not that great. If the craft travels slowly, there will be less of a problem from things like high-speed interstellar dust. Redundancy of systems can overcome some of the losses. If a “craft” is lost, the only “living” thing on it will be a frozen embryo (or zygotes) and so not exactly the loss of a person. If that happens, statistically, other such craft will make it through.

> How is the skeleton crew, human or robot…
There will be no need for a “living” crew human or robotic like there has been no need for such on the Voyager or Pioneer craft.

> The “send embryos” idea doesn’t work, and it’s not even close. Making the skeleton crew (or skeleton robots) fewer just makes the problem worse.
Respectfully, I don’t think that you understand the Embry Space Colonization concept that is being proposed.

Rob Henry February 14, 2012 at 17:13

Joy, your calculations are wonderful, and their result important, but I do not think they prove that without fusion there can be no thriving Oort Cloud community. My reasoning is as follows.

To set the scene, as you have pointed out on several occasions, having a low cost energy source provides resource management dangers that are only apparent in high exponential growth economies. The likely different economic mode of the fusion-less Ort Cloud community is ideal for testing the sustainability necessary for an interstellar voyage.

Adam has pointed out that collecting the suns energy is not a net energy consumer even at that distance, though I don’t think he has thought sufficiently about the potential for human error damaging a collecting array when the break even point might be a hundred years hence.

To me the key is that energy would be a couple of orders of magnitude cheaper wrt artisan human labour in the inner solar system. If sunlight collected in this region could be retransmitted by 1% efficient lasers, it would.

Now because energy is so scarce here, any compact store of it
Would be like a default currency and used sparingly. Economics would dictate the U and Th would diffuse out into the cloud from the Earth’s crust, and luxury goods would diffuse inwards to Earth’s populous. Additionally, any necessary mineral extraction process that would make U or Th extraction economically viable as a by-product, would be reconfigured to extract these.

Thus I don’t think we can rule out a fusion-less Oort Cloud community, and if it does happen there will be ample U and Th squirreled away in their local treasuries to stock an interstellar voyage given the willpower.

JohnHunt February 14, 2012 at 17:34

Eniac, Joy, & Ronald. Glad to see we are agreement. Project Hyperion is tasked with developing a plan for sending humans on an interstellar mission. I think that the Embryo Space Colonization idea is the earliest, most cost-effective first true interstellar mission. If you e-mail me, I would be glad to share with you information about the ESCAPE Mission. johnhunt2001 – gmail.

> Personally I favour a solar sail departure on a sundiving course…
Joy, I am not yet certain what would be the best propulsion option for an embryo mission. The thing that I like most about the sundiver concept is that it would require no in-space infrastructure and so could be the earliest method of launch. I remain open-minded.

Amphiox, Yes I could imagine colonies being spread “organically” as you suggest without guidance. But that is what would happen behind the leading front of low-mass, low-energy seed expansion. The energy cost of sending living, breathing humans to the Oort Cloud would probably be greater than launching a seed in the same amount of time-to-destation for a fully interstellar mission.

> The exciting thing is that we can almost get started on this project RIGHT NOW. (Well not right now, but soon).

I can point to multiple areas of fairly well developed existing technology which are applicable to a frozen embryo mission including: birth after frozen embryo, ectogenesis, android robotics, and expert systems (such as Siri). Granted, those technologies need to be fully matured. We are having a hard enough time getting a colony on the Moon or on Mars. I don’t believe that we will be able to establish colonies in the Oort Cloud any time soon. If the $30 billion to be spent maintaining the ISS over the next 10 years were spent on maturing the technologies I mentioned, that part of the mission could be completed. Beamed power from the Moon? That would be harder.

> Thus to erode a square meter of area, the reflectors need to be exposed to the dust for ~6 million years.
Adam, Impressive calculations. Thanks.

Daniel Suggs February 14, 2012 at 19:58

As I have read through all these comments, I can’t help but keep coming back to thoughts of SciFy channel’s ‘Firefly’ series. Wild west KBOs, Principalities, Empires, etc, etc. Wow!

Bounty February 14, 2012 at 20:39

I think a lot of this depends on the specific Ort Cloud object and it’s origin. They’re not all clones. How many Sedna and Eris like objects might be out in the ort cloud? Possibly ejected by the larger planets.

kzb February 15, 2012 at 8:43

@Adam
I’ve seen a few papers where the number of PMOs in young star clusters is surveyed. The conclusion seems to be that PMOs are formed less often than stars (about 14% from memory). Now reading these papers, I have to admit it is not absolutely clear if these PMOs formed as planets (in which case the surveyed PMOs would be the high-mass tail of the planetary mass function) or be the stellar formation mechanism (in which case they are the low-mass tail of the stellar IMF). Certainly one of the stated aims of these papers is to tie down the stellar IMF, so the latter explanation seems most likely (I also assumed, perhaps wrongly, that PMOs in circumstellar discs would not be resolved separately from their host stars, and hence would not be counted in the survey).

kittlej February 15, 2012 at 12:05

Bounty- you remind me – Eris has a high density and is a strong indication that these worlds are not all made solely from water ice . So we can expect both rocky materials and froze volatiles ( liquid or gas are 25 degrees celsuis)

Adam February 15, 2012 at 15:59

Hi kzb
Studies are only looking at the high end of the mass range for pragmatic reasons – they typically want to try for a direct detection of a hot young planet or PMO in a young cluster. Below a certain mass and PMOs are invisible, only observable via transits (but non-periodic dimming can’t be confirmed as an object vs a sunspot) or microlensing. The latter is where present number density constraints are derived from, with large uncertainties due to the few data points at hand. Thus the original paper discusses ways of piggy-backing further observational studies onto existing survey programs.

Ole Burde February 15, 2012 at 16:03

JohnHunt
The idea of Embrio Space Colonization is a very temptating shortcut to the stars . The only way i can imagine it sucseeding , is a scenario where android robots play a role in some of the critical moments of the mission , such as the startup of the engines (for braking) after several hundred years of shut-down . Another critical role for the androids ofcourse , would be the production of humans from embrioes , and playing the role of parents for the first generation of humans .
Alltogether this would demand some VERY capable androids …the only scenario where it could sucseed seems to be one including an earthlike planet with a breathable atmosphere and nontoxic local lifeforms . If the androids were smart and independent-thinking enough to start a spacebased culture at the target star from scratch , they wouldnt need any humans to slow them down would they ?

Eniac February 16, 2012 at 0:57

@Ole: I think embryos would be sent in addition to a human crew, in order to send a large geen pool with a minimum of energy expenditure. You could, for example, send 50 women and 1,000,000 embryos, to get a genetic population of 1,000,050 at the fare for 50, plus some freight. Both the adults and the embryos would likely be frozen, to avoid the much discussed issues of long-term voyages in living ships. In transit, the ship would probably be in a deep freeze, to minimize deterioration. The crew would be thawed upon arrival, and the embryos for many generations onward on demand for procreation.

As you allude to, it is also very likely that initial missions will dispense with humans entirely, and robotic seeds will be sent to establish an autonomous industrial capability in the target system. This would be used for the purposes of creating more seeds, exploring the system, relay information back, and prepare habitats for eventual human occupation if and when any arrive.

Ole Burde February 16, 2012 at 19:05

Eniac
“Both the adults and the embryos would likely be frozen, to avoid the much discussed issues of long-term voyages in living ships”
Lets hope so , but it might proove imposible or at least too unreliable to freeze adult humans . Theoreticly this might be fixed by genetic engineering , (some siberian fish can survive being frozen and we could perhabs borrow their genes) , but as I discovered last time we talked about similar things , the psycologic resistanse to enginering ourselves is a deeprooted and formidable enemy , EVEN here among the suposedly rational spaceenthusiasts !
So until proved otherwise , a starship will have to have either a live component to its human crew , or a crew of robots capable of producing humans from frozen embrioes , or perhabs a combination of both . The great advantage of the robots-only crew, is that the ship can lie almost “dead” for hundreds of years , while only a bare minimum of automatic funktions , like charging of batteries ,are carried out by relativly simple but longterm-reliable mekanisms.
The main bottleneck for JohnHunts scenario is probably of psycologic-cultural nature… just imagine what kind of experiments would be necesary to verify the main strategy… imagime sombody sugesting we take alot of human embrioes and let a bunch of japaneese plasticrobots be their PARENTS !
Whoever sugested such a thing would imidiately and permanently become a social outcast…. so I wont !!!

Rob Henry February 16, 2012 at 19:39

Sorry if I seem fixated by my own models, but I can’t wait for criticism to see if it would work. The possible existence of a future thriving Oort Cloud community WITHOUT fusion seems to hold up to further back of the envelope calculations I will just look at it being underwritten by thorium supplied from Earth.

The supply chain for thorium would be superlatively long, since the first choice would not be to supply the Oort cloud at the limiting velocity of 1% c due to energy density considerations, and a more likely maximum velocity for trade items would be 0.1% c. This would imply a thousand years for trade items to reach the very outer limits of the cloud as postulated in this article. Over such times direct trade would not make sense, but a diffusion outwards of such items as trade would. However the maths are the same.

At current wholesale electricity prices, the energy content of a kilogram of thorium at typical conversion efficiencies is about $100,000, and its price on Earth is about $100. Power satellites would probably render the Earth based production of electricity from thorium completely uneconomic. At real interest rates of 2%, Economic forces would powerfully drive its outward diffusion to energy poor regions along supply chains that have a cumulative length of up to 350 years. Of cause, because of dramatically varying local supply and demand thorium would have an easily observable NET outward migration over very much longer chains of trade routes – it is just that the drive would not then be so strong that an outermost community could depend on a sufficient supply of Earth crust thorium to supply their entire needs.

However, at this price gradient, much faster speeds of delivery would be considered by entrepreneurs along long, atypical, trade routes (well?) before that 1000 fold increase is attained.

Now for the reason that much of this thorium will actually be stockpiled in the outer system…

We can calculate its value as an insurance if methods of solar energy collection catastrophically fail, and fusing thorium is the only way to save the entire colony. I will calculate the value by a slave colony of software development model. Here the Th energy content is turned into human biological activity at the incredibly high efficiency of 1% (I can defend that this is almost possible even with today’s technology), everyone works a 40 hour week at $10 an hour, which is all exported. This values the thorium at half a trillion dollars per kilogram.

Once all catastrophic failure contingencies are covered by Th stocks in local Oort treasuries, the marginal extra value of a kilogram of thorium to the community as a whole would drop precipitously, and there would be no problem using any additional stock for economic expansion, as long as it is not used for maintaining their existing economic level there would never be a reason to panic.

Note how this can also explain why Adam really can be content that such a long payoff time for a solar energy collecting array.

Mean while, back on Earth, power satellites would have made energy ridiculously cheap, and a cubic kilometre of typical crust, let alone an ore grade, would begin to look interesting with $3 trillion, worth of thorium (at Earth price of $100/kg). The price of all other minerals would have plummeted to near zero. Actually, in the inner solar system, thorium mining does not even have to produce more energy than takes to extract to be economic.

Notice that that economics dictates that no thorium would actually be burned in the inner system, and the top 100m of Earth’s crust should contain half a trillion tones of it. That would represent at least 50 quadrillion dollar of economic expansion (the value of that expansion must exceed $100,00 per kilogram Th used), all of it in the outer system.

I put it to you all that even in such a low energy environment, most human generated permanent capital might end up in the Oort Cloud, even though there is no question that most human economic activity occurs closer to the sun.

Rob Henry February 16, 2012 at 21:49

Oops, I used Earth price for that capitalised cumulative growth figure. It should be 50 quintillion dollars, and since a million dollar is about the capital infrastructure to support a human at very high wealth levels, that should represent the potential to support 50 trillion people permanently.

Eniac February 16, 2012 at 23:31

@Rob: I find it a little hard to believe that fission fuels should be available only in the Earth’s crust. If anything, I would expect the crust to be depleted in heavy elements, since they should tend to accumulate in the core. The other rocky bodies in the solar system should contain at least similar amounts, so should the rocky fraction of the icy bodies further out, including in the Oort cloud. So I would question the premise that Earth will remain the sole source of nuclear fuel for long. Plus, likely there will be fusion, which should make fission fuel obsolete pretty darn quick.

I also disagree with your assertion that solar power satellites will make energy cheap on Earth. Energy is already very cheap, and it is very difficult to imagine that beaming it from space to the ground, even if it were feasible and efficient somehow, could ever be economical compared to terrestrial photovoltaics and nuclear power.

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