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Riding on Comets’ Coat-tails

Voyager 1 is now an interstellar spacecraft, according to the latest reports (and I’ll have thoughts on Voyager, its progress and its implications, on Monday). For today, though, Keith Cooper is envisioning other ways of going interstellar, methods that take advantage of natural objects like comets. Can we harness their resources and change the paradigm of deep space flight? Keith has written often for Centauri Dreams despite a busy schedule as editor of the British monthly Astronomy Now and equivalent duties at Principium, the newsletter of the Institute for Interstellar Studies. This look at how we might expand into the Oort Cloud and beyond takes us into a future in which our species may well differentiate as we explore different ways of reaching the stars.

by Keith Cooper


Amidst the clamor for giant metal-hulled ships, fusion engines and warp drive, our interstellar pioneers may be missing a trick. Why go to all the trouble of building a starship when there are trillions of natural ones right on our doorstep?

After I reviewed W. Patrick McCray’s book The Visioneers in my last column I picked up some old copies of Omni magazine from eBay on the strength of the magazine’s portrayal in the book. Flicking through the June 1984 issue I stumbled upon an article about using comets ejected from our Solar System as vessels to transport us through interstellar space. This instantly took me back to a few months earlier and a conversation with space artist Jon Lomberg, who mentioned artwork he did for Carl Sagan of ‘treeships’ – living, hollow trees grown out of comets, riffing on an original premise by physicist Freeman Dyson.

Comet ships are an old idea that does not seem to have received much airtime recently, but with all eyes on Comet ISON this winter it is the perfect opportunity to revisit this novel notion. ISON itself is a five-kilometre wide chunk of ice, rock and dirt that is racing towards the Sun. It hit headlines last year with claims that it could, for a short time, outshine the full Moon, but expectations have been significantly downgraded since then. Nevertheless, there is still hope that it will grow as bright as Venus near perihelion on 28 November – assuming it isn’t vaporised in the heat of the Sun as it flies within 1.8 million kilometres of our star.

If it survives its solar encounter, the comet will be flung back into the Oort Cloud – the distant realm of the comets – and it may even be ejected from the Solar System entirely on a parabolic trajectory. That’s a remarkable thought: the comet would enter interstellar space, to wander lonely between the stars and to maybe, one day, be picked up by another star. The comet highway could work both ways – the short period comet 96P/Maccholz is suspected by some scientists to have originated from another star system, based on what appears to be a carbon depletion seen in the abundance of molecules like cyanogen compared to other comets in our Solar System (although it should be stated that there are also more mundane explanations for 96P’s weird chemistry). Computer simulations of the formation and evolution of the Oort Cloud indicate that up to a hundred times more comets are ejected into interstellar space than remain in the Sun’s grasp. The Solar System is literally shedding comets in their billions.


Image: Hubble’s view of Comet ISON (C/2012 S1) on April 10, 2013. This image was taken in visible light. The blue false color was added to bring out details in the comet structure. Credit:NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team.

Cometary Trajectories Outward

No astronomer has ever observed the Oort Cloud directly – it is too distant, its inhabitants too small and too faint – but we know the cloud must exist there as the orbits of long-period comets indicate they originate in all directions from a region that exists between as close as 2,000 and as far away as 50,000 astronomical units. Models suggest that the Oort Cloud comes in two parts – an inner doughnut shaped region and an outer spherical halo, together containing more than a trillion icy bodies. It is from the loosely bound outer cloud that the long-period comets like ISON hail – and which interstellar comets must also originate.

We could take advantage of these escaping comets, eschewing the need to scratch-build complete starships and loading millennia’s worth of supplies onto them when we can just hitch a ride on a comet. After all, comets come ready-made, endowed with resources and volatiles: plenty of water, metal ores, even large stocks of deuterium to help fire a fusion reactor. One downside is that we might not have too much say about where the comet is heading, but given that the Solar System will eject comets randomly in all directions, hopping aboard them may pave the way for the full blown diaspora of humanity into the Milky Way at large.

Catching a comet when it is near the Sun might not be the best option. As a comet receives greater solar energy it warms, developing the familiar fuzz of the coma and a trailing tail as ice on its surface sublimates into vapour. More violently, gases trapped within pockets inside the comet’s porous structure begin to expand, often bursting out explosively. The comet would be unstable, even out beyond the orbit of Mars, as 2008’s outburst from Comet 17P/Holmes proved. Instead the best time to latch onto a comet could be when it is receding back into the outer Solar System and things have settled back down on its surface.

There’s an added problem. Human activity on a comet could kick the activity off again, so actually building habitats on or inside comets is not going to work. Consequently, the best tactic might be to live in habitats that are tethered to and which siphon materials from a comet. It is not yet clear how we could mine a comet safely, but the comet’s deuterium, locked in its water-ice, will be essential to the interstellar travellers if they want to survive their journey.


Image: Comet Halley, as seen in 1986 by the European spacecraft Giotto. Data from Giotto’s camera were used to generate this enhanced image of the potato shaped nucleus that measures roughly 15 kilometers across. Some surface features on the dark nucleus are on the right, while gas and dust flowing into Halley’s coma are on the left. Credit & Copyright: Halley Multicolor Camera Team, Giotto Project, ESA.

Energy can be derived from nuclear fusion using deuterium. It is particularly abundant in comets – Comet Hale-Bopp, which graced our skies in 1997, was shown to contain an abundance of deuterium twice that of Earth’s oceans, at a ratio of 1:6,410 compared to regular hydrogen. This is three times as much as astrophysicist Eric Jones predicted when writing on the subject in 1985’s Interstellar Migration and the Human Experience, where he speculated that one deuterium atom for every 20,000 hydrogen atoms in a comet should be sufficient to power fusion to sustain a significant population of interstellar voyagers for many centuries. We see this scenario being played out in [be warned, spoilers] Greg Bear’s novel Hull Zero Three, where three cylinders are affixed on stanchions to a central comet from which resources are being drawn. Alas, interstellar travel times for comets are measured in millennia, not centuries, so an additional power source will also be required.

Deep into interstellar space the singular power of the Sun fades into insignificance. However, despite being at great distance the collective starlight of all the stars could, says Jones, produce several hundred megawatts of power. Huge parabolic mirrors built out of aluminium mined from a comet could be constructed to capture and focus diffuse starlight. Inspired by Robert Forward’s book The Flight of the Dragonfly, in which the main spacecraft used similar mirrors to capture and focus the laser light that is beamed towards it from the Solar System and which is driving it forward to Barnard’s Star, Jones suggests that, at around 1.6 light years from the Sun, the mirrors would require a diameter of 3,000 kilometres. Cluster a bunch of comets together, says Jones, and the inhabitants could tend to a farm of mirrors 30,000 kilometres across. Combined with fusion reactors, these mirrors should be able to supply power to the comet cluster’s inhabitants for as long as the community exists.

Biotechnology and the Interstellar Diaspora

Nevertheless, it all sounds like a terribly hard way to live, a subsistence existence. Small communities in tiny tin cans eking out their lives surviving off a trickle of energy derived from raw starlight and a few centuries worth of fusion power, light years from any planetary system. It is difficult to think of a reason why anybody would want to travel to the stars this way. Jones suggests that bands of nomads, clans and tribes residing in the Oort Cloud in the future would be most likely to embark on such a lonely journey, mirroring the voyage into the unknown Pacific made by the Polynesian explorers a thousand years ago. It is a thought that Freeman Dyson concurs with, a thought that he voiced at his talk at the recent Starship Century symposium in May this year. For Dyson, in a few centuries there could well be more people living in the Oort Cloud than are living on Earth. If this is to be the shape of the future diaspora it couldn’t be further from the organised, planned launch of gun-metal grey starships with hand-picked crews that we envisage today.

Before dismissing cometary travel as a venture only for a rabble of humans, Dyson perhaps has a better idea, one that raises itself above the mundane toil of living off a comet in the conventional way, to arrive at a level of magnificent eloquence and beauty.

During his Starship Century speech, Dyson revisited an old idea of his dating back to 1972. He postulated with a degree of confidence that biotechnology will come to dominate the future, inevitably instigating a number sociological changes in humanity, some that we might predict, others that we cannot. One possibility that Dyson does predict is the existence of ‘big trees’ – genetically modified plants designed to survive in space that would grow to enormous sizes, many kilometres across, with their roots ensconced within a nutrient-rich comet. The tree would be so huge that it would contain hollow, airtight spaces where people could live. Furthermore, the plants could grow their own greenhouses, “just as turtles grow shells and polar bears grow fur and polyps build coral reefs in tropical seas,” according to Dyson. They would produce their own oxygen supply from photosynthesis, like any other plant, while an array of lenses and mirrors could focus light from the distant Sun and stars into the greenhouse. It is an idea that has had a little play in science fiction, such as in Dan Simmons’ novels like Hyperion:

“The Consul remembered his first glimpse of the kilometre-long treeship as he closed for rendezvous… it’s leafy bulk clearly ablaze with thousands of lights which shone softly through leaves and thin-walled environment pods, or along countless platforms, bridges, command decks stairways and bowers. Around the base of the treeship, engineering and cargo spheres clustered like oversized galls while blue and violet drive streamers trailed behind like ten-kilometre roots.”


Image: This painting by Jon Lomberg was commissioned for the book Comet by Carl Sagan and Ann Druyan, to illustrate a concept proposed by physicist Freeman Dyson. Dyson suggested that in the future, comets could be used as the source of minerals and water to support the growth of gigantic, genetically engineered trees. These trees would use sunlight to grow to large size, and provide a habitat for space colonists. Credit: Jon Lomberg.

Biotechnology could play a significant role in adapting life for the harsh conditions on the rim of the Solar System. Rather than toiling away on comets, Oort Cloud denizens could use biotechnology to adapt to be more efficient with limited resources and create a balanced ecosphere in their habitats. While we are currently busy tinkering with our paper designs for starship drives and metal spaceships, are we blind to the oncoming biotechnological revolution and could it be this that leads us to the stars rather than fusion engines, antimatter or warp drive?

Perhaps, but there would be a downside. Sticking engines on a comet or a ‘treeship’ as in Simmons’ novel is probably impractical if we are limiting ourselves to the resources available only on the comet or via captured in starlight. Instead, riding on a comet will mean taking the slow road to the stars. We shouldn’t expect to get anywhere on them for tens or even hundreds of thousands of years. By necessity they would become generation ships, at their greatest a fleet of mobile O’Neill colonies, or a literal floating forest spurring the “greening of the Galaxy” where starships are grown, not built.

If we are to pursue the faster route, constructing engines of enormous power settled into vast metal hulks, then perhaps the the comet travellers play a different role. Maybe they will never reach the stars, but instead settle into the dark spaces between, way-stations for lost or weary travellers who have followed later in their faster metal starships. Two different populations moving out into the Galaxy and occasionally meeting, each pursuing distinctly different technologies, each growing and changing in their own different ways, one driven by the green efficiencies of biotech, the other by the gleam of a metal hull and the hum of a starship engine. If you could choose, which would you be?


Comments on this entry are closed.

  • Rob Flores September 13, 2013, 12:08

    I am not sure about that our mastery of DNA and genomes will come before the industrialization of space obejcts. But even if some expertise in bio engineering arises in the next century, just how fast would such biological space ‘trees’ grow. For significanlty sized objects housing a small colony of 300 or so, would it take a few decades like a redwood, or are we talking more like a pernicious weed that grows with gusto. As always cost will be main driver. Nanobot materials processing and assembly versus bioengineered self repairing entities….Hmm a dicotomy maybe even a societial one, like Hamilton’s Nightsdawn Trilogy.
    As I might have mentioned before, 1 mi comets would make convinient emergency space arks. They would be light enough to be able to change their course and acclerate them . They would shield you from most space debris. You would need to hollow out only what you needed, the rest being created in-situ. Question is, is a 1mi comet provide enough resources for 3-5,000 year journey for a few hundred refugees.

    What kind of event would percipitate such emergency?, use your imagination.

  • ljk September 13, 2013, 13:06

    The European Space Agency (ESA) is currently considering a proposal for a true interstellar probe here:


  • David Cummings September 13, 2013, 13:45

    I’m not so sure we’ll ever ride comets to the stars, though we might end up commandeering them and taking them with us.

    It’s undeniable that between comets and asteroids there is a huge amount of floating wealth in the solar system, wealth that can be gathered by space colonists without the need to lift heavy loads up out of deep gravity wells.

    I personally believe that — except for probes — a serious flight to a nearby star will be launched not by an earth-bound human race but by one that is already at home in the solar system, freely moving about among the various moons and asteroids that abound within fairly easy reach. (There will be Matians, too, though that group will probably be more preoccupied with terraforming and not so interested in interstellar flight).

    As for the closing question of this article, I personally would opt for the “the gleam of a metal hull and the hum of a starship engine”.

  • Alex Tolley September 13, 2013, 13:58

    Jones suggests that bands of nomads, clans and tribes residing in the Oort Cloud in the future would be most likely to embark on such a lonely journey, mirroring the voyage into the unknown Pacific made by the Polynesian explorers a thousand years ago.

    It is a pretty cracked mirror. There is a huge difference between the short Polynesian island hopping journeys and these Oort Cloud colony journeys of long duration. It would be like permanently living on the boat for generation after generation.

    As for:

    …way-stations for lost or weary travellers who have followed later in their faster metal starships.

    Well who better than to quote than Douglas Adams in THHGTTG:
    “Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
    Those way-stations will be lost in space and time.

    The Dyson tree idea is enchanting, but consider does it solve any particular problem better than mechanist engineering? Is is really easier to grow mirrors and membranes than manufacture them? Are “wooden” hulls going to be better at containing ecosystems than metal or plastic ones? To me, invoking “synthetic biology” is just the new nanotech synonym for “magic pixie dust”. Once we made dug out canoes from tree trunks. Much later we grew trees and cut and shaped their lumber to make maritime ships. The largest ships today are made of steel, a material that has been favored in shipbuilding for about 150 years. Would you really want to resort to growing ships today, assuming you had the right technology? It is exactly the same problem we have with fanciful ideas of “biotecture” – fun to speculate about, but unlikely to be practical.

    As for choosing a comet anchored tree or a slow worldship to live in, I would choose neither. Life will be much more interesting in the solar system, with a range of inhabited planets and a vast number of space habitats to pick and choose from, each exhibiting their own unique environments and culture.
    If I am going to be part of the human interstellar diaspora, I would rather be born at the target star, preferably when it already has vibrant space colonies and inhabited worlds. Getting on a boat, whether of wood or steel, that I will never get off, is not an attractive proposition in most scenarios I can think of.

  • Andrew Palfreyman September 13, 2013, 15:14

    If the idea is to hitch a ride on a comet which will shoot out of our solar system, this presupposes we already have the deltaV to rendezvous with it – in which case we can choose our own craft. The SunDiver concept is such a one.

  • Abelard Lindsey September 13, 2013, 18:54

    I am not sure about that our mastery of DNA and genomes will come before the industrialization of space objects.

    I expect it to come far sooner. Science and technological innovation is a cost driven enterprise. Bio-engineering experiments are far cheaper than space projects. Hence, more people can do them and progress is faster.

  • NS September 14, 2013, 4:37

    Had the same thought as Andrew Palfreyman — if we have the energy to catch up to a comet then we don’t need the comet. Also hoping for a sundiver/beamed sail/something else that will get us to interstellar space within the next 30 years so I’ll have a chance of being here to see it!

  • David Cummings September 14, 2013, 7:00
  • Alex Tolley September 14, 2013, 14:00

    @NS – surely the point of comet rendezvous is to utilize its resources without needing the energy to accelerate it to interstellar speeds.

    Alternatively, it could be used as a “free stage” for further acceleration using its resources to reduce the demands of the rocket equation.

    I’ve wondered in the past if a rocket rendezvoused with an inbound comet at some delta-v, then used solar power to generate new LH2LOX propellant, it could then depart around perihelion (like a sun diver solar sail) to even faster velocity.

  • Brett Bellmore September 14, 2013, 19:47

    “There is a huge difference between the short Polynesian island hopping journeys and these Oort Cloud colony journeys of long duration. It would be like permanently living on the boat for generation after generation.”

    Surely it would be more like permanently living on the island for generation after generation?

  • Brett Bellmore September 14, 2013, 20:35

    ” Question is, is a 1mi comet provide enough resources for 3-5,000 year journey for a few hundred refugees.”

    Assuming a 1 mile diameter spherical comet, that’s about 2 billion cubic meters. Divide that by 5000 years and a thousand refugees? 429 cubic meters per person-year, a couple hundred tons of dirty ice a year. You certainly wouldn’t lack for water, you’d have enough carbon and nitrogen to have no trouble producing all the organic compounds you might need even without recycling. But you’d find higher elements in short supply, you might want to bring along a lot of essential trace elements and metals.

  • JB September 14, 2013, 20:44

    Maybe there isn’t a dichotomy between bioengineered ships derived from comets and classic metal ships. Nanotechnology of the type envisioned by Eric Drexler etc. might allow both types to be grown from resources found in space. Indeed, the end result might be a hybrid between the two. Imagine something that operates like a very large plant, obtaining energy from starlight when possible, but can also obtain energy from fusion, is made of the right materials to block radiation, can withstand acceleration from fusion engine pods grown as an integral part of its structure, and can repair itself and even grow and replicate when new sources of raw material are encountered.

  • Gregory Benford September 14, 2013, 22:13

    I like Brett’s “Surely it would be more like permanently living on the island for generation after generation?” — quite true.
    David Brin and I treated using cometary resources in HEART OF THE COMET and implies at the end that they were all bound for the Oort cloud. We intended to write a sequel about the growth of a cometary civilization, eventually migrating to the stars–and even started a few chapters…but lost interest and never finished it. Too bad! That future looks even more plausible now.
    FYI, HEART OF THE COMET is still in print after nearly 30 years.

  • Rob Henry September 14, 2013, 22:36

    “The Dyson tree idea is enchanting, but consider does it solve any particular problem better than mechanist engineering?”

    Yes, it speaks to the following problem:
    In the Oort Cloud light levels are so low that any habitat growth driven by photosynthetic processes may well have a minimum doubling time that is much greater than the natural human generation time of about a quarter century. We have done the analyses before on these pages, and found any photosynthetic process would be forced to use the bulk of its energy creating light collecting mirrors, and these mirrors virtually have to be made of ice to bring that doubling time anywhere near that human-time-scale mark. The point follows…

    To live via light utilization in the Oort Cloud you must uncouple the manufacturing process from habitation itself. For every occupied comet, there must be dozens, possibly hundreds of others, that are both unoccupied and growing. Only fully selfreplicating process can provide that.

    However, one thing has subsequently occurred to me. If we use a mirror manufacturer that uses a fluid with a very low melting point and room temperature boiling point (for biology in a xylem-type setup, that would be the fluids critical point) and utilizes countercurrent flow to recover most heat from the setting ice, and transfer it to the point at which it is being melted, then the manufacturing of these ice mirrors might be several times more efficient than we previously supposed. I’m thinking there is no way that that countercurrent fluid could be water, and so much for that idea of warm blooded plants.

    Anyhow I was wondering how on earth Dyson expected these plants to manufacture anything when their every surface would have to be covered by a thick layer of aerogel (is there any other way of preventing their entire energy budget, and more, being spent on just keeping warm??)

  • Eniac September 14, 2013, 22:42


    However, despite being at great distance the collective starlight of all the stars could, says Jones, produce several hundred megawatts of power. Huge parabolic mirrors built out of aluminium mined from a comet could be constructed to capture and focus diffuse starlight.

    Sorry, but no.

    This does not work on at least two levels: Optically and thermodynamically. It is impossible to focus diffuse light with mirrors, and the equilibrium temperature (The temperature at which an object would be able to absorb as much energy as it emits) is just a few Kelvin for starlight.

    Let’s not create another one of these wide-spread erroneous notions that are so very hard to root out later.

  • Abelard Lindsey September 15, 2013, 11:43

    Is is really easier to grow mirrors and membranes than manufacture them? Are “wooden” hulls going to be better at containing ecosystems than metal or plastic ones? Whichever has lower capital costs and requires less human labor is what will be done. Its all about reducing capital costs.

  • Alex Tolley September 15, 2013, 13:34

    @Brett – The island is the equivalent of a new world, the worldship is the transport. Only if you think that the worldship will be as relatively unconstrained and stable as an island ecosystem and that the target world is grim would your analogy work in any way. Worldships are going to be very constrained in nature, and I would not bet on them being the “paradises” they are 0ften depicted since the O’Neill era (or even the earlier Star Trek episode “For the World Is Hollow and I Have Touched the Sky”.

    Would you rather live your life on the boat, even a cruise ship, or Maui? The more interesting question would be, would you rather live on a well run L3 sized colony ship or a world like Mars?

  • Brett Bellmore September 15, 2013, 16:32

    Alex: Why can’t it be transport AND home? Polynesian islands don’t move around on a human, or even human historical, time scale, but this would be like an island that traveled from Europe to North American between the bronze age and today. In terms of size and resources, it’s a fair sized “island”, permitting neither the illusion of unlimited expansion, nor a dire shortage of resources for a fixed population. Assuming a 5000 year travel time, you’d easily have the resources to maintain a largish O’Neil colony using fusion power for that time frame.

    I think your biggest problem would be maintaining an industrial society with such a small population, not energy or material resources. It takes a lot of people, the way we’re presently organized, to have all the specializations we need, and pass them on.

  • Alex Tolley September 15, 2013, 17:26

    @Eniac – couldn’t a mirror be focused on 1 star, e.g. Vega. Assuming 25 magnitudes difference, would that mean that we would need about 10^10x surface area to get the same intensity as the sun from earth. So for each 1 m^2 collecting area, the mirror would have to be 100 km on a side. If the mirror were made of Aluminium, 100 atoms thick, that would be ~ 10^10 tonnes for the structure (my BoE calc could be off). Extremely large and delicate, but possible? Of course we would need very many of these mirrors to generate the power needed for a colony, which suggests it isn’t practicable. As I recall, O’Neill calculated that a mirror for a colony that was the same mass as the colony habitat would allow the colony to enjoy earth level energy input 2.7 light days from the sun. The sun’s energy there is low, but much more than Vega, so that might be getting towards the possible limit.
    Any mirror would probably have to be manufactured much closer to the sun, which suggests to me that it would no be possible to maintain it in interstellar space using starlight.

  • Astronist September 15, 2013, 18:30

    Keith and Eniac, I think the idea would be to focus the light of a single star, the brightest in the sky at the time, onto a photoelectric array. The adjective used should have been “faint” rather than “diffuse”. So going say to Alpha Centauri, for half the journey you’d be focusing the Sun’s light, and for the rest of the journey Alpha Centauri’s light.


  • Astronist September 15, 2013, 18:38

    Brett and Alex: I recommend Jared Diamond’s book “Collapse: How Societies Choose to Fail or Survive” (2nd edn, Penguin, 2011). Diamond describes the remote and tiny Pacific island of Tikopia, which has supported a population of around 1000 people for the past 900 years. Apart from the fact that these Polynesians were living at a very primitive level, the dimensions of their living space and the size of their population are to my mind an interesting model of a starship on a slow interstellar crossing.


  • Alex Tolley September 16, 2013, 11:01


    This is from the Wikipedia entry for Tikopia: Raymond Firth speculates about the ways population control may have been achieved, including celibacy, warfare (including expulsion), infanticide and sea-voyaging (which claimed many youths). Currently, many young men leave the island, heading to either the Russell Islands or the national capital, Honiara, in search of work. As a result of this outflow, population control is less necessary.

    I hope our worldship could do better than that but who knows.

    I cannot recall the details, but didn’t Diamond talk about this island as part of a 3 island trade block, rather than self contained?

    While the island does have a sustainable agriculture, one needs to bear in mind that they are not bottle ecosystems. Volcanic dust is a major source of vital micro nutrients in the Pacific.

  • Alex Tolley September 16, 2013, 11:10

    @Brett – my point was that the analogy of Polynesian island hopping to slow ships using Oort cloud objects to reach another star does not map well.

    Even the idea of converting these icy bodies to habitats may not make sense in the long term as they will die due to imperfect recycling.

    Culturally, any worldship migrating in the Oort will be more isolated than any Pacific island.

  • Sedjak September 16, 2013, 14:53

    Why worry about focusing light (far, far away heat/energy) for growing the Treeships’ trees when the path is to create biota that can extract all the needed energy from the local resources of the comet? Surely, there will be some form of fusion (cold or damn hot) that can be bootstrapped from cometary resources within 100 years. Biotech will by then also be able to create chimera plants that can extract energy from the mulch laden in the comet plus this aforementioned heat, much like microbes do kilometers below the earth’s surface (hydrogen sulfide) or next to undersea black smokers. Propulsion may be a different matter, though.

  • Brett Bellmore September 16, 2013, 16:00

    “So going say to Alpha Centauri, for half the journey you’d be focusing the Sun’s light, and for the rest of the journey Alpha Centauri’s light.”

    Given a really large parabolic mirror, you will have multiple stars with different focal points, I suppose you could place a collector at each of a hundred or so points. But that’s a flatly enormous mass investment to garner a real trickle of energy. And metals do not comprise a very large proportion of comets, so you’d be using material you brought along. Why not just bring along an equivalent mass of U238 or Thorium, if for some reason you don’t want to rely on fusion?

    I think the effort to extract energy from starlight in interstellar space is so great compared to the payoff, that any rational being is likely to forget about it, and live on nuclear energy while spending a few thousand years getting closer to a star, where you can obtain your energy by less heroic means.

  • Brett Bellmore September 16, 2013, 17:39

    Alex: Based on historical colonization practices, cultural isolation is likely to be seen as a feature, not a bug, by at least the first generation colonists. After that? I’d be shocked if such a colony ever got more than a few light months from another outpost of civilization, and communication over that distance isn’t instant, but it’s hardly total isolation, either.

  • Rob Henry September 16, 2013, 18:02

    So going say to Alpha Centauri, for half the journey you’d be focusing the Sun’s light, and for the rest of the journey Alpha Centauri’s light.”

    By my calculations, the vast majority of the time the mirror would be aimed at Sirius.

  • Rob Henry September 16, 2013, 18:43

    I’ve just had an epiphany. Given the timescale of this endeavour, we could play cosmic pinball, with a thousand year gravitational assist trajectory to maximise pointing accuracy and hyperbolic excess, and still arrive quicker than those who wait for nature to take its cause. Now to the point…

    The leaving point from our system will be a Jovian slingshot, whether that object be asteroid of comet. The epiphany was, given those circumstances, it must be possible to manufacture a simultaneous departure of an iron meteorite and volatile rich comet, such that our community will retain the resources of both (but without Brett’s massive inventory of U or Th)

  • Rob Henry September 16, 2013, 19:10

    In my first post above I made an error. The time when it’s best to point at Sirius looks brief, but its complex. It makes me wonder though whether the above mode of travel would turn practical when an O or B star passes within a couple dozen parsecs of an ETI home star. If so, a whole new sphere of colonising potential would open up to it, and remain to their colonist descendants till the death of that star.

  • Alex Tolley September 17, 2013, 0:08

    @Brett – a few light months separation is an interesting thought. Enough for effective physical isolation, yet close enough for communication. No doubt by then almost any physical artifacts could be transmitted by information and created locally. The environment is going to have to be very rich, otherwise teenagers will get restless when they learn how much they have been separated from the richness of the solar system culture they can never join.

  • Keith Cooper September 18, 2013, 18:10

    Some great comments here, sorry to be late to the discussion.

    @Rob Flores – yep, the distinction between Hamilton’s Edenists and Adamists was in the back of my mind whilst writing this essay.

    @ Eniac, Astronist, Alex Tolley – yes focusing on individual stars does make more sense and I probably shouldn’t have used the word ‘diffuse’. However, one could have a ‘farm’ of mirrors, each mirror focusing the light of a different bright star. Of course, such a mirror farm would be gigantic, around 150,000km according to Eric Jones’ estimates, and possibly impractical. In his essay in the book Interstellar Migration and the Human Experience, Jones talks about how just getting from one end of the farm to the other in fair time could use up significant amounts of the power available to the comet colony. He writes: “A one-day trip across such a city would cost about a megawatt-yr. if, as we have assumed, power generation is of 1MW, frequent trips across such a city would strain the resources of the community.” He suggests instead that the comet dwellers actually be spread across a cluster of comets interspersed amongst the mirrors.

    @ Alex Tolley – on Tikopia the infanticide and one-way trips out into the ocean were seen as altruistic means to benefit the good of the majority from the dangers of overpopulation. Certainly they are not something that Western sensitivities can easily appreciate. Ethics on a colony ship or comet cluster may be similarly alien to what we know in our cosy homes on Earth.

  • Eniac September 18, 2013, 22:05

    I don’t think we have to worry about infanticide and one-way voyages if a good supply of birth control pills will do the trick. Although, there’d have to be a lot of them for 1000 years. I wonder if they keep this long…

  • Eniac September 18, 2013, 22:09

    I can’t believe you guys are hoping to extract energy from the light of an individual star light years away. The lunacy of it should make any decent physicist cringe!

  • Alex Tolley September 19, 2013, 11:54

    @Eniac – that’s the point, it doesn’t really make any sense to collect starlight. Armstrong raised it a few topics back too. Elsewhere someone pointed out that the energy to make metal mirrors would exceed the energy recovered [colonizing in the Oort topic?]. I don’t think that changes with living systems.
    Coincidentally, I just finished reading Dyson’s “A many-Colored Glass” in which he expounds on life using large mirrors to concentrate energy from Europa to the Oort. This is a thought experiment about detecting macrolife by using the :approach called “pit lamping” (detecting the highly directed reflections). But again, no sense of the energetic cost of the mirrors vs the solar energy extracted.
    Beamed energy would make more sense, but it does require that the energy source is maintained for most of the journey, which a number of commenters here have expressed skepticism about.