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Two Ways to the Stars

I often cite Robert Forward’s various statements to the effect that “Travel to the stars is difficult but not impossible.” Forward’s numerous papers drove the point home by examining star travel through the lens of known physics, conceiving of ways that an advanced civilization capable of the engineering could build an interstellar infrastructure. But while Forward was early in this game, so were Iosif S. Shklovskii and Carl Sagan. A Russian astronomer, Shklovskii had written a book’s whose Russian title translates roughly as ‘Universe, Life, Intelligence’ in 1962. Four years later, Sagan would join Shklovskii as co-author and the two would tackle the original book afresh, adding new material that reflected on and expanded the 1962 version’s ideas.

The result was the volume now called Intelligent Life in the Universe. I sometimes recommend books that are essential parts of a deep space library, and this is surely one, significant not only for its historical treatment of starflight but a compelling source of ideas even today. 1966 was early times for the interstellar idea, with the pioneering 1950s papers of Les Shepherd and Eugen Sänger still fresh in the memory; the work of Robert Bussard on ramjets that could move at a high percentage of the speed of light was very much in play. Shklovskii and Sagan thought there were two ways of achieving human interstellar flight, the first of which involved slowing down the human metabolism to allow it to survive long voyages.

Slow Journey, Frozen Time


Automated interstellar probes were still a reach for most scientists in this era, but Shklovskii and Sagan thought that velocities of up to 100,000 kilometers per second — one-third the speed of light — were not beyond reach as our technology advanced. Given that, humans could be frozen for the duration and awakened upon arrival. The challenge was obvious: The density of ice is lower than the density of water, so that freezing a human being, who is composed largely of water, works serious damage to the cells during the freezing and thawing process. Delicate cell structures are disrupted in each case, and anti-freezing chemicals would kill the subject.

Image: The Russian astronomer Iosif S. Shklovskii, whose collaboration with Carl Sagan produced a classic of interstellar studies.

As so often in Intelligent Life in the Universe, Shklovskii and Sagan thought about possibilities that most in the scientific community hadn’t yet considered, particularly Sagan, whose contributions are flagged in the text to make it clear when he is speaking rather than Shklovskii. Sagan had been working with a Swedish biologist named Carl-Göran Hedén, who specialized in microbiology and biotechnology (Hedén would go on to become the founder of the first chair in biotechnology in Sweden, and the first president of the International Organisation for Biotechnology and Bioengineering). Sagan’s conversations with Hedén focused on the difference in density between water and ice, noting that at high pressures different crystal structures and different densities emerge, producing ice that may have applications for human freezing.

Thus a pressure of 3000 atmospheres and a temperature of -40 degrees Celsius turns ordinary ice into ice II, a kind of frozen water that has nearly the same density as the liquid. The conclusion seemed obvious to Sagan, who wrote:

If a human being could be safely brought to and maintained at an ambient pressure of several thousand atmospheres, and then quickly and carefully frozen to very low temperatures, it might be possible to preserve him for long periods of time. This is only one of many possible alternatives. It seems possible that by the time interstellar space vehicles with velocities of 1010 cm sec-1 are available, techniques for long-term preservation of a human crew will also be available.

Sagan thought that with these means, journeys of up to 100,000 years would be possible. A spacecraft moving at 100,000 kilometers per second could reach a star 1000 light years away in 3000 years, with whatever adjustments would be needed for acceleration and deceleration. A trip to the galactic core would take 60,000 years. He went on to say: “If such voyages are to be feasible, the lifetime of our civilization should perhaps exceed the length of the voyage. Otherwise, there will be no one to come home to.” True enough, but it’s hard to imagine coming back to Earth after 60,000 years thinking that the place you returned to would still be ‘home.’

The Relativistic Solution


The second way to make manned interstellar missions happen was, Sagan believed, the use of relativistic spacecraft, which would, because of time dilation, act as a different kind of metabolic inhibitor. This was mind-boggling stuff back in the 1960s, though it followed as a consequence of the by then well established special theory of relativity. Continue to accelerate at 1 g and you reach the nearest stars in a few scant years of ship time. 21 years take you to the galactic center, while 28 years get you all the way to M31, the great galaxy of Andromeda. The ship nudges up ever closer to c, 300,000 kilometers per second, but never reaches it. Poul Anderson explored all this in his wonderful novel Tau Zero, which has blown minds and inspired science careers since it first appeared in Galaxy in 1967.

Shklovskii and Sagan saw such trips as a communications tool. A radio signal would take 2.5 million years to reach Andromeda, and another 2.5 million years would elapse before any possible response. At the time Intelligent Life in the Universe was being written, SETI seemed to solve the intractable propulsion problem by allowing us to ‘explore’ — i.e., to listen — with radio waves that move at the speed of light. Shklovskii and Sagan reversed the paradigm when it came to destinations well beyond the nearest stars. Now it would be actual journeys to these places that allowed a human presence to be known. As Sagan wrote:

…if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after perhaps 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization. Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their world, an adventure and a challenge certainly difficult to duplicate.

Surely this passage is the source of the title for Arthur Clarke’s The Songs of Distant Earth, published in 1986, which explores the effects of long-term interstellar flight. Addendum: See Adam Crowl’s comment below — the influence evidently flowed the other way. My mistake.


Given that reaching M31 within the lifetime of a human crew would require a velocity of 0.99999 c, the only solution that fit the bill was Robert Bussard’s interstellar ramjet, which feeds off the interstellar medium to gorge itself with reaction mass, burning a fusion torch aboard a vessel that becomes more efficient the faster it moves. Sagan liked the Bussard concept and thought it violated no physical principles — he even expected it to be achieved in prototype form in no less than a century — but as we’ve seen (see Catalyzed Fusion: Tuning Up the Ramjet), the problems of lighting proton-proton fusion are immense, and so are issues of drag.

Much work would go into demonstrating this in the next few decades, obviously unknown to Shklovskii and Sagan in 1966, but Bussard variants using a catalytic cycle called the CNO bi-cycle (carbon-nitrogen-oxygen) are still intriguing, and as you might imagine, we’re not through with them here on Centauri Dreams. We can take Shklovskii and Sagan as our models. Both had a taste for bold venturing and pushing the limits of possibility, a taste confirmed in their choice of epigram to introduce the book, Pindar’s Six Nemean Ode:

There is one
race of men, one race of gods; both have breath
of life from a single mother. But sundered power
holds us divided, so that the one is nothing, while for the
other the brazen sky is established
their sure citadel forever. Yet we have some likeness in great
intelligence, or strength, to the immortals,
though we know not what the day will bring, what course
after nightfall
destiny has written that we must run to the end.


Comments on this entry are closed.

  • David Cummings November 18, 2013, 12:27

    Freezing, relativistic time dilation… but there’s also downloading of awareness to a device (mentioned by other posters on this forum), and beaming that download at the speed of light to some pre-positioned receiving device.

    Is such an idea more far-fetched than freezing/rethawing or relativistic spacetravel speeds?

    Perhaps. But certainly the idea of probe software being beamed across space is within the realm of speculation. Of course, the receiving device would have to be pre-positioned, but future upgrades (or downloaded personalities?) can then be beamed out at the speed of light.

  • Paul W November 18, 2013, 12:35

    It’s too bad Sagan wasn’t cloned (is it too late?). He was inspirational.
    There are never enough brilliant thinkers out there who are also optimistic, cheerful, and filled with excitement and awe when contemplating their goals.
    I know that other, “real” scientists were often chagrined (jealous) of the attention afforded to Sagan, a Johnny Carson favorite.

    Let the cause go forward in the near time with near-earth endeavors. Single stage to orbit (space elevator, even better). TPF array. Give us a target and a means to launch from above the gravity well.
    Make Carl proud of us. Make us worthy of his dreams.

  • Roy E November 18, 2013, 16:11

    We don’t need to send people, we just need to send tiny robots that can make more tiny robots and enough information to create lifeforms including humans. The small payload would enable the probe to be sent either faster or cheaper and it wouldn’t be a problem if the probe was destroyed, no one would have died. If the planets were not suitable or they were already inhabited then no one would have travelled in vain. I believe a few terabytes of storage with redundancy would be very hard against radiation and deterioration and would be tiny.

  • Adam November 18, 2013, 16:26

    Hi Paul
    Evocative stuff! One little quibble factually – Arthur C Clarke’s original novella, “The Songs of Distant Earth”, came out in 1957. Influence would seem to have flowed in the other direction. As for flying to M31 at 1 gee, the turnover speed would be 0.9999999999997 c, but that’s far too many 9s to keep track of…

  • Tarmen November 18, 2013, 16:36

    Also at least one more way….
    AIs or near AIs can be sent on fusion-powered, sail assisted ships packed with frozen seeds, eggs and DNA libraries. Many eons to pass. AIs can be programmed to sprout frozen seedlings and grow frozen sperm and eggs in artificial uterus. Activate the step by step plan when and if AI(s) finds a decent enough new world. Start with plant seeds. They might even tweak the genetics to be more suitable to the new place. Same goes for all the other seeds and animals that will be needed for an ecology. Then finally the human children, when all is ready. Presto Garden of Edan.

    Lots could go wrong, I know. So many of these seed-ships should be sent at once. A portion would succeed. Like dandelion seeds in May.

  • James D. Stilwell November 18, 2013, 17:05

    RE: Paul W:
    Sadly, the dream must begin locked in the gravity well commonly known as earth’s political culture….We can’t even launch Palestine….I put you first, but I’m only number six billion and one….

  • Alex Tolley November 18, 2013, 17:28

    But certainly the idea of probe software being beamed across space is within the realm of speculation.

    This is a relatively straightforward extension of what we do today when we upload software patches to our space probes. A quantitative, not qualitative extension.

    Another wrinkle. Suppose we can create wormholes that have near zero 4D length. I’m not talking about big enough for starships to fly through, but microscopic ones that a signal can traverse. (If the wormhole is uni-directional, then you need 2). With such a device you could eliminate the light barrier. This would allow:

    1. A very low power signal to be used. No massive lasers, just optoelectronic components found in contemporary kit. Which means:
    2. real-time command and control of our robotic probes. Which means that:
    3. a human could experience telepresence on another world. “Walk” on Mars, ‘swim” in Titan’s lakes, “fly” in Jupiter’s clouds. No implausible mind uploading of humans required. Just sufficient bandwidth, of the order of a game console. Which means:
    4. Even if a probe has to carry the terminal ends to the destination, humans could experience being on any world in the galaxy. Any time. For cost of renting the host machine. Our sail ships carry constructors and the wormhole terminal. Which might mean:
    5. No ETC beacons or high power optical/radio pulses. Just aliens on our world using some constructed host body – human/cat/mouse?

    Even a microscopic black hole that was big enough to be usefully stable, might require a pretty massive starship to transport. But once at the destination, no further trips need be made.

  • Paul Gilster November 18, 2013, 18:00

    Adam writes:

    Arthur C Clarke’s original novella, “The Songs of Distant Earth”, came out in 1957.

    Whoops. I put an addendum in the text to point to your comment. Mea culpa.

  • James M. Essig November 18, 2013, 18:39

    Special Relativity is an interstellar pioneers best friend and also permits effectively greatly enhanced forward time travel. Regarding ISRs, a godsend would be the development of an exotic QCD reactor that converts much more than 0.7 percent of the intake mass useful propulsion energy. Not sure how this would work although papers on strange and charmed quark based matter have contemplated exotic hadronic matter having lower quark mass-specific energy states than ordinary atomic nuclei.

  • Christopher Phoenix November 18, 2013, 18:49

    Interesting stuff- Shklovskii and Sagan didn’t mention possibly the first vetted option for interstellar travel, the multigenerational ship, though. I believe Konstantin Tsiolkovsky was one of the first to propose the generation ship, while Robert H. Goddard favored lowering the temperature of his interstellar ark to the background temperature of space in order to preserve the crew. That gives us 3 ways to reach the stars- generation ship, cryopreserved crew (corpsicles), and near-light-speed travel taking advantage of relativistic time dilation.

    Freezing someone into Ice II sounds very intriguing… I can’t help but wonder if the human body can really survive being pressurized to 3,000 atmospheres, but perhaps it is more likely than surviving the lethal action of modern cryoprotectants!

    I’m surprised that suspended animation of this sort hasn’t been researched more thoroughly by starflight researchers (perhaps this is because it is a biological/medical problem, not a physics/engineering problem). Not only is this a potential way to the stars than needs a lot more development before it can be practical, but it can also engage the public, as suspended animation is possibly the most well-known concept for deep space flight after warp drive.

    Think of how much popular media has portrayed suspended animation… 2001: A Space Odyssey, the Planet of the Apes series, Star Trek, the Chronicles of Riddick series, Lexx and more recently Oblivion -freezing astronauts remains one of the most popular and recognizable images of future spaceflight.

    The trouble with relativistic flight, of course, has always been that incredible amounts of energy are required to propel anything to nearly the speed of light. And, at those speeds, even the tiniest bits of debris impact with the force of small bombs, and individual protons are transformed into a intense radiation bath. Couple that with the relativistic rocket equations implications for the mass ratio, and you can see why Edward Purcell believed that travel between star systems is impossible even with matter/antimatter rocket fuel.

    The ramjet promised to change all that by transforming the lethal induced cosmic rays into a fuel source for a fusion engine with its magnetic scoop, thus deftly solving both the fuel problem and the deadly debris problem in one swoop. But it has numerous problems, most notably drag and the difficulty of inducing proton-proton fusion, and also issues relating to the strength of materials and the challenge of igniting fusion in a relativistic mass flow.

    Still , as Bussard said, the possibility of infinite fuel reserves for starships is too good an idea to give up without a fight, so we are not through with Bussard variants just yet!

    By the way, Paul W., cloning Carl Sagan would not have saved us the astronomer… a person’s higher order personality traits are not determined by their genetics. And a clone is really nothing more exotic than an identical twin. A clone of Carl Sagan would be a different person, who would have different formative experiences in childhood, have a different life, and may not be interested in astronomy at all. It is best to turn to a good biology textbook for the definition of “clone” rather than low-rate SF movies!!

  • tesh November 19, 2013, 3:34

    @ Roy E November 18, 2013 at 16:11

    Somehow I “feel” (though not in actuality) you have missed the point. Though I understand that establishing a multi-solar civilisation is the end goal, the how is the important bit. I get the impression that we need to go there ourselves, physically, either over generations or in hibernation. Actual people, alive in the here and now, need to make the journey. Sending a bunch of bots to a distant sun to “build” humans almost seems like missing the point. Sure it will be possible but where is the adventure in that?

  • Mundus Gubernavi November 19, 2013, 5:44

    All matter is basically in a warp drive, since just by existing as matter we are bending and finagling spacetime. It’s really trippy to think about and is also made incredibly mysterious by the diversity of matter extant on Earth alone–as well as our ability to fathom accelerating the warp independently of relativistic means. …Maybe it makes reality more metaphysical than we want to realize (or admit). Or even better, maybe the metaphysical (i.e. our thoughts) are just as physical and real as the systems that allow us to have them.

    It causes me to wonder that maybe there has to be (or had to be) complex material systems (such as life) elsewhere in the universe, albeit now long entered into a sort of hyperspace existence (if they were to survive the death, explosion, or dissipation of their stellar structures). We are still trapped inside of the TV box, while everyone else probably figured out how to use the DVR remote on themselves (or evolved into the DVR remote itself).

    @David Cummings Our knowledge of quantum mechanics has given us a starting foothold in omnipresence, it just seems like we need to figure out how to safely apply it to ourselves. If we can send things now at fiber optic speeds to one another, then I think we are very close to that model (moreso than to what Sagan had in mind). The dream world/imaginative consciousness is proof that its possible.

  • Mundus Gubernavi November 19, 2013, 6:07

    That is, of course, unless there is some type of third party capable of blocking the data being sent, much like a cosmological moderator we don’t know about. Imagine we on Centuari Dreams are our messages and Paul Gilster is that cosmological moderator, with the power to let you (your message) go through, or reject it. lol… like the advanced aliens that had figured out how to access the DVR system can intercept us and pause, rewind, fast forward, or eject us inside of the network, or even worse act as a malware and backdoor us or give our awareness a virus. That would be hilarious and frightening at the same time.

  • Michael November 19, 2013, 7:11

    Perhaps firing a beam of magnetically poled fusion pellets towards the target Star and then lauching a craft after them that will pick these pellets up (magnetically aided) to be fused. The energy to get the fuel pellets moving will have been paid for by the pellet accelerators negating the need for fuel tanks, a sort of hybrid buzzard ramjet.

  • andyet November 19, 2013, 7:15

    There is a third way.

    Maybe we won’t have to make giant leaps to actual stars. Maybe we can take small steps to brown dwarfs between the stars. Who knows, there may be 100s of BDs between the stars for every visible star:


    “The other headline would be the discovery of a brown dwarf that is even closer to Earth than the nearest star, the Alpha Centauri system at 4.3 light-years. Brown dwarfs are objects that form along with stars but do not have enough mass to trigger or sustain nuclear fusion. They are so cool and dim very little is known about their distribution in the galaxy”



    “What if space is littered with these failed stars, scattered between the bright ones like a stellar Polynesia, making interstellar travel a series of short hops, rather than a single gigantic one? What if a simple fusion reactor carried just enough fuel to push a spacecraft to our solar system’s Planet X in reasonable time? What if it could refuel there, harvesting just enough hydrogen or deuterium or helium to limp along to another dark neighbor, and another, and another? Granted, it would take a long, long time to get to Alpha Centauri that way, and probably a much, much longer time to find a planet somewhere that looked even remotely like our rain- and sun-drenched Earth. But given the likelihood of tidally warmed moons, and the obvious possibilities for life there, we may just find that the cold, dark spaces are where most of the action is anyway….Discs around brown dwarfs typically weigh about one-tenth of the mass of the star itself, so in this case it probably contains one or two Jupiter masses of available planet-building material. “I’d speculate that it could build a Saturn, or maybe a few smaller Earth-sized planets,” says Luhman. What is more, these would-be planets could be habitable. The surface temperature of the mini brown dwarf is about 2000°C, which means that any planet 1.5 to 7 million kilometres away could maintain liquid water. The disc probably straddles this range. Luhman hopes to find out whether even smaller objects – perhaps as little as five times the mass of Jupiter – can reign at the centre of nascent planetary systems. “It’s still an open question as to how small you can go, but hopefully we’ll be able to answer that soon.”

    There may be dozens or hundreds of mini-solar systems between Sol and Alpha Centauri. With the discovery of brown dwarfs, free floating planets between the stars, and extrasolar planetoids like Sedna, future space explorers may find plenty to keep them occupied in our own solar neighborhood for centuries to come. While not the galaxy spanning empires and federations of science fiction, it would be enough for our species to explore far into the future without the need for exotic star flight technologies.

    Now if Brown Dwarfs turn out to be scattered by the dozens or hundreds in the space between the stars (and if most of them have mini solar systems capable of supporting life because enough heat is generated by the BD to allow liquid water and photosynthesis based on infrared frequencies), then the old galactic space operas become as obsolete as dinosaurs on Venus.

    Perhaps we’ll find that our kind of life, based on visible light spectrum photosynthesis, is the rare oddity and infrared based life far more common.

    Since these mini solar systems are a stone’s throw away, they can be reached without exotic warp drives or hyperspace. Simple laser sails or nuclear rockets will do just fine. Exploration missions can visit and return in a matter of years, instead of centuries or millennium.

    As for SF story telling, interstellar “empires” and “federations” can be created using slower than light space travel. Maybe Capt. Kirk and Obi Wan Kenobi wouldn’t be impressed, but there is no need to violate the laws of physics to tell a good space adventure. An “interstellar” federation can be created just with the BDs near to Earth, Kuiper belt objects, Oort cloud comets and planetoids, etc. providing more than enough grist for good story telling.

    Only the scale has changed, a BD federation would consider Alpha Centauri to be as far away as Capt. Kirk considered the Andromeda Galaxy.

  • andyet November 19, 2013, 7:19

    Then again, maybe its just a matter of scale.

    Where does a civilization have to be on the Kardeshev scale to allow for fleets of near light speed interstellar manned ships whose cost in energy terms is proportionally equal to the amount of energy currently used by oceanic shipping of a proportional number of people and amount of freight?

    For example, if the current total world energy use is X, what portion of X is currently used to move oceanic shipping? Out of 7 billion people how many are currently at sea at any given time? How many tons of freight are moved each year via oceanic shipping?

    Now convert these people, and proportional tonnages of freight (along with the cost and weight of their life support systems, etc.) into starship equivalents. Assume a standard crew complement of 1000 per starship. So if we can estimate the energy costs to propel such a starship to 0.9C, how high up the K scale do we need to be before a level of interstellar shipping equivalent to current oceanic shipping (as a percentage of the civilization’s overall energy usage) is achieved?

  • Michael Spencer November 19, 2013, 8:18

    I can’t help but think, Paul, that much of the conceptual ‘heavy lifting’ is decades old. Worse, so much of our current forward thinking is reminiscent of buggy whips and automobiles at the turn of the last century.

    Am I pessimistic? A little. On the other hand, I also note the presence of so many unknowns in our current thinking of physics indicates that, at least to me, the eventual answer to the problem of interstellar travel is yet to be teased out of a completely unknown source.

    I have been joyfully plowing through some very old SF books, via audio, and currently on Norton’s ‘Plague Ship’. An alien race has discovered the secret of interstellar travel, but it’s impossible to explain to humans. Norton wonders: could you explain an elevator to a bird? That you can get to the top of a building without flapping your wings?

  • James M. Essig November 19, 2013, 11:21

    Michael, the magnetically poled fusion pellets idea is intriging. Such a mechanism could allow ordinary fusion reaction powered craft to attain highly relativistic velocities with a starting mass-ratio close to one, perhaps equal to one if the craft starts out in our solar system with the pellet drive concept. Fusion applications seem like the way to go being that it is the dominant Standard Model cosmic energy source.

    Insitu fuel sequestration whether of naturally or artificially disposed seems like a key to the cosmos.

    I ran the following numbers on EXCEL for an ideal specific impulse of 0.119 C and the mass ratios look bad for velocities greater than 2/3 C. The pellet stream idea is a much better alternative where high gamma factors are desired.

    Mass Ratio B = v/C Gamma = {1-[(v/C) EXP 2]} EXP (-1/2)
    10 0.267349984 1.037775555
    100 0.499031206 1.153956203
    1000 0.676169385 1.357319523
    10000 0.799068636 1.663229838
    100000 0.878700883 2.094799013
    1000000 0.928036025 2.684632576
    10000000 0.957756616 3.477293107
    100000000 0.975359689 4.53266699
    1000000000 0.985681105 5.930488891

  • David Cummings November 19, 2013, 14:46

    Michael November 19, 2013 at 7:11:
    “Perhaps firing a beam of magnetically poled fusion pellets towards the target Star and then lauching a craft after them that will pick these pellets up (magnetically aided) to be fused.”

    I think that’s a good idea.

    Also mentioned here:

    “Several of the obvious technical difficulties with the Bussard Ramjet can be overcome by prelaunching fuel along the spacecraft’s trajectory using something like a magnetic rail-gun.”

  • David Cummings November 19, 2013, 14:48
  • Michael November 19, 2013, 16:01

    @DavidCummings and @James M. Essig

    It is possible that Plutonium (239) pellets could be used, although they have less energy per kilogram it is easier to make and explode. They can be coated with a superconductor material (coils) that will hold a powerful induced magnetic field allowing the pellets to be propelled from magnetic rail guns ahead of the craft and then caught by the craft as it accelerates using an on board craft magnetic field. Once caught perhaps the superconductor material could be used for shielding as the craft gets faster or be used to increase the grasp area.

  • Alex Tolley November 19, 2013, 16:23

    What if a simple fusion reactor carried just enough fuel to push a spacecraft to our solar system’s Planet X in reasonable time? What if it could refuel there, harvesting just enough hydrogen or deuterium or helium to limp along to another dark neighbor, and another, and another?

    I’m not clear why this makes sense. The ship would need to decelerate to, and then accelerate away from, each brown dwarf. There would be no useful fuel saving from a single journey. I you need a lot of on board power as well, then it might make sense. I’m not clear that such a body would provide useful materials to resupply life support, or shielding.

    “Shore leave” might be somewhat uninspiring. :)

  • Rob Henry November 19, 2013, 21:35

    I have to state the problem here and now. The physical problems of a pressure of 3000 atmospheres should be easily soluble, it’s the new chemical equilibrium here that is the killer. So… the crucial question is how long can mammalian tissue exist at this new pressure before toxic effects are experienced? That would tell how long we have to freeze and thaw a body that is innovated by an intricate lattice of cryogenic fluid.

    To the best of my knowledge, no one has ever bothered to do those simple tests on a living creature. Thus we can still hope that the problem turns out to be trivial!

  • Adam November 20, 2013, 3:57

    andyet, you’re a bit behind the times with your brown dwarf numbers. Current estimates – based on WISE and other sources – tells us that they’re only about 1/5-1/6th as numerous as Main Sequence stars. They’re not filling the void between the stars in any meaningful way. The odds of even “Rogue Jupiters” between the stars are looking pretty poor too.

    At the other end of the interstellar mass-scale, however, there might be ~100,000 Pluto-mass objects per star and maybe ~1,000 Earth-sized objects. If we could figure out how to “pipe sunlight” directly to them, then there’s more than enough sunlight from Sun-like stars to warm them to Earth-like levels. Pluto-mass objects would need World-Shells to stop their air from leaking away, but that’s not an onerous task after piping sunlight to them.

  • stephen November 20, 2013, 11:00

    When we’re piping sunlight, using solar sails with lasers, etc., what happens when something gets in the path of the beam, either accidently or on purpose? Won’t that be a problem? Won’t we need to have patrols all along the path of the beam to take care of any debris or other vessels getting in the way?

  • James M. Essig November 20, 2013, 12:37

    Michael, regarding Plutonium 239 pellets, that is an interesting idea. Another idea would utilize U-238 pellets. The pellets might be captured and injected with anti-protons as they pass through a reactor tube. It has been suggested for relativistic rockets that Project Orion style pulse drives might utilize U-238 pellets containing anti-proton cages. The cages would be nano-tech controlled and would be opened when desired to cause atomic nuclei fissioning to an extent commensruate with a super-critical mass. A blend of U-235 and U-238 may be used instead.

    Another option is thorium pellets, all else being simillar or the same. It is estimated that there is about 10 EXP 14.5 or more metric tons of thorium in the Earth’s crust. Obtaining it from granite could be a little problematic because of its low concentration. However, if renewable energy is used to run the extraction process, then it would not matter whether or not it takes more eenergy to extract the thorium that the yield of the thorium itself.

    Uranium has also been discovered on the Moon, so I would not be surprised if it is also plentiful on Mars, Venus, and Mercury. If uranium, why not thorium there as well.

    A small core of fusion derived neutrons might also be useful. During the hieght of the Cold War, 3 stage nuclear weapons designs existed for which one metric ton or more of U-238 would jacket a two-stage or fission-fusion device. These devices had a staggering yield of about 25 megatons. The fusion derived neutrons would result in the fissioning of the U-238 jacket. The original Tsar Bomba tested at 58 megatons was considered for such jacketing which would have brought its yield up to as high as 100 megatons, perhaps even 150 megatons.

    I like the fission pellet concept in general. For ISRs, it is one thing we should consider in pellet runways.

  • Michael November 20, 2013, 15:13

    @James M. Essig

    I was thinking along the lines of the collected Plutonium 239 pellets been in the form of implosion ready (hollow spheres or made later) that are laser imploded and showered with neutrons from an external neutron generator at the critical time of maximum density.

    The spheres could also be electrostatically charged (negative) to hold cold antiprotons. The antimatter can be injected into the charged sphere just before use, deforming or plugging the channel after injection and maintaining the charge is critical to safe operation though. Lasers could then start the implosion at the same time that the charge is removed/reversed so that at highest density the compressed device explodes as the material interacts with the antimatter. Lining the inner surface of the sphere with material that releases an excess of neutrons when irradiated with gamma rays could aid the explosion.

    Now with the pellets been fired from the mag gun they will have a tendency to scatter, we could perhaps build a microprocessor into them that could use energy from the magnetic field that was induced into them to make decisions. For instance as the space craft approaches the line of pellets the pellet computers calculates when to release a gas (propellant) to push it into the path of the catcher for example.

    As with complex ideas the devil is in the detail


  • James M. Essig November 20, 2013, 16:57


    Interesting concepts indeed.

  • Brett Bellmore November 21, 2013, 7:33

    “Obtaining it from granite could be a little problematic because of its low concentration. ”

    The “low” concentration is, IIRC, plenty high enough to make extracting Thorium from granite very feasible from a EROI standpoint, though not economically feasible until more concentrated sources, (Such as coal flyash!) have been exhausted.

    “Island hopping” to the stars does seem to me the most practical way to go. But it only makes sense if you’re stopping at each body to colonize, and your descendants are the ones going on to the next island. It makes no sense to spend fuel stopping just so that you can get fuel to accelerate again.

    Though with the smaller “islands”, you actually have the option of taking them along. Set down on a comet, mine it for heavier elements, and ‘burn’ the hydrogen over a very long time for energy and to speed you towards your destination. You ought to be able to arrive at the next star in due time, having converted a comet into a habitat, and saved enough of the good stuff for a burn at the end to come to a stop in the new system.

    It would work much better if we can solve the problem of proton-proton fusion, but even that is far more likely if you’re starting with condensed matter, instead of trying to collect interstellar gas.

  • Eniac November 21, 2013, 21:13

    If you really want to get to relativistic velocities with pellets, you might as well make them from rock or lead or water: At relativistic velocity, their kinetic energy will dwarf any nuclear energy they may hold. This makes extraction of nuclear energy an unnecessary complication.

    The trick is accelerating them in the first place. There is no known way to do this.

  • Michael November 22, 2013, 14:37


    ‘The trick is accelerating them in the first place. There is no known way to do this.’

    Acceleration of a pellet surround by superconducting coils is quite possible, magnetic induction is easily done, materials such as plutonium are quite able to survive very high accelerations before deforming and plutonium is also a super conductor at very low temperatures (but not a very good one though).
    With the pellets the energy of moving the fuel is paid for efficiently and the mass of the craft is reduced by a large margin, little or no storage tank mass.

    A concept I also looked into was to use pellets made from a subliminal gas which would impact a sail type craft and push it along, it would use a microcomputer on each pellet to calculate when the craft was in range and eject some gas to guide it onto the crafts surface in effect pushing it along like the sailing ships of old.

  • Eniac November 22, 2013, 21:02

    @Michael: All of these methods you mention have great difficulties reaching even orbital velocity, today. There are fundamental reasons why they may never go much beyond that. Put simply, it gets very hard to accelerate something with stationary equipment that is already whizzing past in microseconds. There are limits to how fast you can change electromagnetic fields.

    On top of that, when you calculate what barrel lengths and accelerations would be needed to approach light speed, the results are quite mind-boggling. We have discussed this a lot in these pages, a search for “accelerator” or similar will bring up some lively discussion.

    It seems you do realize that any such pellets would have to have guidance systems and propulsion, otherwise they could never be targeted with sufficient accuracy. They would not need to be plutonium, because their kinetic energy far outmatches what they could yield as fuel, even if you could catch them and put them in a reactor, somehow.

  • Alex Tolley November 22, 2013, 22:50

    @Michael – to accelerate pellets to 0.1 c, at a “modest” 10,000 g would take 5 minutes with an acceleration distance of 4.5 million km. Maybe the pellet can be kept in a circular track with a much smaller circumference to make this feasible. Have you worked out the likely forces on, and energy requirement of such a device? Would the ship use magnetic forces for gas/particle capture, rather than a physical surface?

    One wrinkle that might be useful is to send pellets ahead at a much slower pace than the ship, so that it could decelerate at the target star. If the ship could swing around the star, then fast pellets could also decelerate the ship.

  • Dmitri November 23, 2013, 17:35

    “The original Tsar Bomba tested at 58 megatons was considered for such jacketing which would have brought its yield up to as high as 100 megatons, perhaps even 150 megatons.”

    Sakharov proposed during design phase of building an H-bomb around 100 Mt as a joke because no one had knowledge how to build a 50 Mt one. From that on they came up the layered H-bomb design. 150 Mt yield was another jokingly proposed milestone but it was not more than a thought. When they made calculation for 100 Mt yield they backed off. That would have been capable severely damaging the planet’s atmosphere. Even 50 Mt was a though goal and they didn’t know what the excact yield would be. Initial was around 45 with possible goal 50 but turned out to be 59. Indeed the design was made in such a way that the yield of a bomb could be scaled up but it’s unknown how complicated a 150 Mt bomb would have been. Tsar Bomba was the first compact H-bomb with such yield and already weighing 27metric tonnes.

  • Eniac November 23, 2013, 18:30

    Alex: Keeping a pellet on a circular path is not much easier than accelerating it, and the radial acceleration vs. radius trade-off is no better than that for the barrel length of a straight path.

    It is not possible to swing around a star gravitationally before decelerating, stellar escape velocity is far too slow. Unless, that is, there is a black hole or neutron star nearby.

  • Alex Tolley November 23, 2013, 21:33

    “It is not possible to swing around a star gravitationally before decelerating, stellar escape velocity is far too slow.”

    Which is why I said:

    “send pellets ahead at a much slower pace than the ship, so that it could decelerate at the target star.”

    IOW, rather than trying to just use the local materials to decelerate, use pellets sent on ahead to transfer momentum. With enough deceleration, ship should be able to make a tight parabolic orbit and then use more pellets in the system to fully decelerate into the star’s orbit.

    I’m literally brainstorming to see if it is possible, rather than if the idea makes sense. The problem of launching pellets is high, as is momentum transfer. Pellets that miss the ship would be relativistic and potentially dangerous. Since the ship only needs a 1.7 tons of fuel per ton of structure if the exhaust velocity is to be the same as the peak ship velocity, it seems to me that the cost of having the fuel on board is not such a disadvantage to the pellet approach.

  • Rob Henry November 24, 2013, 21:03

    Alex Tolley’s ideas give me this fantastic image of what a real response of an ETI to a METI message would be. Rather than the two usual options depicted, ie. advanced aliens caring nothing for themselves and only trying to help us, or deliberately destroying us, a more practical response seems more likely, where they send a stream of relativistic pucks aimed at our inner system to help their starship decelerate, at a destination that has become of interest! … Sorry, I couldn’t help but share that.

  • Michael November 25, 2013, 14:01


    ‘Put simply, it gets very hard to accelerate something with stationary equipment that is already whizzing past in microseconds.’

    Agreed, keeping the barrel aligned will be a real challenge but it can be bent by a fair margin as it is quite thin and flexible which could be an advantage for pointing. Handling the forces that could deform the barrel will be an issue requiring active thrusters to keep it aligned.

    ‘There are limits to how fast you can change electromagnetic fields.’

    A rail gun could use superconducting transistors to switch in energy transmitted through two superconducting rails that run along the length of the barrel. A laser that shines alongside the barrel is used to time each switch. Protecting the electronics from such a hard magnetic impulse needs some serious thought.

    ‘On top of that, when you calculate what barrel lengths and accelerations would be needed to approach light speed, the results are quite mind-boggling.’’to accelerate pellets to 0.1 c, at a “modest” 10,000 g would take 5 minutes with an acceleration distance of 4.5 million km.

    With a 3 million G force and a 30 000kms final velocity (last pellet) from a standing stop would require a 15 000 km barrel and the pellet will complete the journey in 1 second. The barrel length is inversely proportional to the G force so the higher the acceleration the smaller the length of barrel that will be required. Light speed or approaching it would rightly require enormous amount of energy, well beyond what we can effectively focus at the moment by a large margin.

    ‘It seems you do realize that any such pellets would have to have guidance systems and propulsion, otherwise they could never be targeted with sufficient accuracy. They would not need to be plutonium, because their kinetic energy far outmatches what they could yield as fuel, even if you could catch them and put them in a reactor, somehow.’

    With the sail concept a solid pellet that sublimes into a gas or a propellant in a nozzle shaped container (ejected clear) could be used to change the direction of the gas cloud so that it hits the sail to compensate for inaccuracies, the chemical thrust or explosive molecules could also aid propulsion. A nano-computer will be used to calculate when to change the orientation of the pellet aided by an on-board laser which could also aid sublimation of the pellet when in range. The plutonium pellet idea could also use a propellant to guide it into the path of the oncoming craft to be used as fuel by a fission implosion drive, been magnetised they would be easier to manipulate due to the low relative velocity between the craft and pellet, timing is everything.

    @Alex Tolley

    ‘Maybe the pellet can be kept in a circular track with a much smaller circumference to make this feasible.

    Centrifugal forces would be very large indeed at high velocities. I have not calculated this if for instance it was to be wrapped around the moons equator or simply in space.

    ‘Have you worked out the likely forces on, and energy requirement of such a device? ‘

    The energy required depends on the probe mass primarily and efficiencies, the machine would be using tens of terawatts of power at its peak. The nice thing about the idea it is that it is scalable and the machine could be used to propel other craft around the solar system when not been used on interstellar missions, maybe even asteroid impact deflection.

    ‘Would the ship use magnetic forces for gas/particle capture, rather than a physical surface?’

    The sail concept would use direct impact by an electrically neutral gas (limited by material design strengths) in effect the impacting gas would hit the surface and be deflected off to the sides. I have thought about adding other elements (molecules) to the pellet that could be used to build the craft catchment area on route using micro-machines. The plutonium/nuclear option could use gas propellant and/or magnetic attraction to catch the pellets as there will be little relative velocity difference between the craft and pellet if timed correctly.

    P.S Just ideas, I would need to take a closer look at the finer details of the concepts to see if they have any merits

  • Eniac November 27, 2013, 0:24


    Agreed, keeping the barrel aligned will be a real challenge but it can be bent by a fair margin as it is quite thin and flexible which could be an advantage for pointing.

    Bending the path of the pellet is, as we discussed, associated with huge accelerations and would destroy the barrel, especially if it was thin. Also, how thin could a barrel be that needs to have superstrong electromagnets (or whatever) along its entire length to keep up the acceleration of enormous magnitude you are contemplating?

    You seem to correctly understand the troubles that railgun designers go through to even reach velocities of a few km/s. Can you do better by 4 orders of magnitude in speed? 8 orders in power? I think the possibilities here are incredibly remote, if any.

  • Eniac November 27, 2013, 0:33

    @Alex Tolley:

    use pellets sent on ahead to transfer momentum. With enough deceleration, ship should be able to make a tight parabolic orbit and then use more pellets in the system to fully decelerate into the star’s orbit.

    I see two major problems here:

    1) The pellets that are sent ahead have to be slower than the ship. So, those pellets that are slow enough to help brake to a velocity allowing a parabolic orbit (let’s say 300 km/s) would have to have been launched to Alpha Centauri 4000 years ago.

    2) Once you have managed to go from relativistic to escape, the remaining deceleration is trivially small in comparison and it hardly matters how it is achieved.

  • David Hoskins November 28, 2013, 14:24

    What about the crew on our hypothetical ship? How much shielding and of what type would they need to be kept safe from first the reaction mass accelerating this thing, and secondly from cosmic rays hits when traveling at a good portion of c?

    I am assuming that this is a manned mission, however shielding would still be required to protect the electronics on a robotic mission albeit significantly less.

  • Dmitri December 3, 2013, 9:25

    I stray away for a moment from the interesting discussion here. I’ve noticed a detail what started seriously bother and it’s not something have always followed but just with this article this especially emerged and all the gleaned knowledges from previous related Centaury Dreams articles suddenly coming together. I’m talking about Sagan and Shklovskii collaboration. It seems oddly peculiar and when you research it becomes same oddly ordinary and clearly predictable as a hindsight. And it’s both at the same time raising question is this destiny as we have know it over the course of history?

    If you look at the context of the era when Sagan and Shklovskii met, it does not make sense. They are well renowned and expelled persons from the both side of the stand off line, yet they seem to collaborate seemingly naturally.

    Shklovskii books represented to events of the time:
    *) Cosmic Radio Waves (1960) – Francis Gary Powers shot down with U2 over the USSR / The iconic picture of Che Guevara is taken
    *) Universe, Life, Intelligence (1962) – The Cuban Missle Crisis.
    *) Physics of the Solar Corona (1965) – A year back Khrushchev was replaced by Leonid Brezh­nev./ First US combat troops arrive in South Vietnam.
    *) Intelligent Life in the Universe (1966, with Carl Sagan) – North Vietnam declares general mobilization. / Warsaw Pact summit ends with a promise to support North Vietnam.
    *) Supernovae (1968) – Soviets and Warsaw Pact countries invade Czecho­slo­va­kia.
    *) Stars: Their Birth, Life, Death (1978) – One year to Soviet Afghanistan invasion. / The first Unabomber attack.
    *) Five Billion Vodka Bottles to the Moon: Tales of a Soviet Scientist (1991, memoir) – 6 years from death of Shklovskii / The coup attempt in Moscow and subsequent independence of the Baltic republics and dissolvement of the USSR / 5 years to death of Carl Sagan.

    If you delve deeper into Shklovksii and Sagan’s lineage then surprising details start to emerge. Sagan’s mother’s side grandfather came to the US in 1904, bringing her wife w/ him the next year. Sagan’s mother and her sister both were born in the States. Sagan’s mother’s father was from Zolochiv, near Lviv, area now know as South-West of Ukraine. At the time it was part of Austro-Hungarian Empire district of Galicia. Due to Carl’s mother’s mother early death at 35-years, her father sent her back to relatives in Austro-Hingaria. At age of 4 due to complications in Europe and raise of anti semitic movement, especially in Austro-Hungarian Empire, her father brought her back to the States.

    Carl’s father in other hand came to the States in 1910 at age 10 by invitation of uncle who made the journey earlier. He was from Kamianets-Podilskyi, near Chernivtsi, which is mere 230 to SE from Lviv. It might look arbitrary to connect places of origin as connecting thing between Sagan’s parents but it’s definitely a strange factor and definitely a driver in their successful 46-years marriage as Carl Sagan itself has described in his biography that his parents, especially mother, never severed their ancestry and Carl was very aware of his Ukrainian Jews origin and relatives in Ukraine. In biography this is referred as “in Europe” which is so vague argument that it almost has lost its importance. As mother’s side grandfather Carl’s both parents were Ukrainian Jews. Yet they have very clear American upbringing, which definitely was the decisive factor for them when they met rather than the place of origin. Of course as an emigrants in other country finding a countryman who happens to be next to your neighborhood makes that magical factor X on the relation.

    To put Carl Sagan’s parents’ places of origin into perspective from modern day Ukraine to US it’s like being origin from California, Santa Clara, from father side and having a young man from Madera, California.

    Iosif Shkvolskii on the other hand was born in Hlukhiv, NE of modern Ukraine, which is respectively Wisconsin in US. He was born 18 years prior Carl – 1916 and 1934. If Carl parents were fortunate to not see the Red Revolution, WWI, and WWII, and Carl fortunate to miss Great Depression then Shklovskii was born right before the big thing started to happen in Russina Empire, especially in Ukraine, but Iosif *was* very fortunate to miss Holdomor (man made famine in Ukraine in 1932-1933), which actually was so horrific that the siege of Leningrad looks compared to that like child’s play.

    I don’t know how Shklovskii and Sagan came to know each other or how they communicated. Probably they exchanged (snail) mails. Shklovskii was an outcast in the Soviet Academy of Science due to his Jewish origin but especially because he was a strong opponent for prosecution of Jews in getting higher education, on making careers, and defender of Sakharov *at that time*. Travelling abroad for a soviet person was something unheard of. It was allowed only for privileged ones. It only could have happened if KGB gave a go-ahead and instructed you properly. Shklovskii was actually prohibited to travel on many occasions.

    I was suddenly perplexed by the way how this article connected and glided over Sagan-Shklovskii collaboration as for me the many things in past which have suspended somewhere in head suddenly started to recalling itselfes how it could have happened and why *this* collaboration. There is not much information how Sagan-Shklovskii met and how the idea of Sagan publishing Shklovskii’s book in US was communicated but I’m pretty convinced that the Ukrainian Jewish origin and that 2 man from both side of Iron Curtain actually shared similar faith in sciences and by peers, connected on some mysterious ways instantly. As far as I understand Sagan was not well greeted in the States by peers and he was not warmly received in American Academy of Science. You can’t put equation sign between Shklovskii and Sagan but definitely they both share similar faith and interest. I actually find this detail and connection intriguing as in past people would have called it faith, destiny or by any other divine name.

    I some how came to understanding that they were set to meet and make what they made, especially when you lay out what actually connects them.

    There are many theories how Carl got his name but one of them stands particularly out. As Carl’s mother returned to US and her father remarried she did not got well with her stepmother. She wanted to name her first child by name of her mother to honor her. To Americans she is known Clara Gruber. Clara is her Americanized name. Her actual name was Chaiya. Peculiar name for an Ukrainian, but maybe not for Ukrainian Jew. Same name Americanization happened to Carl’s mother’s grandfather Leib Gruber -> Louis Gruber, and father Samuel Sagan -> Sam Sagan.

    The Carl’s name origin from grandmother have been stated in several sources, so this Ukrainian (Jewish) connection is not an arbitrary thing and should not be dismissed in light of other coincidences (facts / circumstances) . If they were coincidences.

    How cool is that? :)


  • ljk January 2, 2014, 11:48

    Triple-threat method sparks hope for fusion

    The secrets to its success are lasers, magnets and a big pinch.

    W Wayt Gibbs

    30 December 2013

    The Z machine at Sandia National Laboratories in New Mexico discharges the most intense pulses of electrical current on Earth. Millions of amperes can be sent towards a metallic cylinder the size of a pencil eraser, inducing a magnetic field that creates a force — called a Z pinch — that crushes the cylinder in a fraction of a second.

    Since 2012, scientists have used the Z pinch to implode cylinders filled with hydrogen isotopes in the hope of achieving the extreme temperatures and pressures needed for energy-generating nuclear fusion. Despite their efforts, they have never succeeded in reaching ignition — the point at which the energy gained from fusion is greater than the energy put in.

    But after tacking on two more components, physicists think they are at last on the right path. Researchers working on Sandia’s Magnetized Liner Inertial Fusion (MagLIF) experiment added a secondary magnetic field to thermally insulate the hydrogen fuel, and a laser to preheat it (see ‘Feeling the pinch’). In late November, they tested the system for the first time, using 16 million amperes of current, a 10-tesla magnetic field and 2 kilojoules of energy from a green laser.

    “We were excited by the results,” says Mark Herrmann, director of the Z machine and the pulsed-power science centre at Sandia. “We look at it as confirmation that it is working like we think it should.”

    The experiment yielded about 1010 high-energy neutrons, a measure of the number of fusion reactions achieved. This is a record for MagLIF, although it stillfalls well short of ignition. Nevertheless, the test demonstrates the appeal of such pulsed-power approaches to fusion.

    “A substantial gain is more likely to be achieved at an early date with pulsed power,” says nuclear physicist David Hammer of Cornell University in Ithaca, New York, who co-wrote a 2013 US National Research Council assessment of approaches to fusion energy.

    Full article here: