Built on Facts

VERY Long Distance Rocketry

I don’t know if you’ve been reading Photo Synthesis, but if you haven’t you’re missing a real treat. Currently there’s some incredible photography and video of amateur rocketry at fairly large scales. Amazing stuff. A hundred years ago the idea that an amateur of comparatively middle-class means (well, after a lot of saving anyway) could send rockets to the edge of space would have been laughable.

But we’ve got a long way to go as well. Not long ago Gene Expression commented on a Seed Magazine piece on planetary astronomy (do you subscribe?) and noted that the problem of interstellar exploration is pretty far out of reach. Of course the goal of exploring the galaxy has been around as long as we’ve known that there were galaxies. Now however we’re on the verge of astronomical instruments that might be able to tell us something meaningful about potential earthlike planets elsewhere in the nearby interstellar universe:

“If planets are found around Alpha Centauri, it’s very clear to me what will happen,” Marcy said. “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital camera, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”

So let’s say it happened. Alpha Centauri is about 4 light years away, so at a tenth the speed of light you could reasonably expect to get a signal back in your lifetime if your life expectancy has still got around 50 years on it. That’s still something of a sketchy proposition for a lot of scientists, but there’s plenty of scientists working diligently on proposals for all kinds of projects they’ll certainly never live to see. Especially those poor high-energy guys looking to build the post-LHC collider.

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Second star to the right, straight on ’till morning.

But could it be done at all? The top speed of a rocket in a vacuum is directly proportional to the velocity at which its propellant is being blasted out of the engine. A good rule of thumb is that your rocket can reach twice the speed of its propellant*. A liquid fuel rocket spits out its blast of fire at around 4-5 km/s, which leads to a pathetic 0.003% of the speed of light. So much for NASA’s normal methods. Ion engines have been tested with real success and generally end up around 30-60 km/s. This is maybe 0.04% of the speed of light. Still way too slow. VASMIR is another potential kind of ion engine, though not one that has yet seen deployment. It claims theoretical jet maximums of around 300 km/s. And that’s actually a solid 0.2% of the speed of light for the speed of the spacecraft. But incredible though that is, it’s still not 10%. A travel time of 40 years is long; a travel time of 2000 years is just depressing.

Can we do better? Probably, but it’s not likely to be popular. Get a spacecraft, fill it up with nukes, throw the nukes out the back one at a time, and let the blasts propel you forward. Crazy as it sounds, it’s been extensively studied and is not at all beyond the reach of today’s technology. It probably wasn’t beyond the reach of 1970s technology – overall it’s a very simple system. Theoretically you could probably squeeze close to 1000 km/s out of a rocket thus propelled. Still just 0.6% of the speed of light.

In terms of current feasibility that’s probably about it. There are some undeveloped technologies that can in theory do better. The fission-fragment rocket uses nuclear energy by directing the actual fissioning nuclei out as the rocket exhaust. Since no explosions are involved it wouldn’t be as controversial and theoretically it can give exhaust speeds of up to 5% of the speed of light. And that’s exactly where we need it to be to get 10% for our spacecraft. Fusion engines could do even better, but at that point we’re straining the bounds of what NASA may plausibly be able to do in the real world.

Further thoughts? My personal opinion is that it would certainly be worth spending a large fraction of the space exploration budget on such a project if a potentially life-supporting planet were found around Alpha Centauri, which I’m certainly not holding my breath over. Even if it were, we can only do what we have the technology to do. But it would certainly be worth a shot.

*Formally it can reach the speed of its propellant times the natural logarithm of the ratio of fuel mass to non-fuel mass. The 2x number assumes a mass ratio of a little over 7, which is reasonable.

Comments

  1. #1 D. C. Sessions
    June 5, 2009

    Those propulsion schemes are high specific impulse, but low thrust. At some point the losses to interstellar media become high enough to limit peak velocity, and you’re heading into Bussard ramjet territory.

    Which you’re going to have to confront anyway. The energy of an impact with interstellar gas at 60 km/s is bad enough that spending years under that kind of bombardment is prohibitive for organic systems; cranking up an order of magnitude is going to play Hobb with the electronics too.

    IMHO our best bet for interstellar colonization (not exploration, note) will be seedships at relatively modest speeds in the 30-300 km/s range loaded with massively redundant self-repairing AI systems which also carry biological genome recordings and equipment capable of building more elaborate facilities on arrival (self-replicating, of course.)

    So what if one takes millennia to arrive? What counts is getting there in reasonable condition.

  2. #2 Austin
    June 5, 2009

    I really think this is one of, if not (the) most important missions NASA, (only because it has the resources) should seriously focus a concentrated effort into. I know its extremely premature, we don’t have the tech, experience, the political will, etc. But we’re close to 7 billion people, and people, not unlike atoms tend to get reactions as we get squeezed close together. Aiming for the stars is the only option for our continued existence. Simple logic: All of mankind is on one planet, we must expand to avoid extinction. But knowing us, American Idol is much more important than serious discussions on the future of mankind.

  3. #3 D. C. Sessions
    June 5, 2009

    But we’re close to 7 billion people, and people, not unlike atoms tend to get reactions as we get squeezed close together. Aiming for the stars is the only option for our continued existence.

    Short of Doc Smith-grade magic technology, the number of people we could remove from Earth to some other planet is simply too small to affect population dynamics.

    Space colonization may well be worthwhile (hey, I grew up on science fiction and the Mercury program!) but not as a population-pressure relief valve.

  4. #4 Brando
    June 5, 2009

    Propulsion technology should be a huge focus, in my opinion, as it’s one of our biggest limitations in space exploration.

  5. #5 Matt Springer
    June 5, 2009

    I’m not too worried about population. Material prosperity and technological advance slash growth rates all by themselves. Much of Europe and Russia is or is about to experience cratering populations. Japan’s population is falling at an amazing speed. The US is just at breakeven, and China’s increasing population is almost entirely driven by longer life expectancy – their fertility is plummeting as well. India probably won’t be far behind.

    Africa and the Middle East are growing very quickly and this will probably cause serious problems in those regions. But what else is new?

  6. #6 llewelly
    June 5, 2009

    Laser Sail.

    Why carry your fool, er, fuel with you?

  7. #7 Ray Ingles
    June 5, 2009

    I’m partial to the Nuclear Salt-Water Rocket, just for the name alone.

  8. #8 Jim C
    June 5, 2009

    We do need to expand into space and eventually into other solar systems due to population, not because of over population, but because of under population. A larger impactor, a large volcano (Yellow Stone), even a simple virus could reduce our population by a large fraction tomorrow, over even totally eliminate us. It certainly could destroy civilization as we know it. We need to move our eggs out the the single basket they are in.

  9. #9 Art
    June 5, 2009

    Lets, for the sake of argument, assume we wake up tomorrow and find that astronomers have spotted a spacecraft of unknown origin coming at us at 10% of the speed of light. It seems to be driven by a continuous series of explosions of some type of nuclear bomb. It arrives in our solar system and homes in on earth. Then, by design or mishap, it flies toward and ultimately into our planet at roughly 10% of the speed of light scattering several thousand nuclear bombs across the globe.

    Do we request the phone number of the local agency in charge of interstellar environmental cleanup? Do we send a spacecraft full of lawyers back at them to sue for reparations? Would that be considered an act of war? Do we declare war?

    A spacecraft full of bombs showing up unannounced seems like a rather poor way of starting a relationship. Unless your a Vogon. In which case it is pretty much expected and something of a relief that they don’t arrive and start reciting Vogon poetry.

  10. #10 D. C. Sessions
    June 5, 2009

    Then, by design or mishap, it flies toward and ultimately into our planet at roughly 10% of the speed of light scattering several thousand nuclear bombs across the globe.

    Never mind the bombs — anything much bigger than a breadbox arriving at 0.1 c isn’t going to leave the planet interested in much outside the atmosphere for a loooong time.

  11. #11 Uncle Al
    June 5, 2009

    Artifact transportation is limited by the energy it contains and the momentum (mass) it can throw. Even if you luck into a series of planet-grazing gravitational slingshots, erosion by the interstellar medium will eat you and (blue-shifted) radiation will chew on your electronics. Matter-antimatter annihalation is at best HALF as good as advertised: 50% of hadron-antihadron annihalation energy will be carried off by neutrinos.

    We are stuck here good and hard unless somebody invents some wildy different physics. The universe voices strong opinions about causality in counterpoint. One can look and presumably talk (continuous streaming conversation in both directions). Travel is not on the menu – not even if you have the stones,

    http://www.usuhs.mil/pat/deinococcus/index_20.htm

  12. #12 KoKo
    June 6, 2009

    Is there an upper speed limit for light sails? They have the advantage of providing a zero-cost way of slowing down the vehicle once it gets to Alpha Centauri, so it doesn’t scream through the system at 10% lightspeed, frantically snapping pictures.

    Or could a lightsail be used as a launch booster, just at the outset? We could use lasers to help it along. Then, when the distance becomes too great for the sun and our lasers to help much, you could jettison the sail to reduce drag. When it arrives, it could deploy a new sail for braking and steering.

  13. #13 travc
    June 6, 2009

    Propulsion research is a akin to “basic science” in a lot of ways. We have ideas, but always seem to end up discovering much more on our way to testing those initial ideas than is expected. Actually trying to build things we don’t quite know how to build ends up doing that… which is good.

    There are two big propulsion challenges I see…

    The one you point out is “how to get a probe going very very fast”. That is a good question worth answering. It seems a multi-modal system for different velocity regimes is going to be most effective. Chemical to ion to some sort of photonic drive?

    A more immediate challenge is just getting mass into orbit much more efficiently. It isn’t as sexy, but until we get a cheaper way to boost payloads actually trying the more far-out-there long range propulsion ideas is going to be very slow. As any half-decent engineer knows, development goes a hell of a lot faster once you actually start making prototypes.

  14. #14 Wilford M. S.
    June 6, 2009

    Space colonization is an american fantasy that rationalizes the destruction of this planet. It’s the LAW of PHYSICS that says we are not going anywhere of this planet. So lets get that straight.

  15. #15 Tim Gaede
    June 7, 2009

    I am a little pessimistic about the prospects of crewed interstellar missions. Practical power limitations could severely limit the thrust of extreme high speed exhaust propulsion systems. On top of that, there is the issue of cosmic rays (mostly high speed protons) which, unless averted, would mean a persistent radioactive environment for the occupants. The issue of cosmic rays may be as important as propulsion.

    There are a couple of ideas which may divert cosmic rays away from an interstellar–or interplanetary ship. One would involve maintaining a very high positive charge on the vessel and possibly use a ring of material around the ship as a sink for electrons. The other would use a magnetic field to steer the protons. Perhaps a scheme in which a central solenoid-like magnet provides the field necessary for manipulation of a plasma fuel and exhaust which also shields against cosmic rays could work. The crew modules would be aligned longitudinally around this center solenoid. The effects of an intense magnetic field on humans is not completely known so an outer solenoid could partially cancel the magnetic field for the crew as well as contribute to the magnetic field inside the center solenoid. To complete magnetic shielding for the crew, the modules may need to be coated with a material with high magnetic permeability.

    The only practical idea for colonizing space may be to send vitrified (“frozen”) embryos. The embryos would probably be less susceptible to cosmic rays and could survive a journey lasting centuries. Upon arrival at the destination, an embryo would be grown in artificial uterus. Raising the colonist after that presents some technological and ethical issues.

    Unfortunately, Alpha Centauri may be an unlikely candidate for colonization. Although the results of one study on the formation of terrestrial planets in that system was promising, follow-up studies have been far more pessimistic and suggest that the two primary stars needed to be separated by more than double their distance.

  16. #16 Tim Gaede
    June 7, 2009

    This discussion of Alpha Centauri had me wonder how astronomy and the space programs on Earth would have progressed if the Sun had a twin companion star with an Earth-like planet.

    For our inner solar system to remain nearly the same, the periapsis of the stars may need to be 50 AU distant (Neptune is 30 AU away) and for a typical binary orbital eccentricity of 0.6, the apoapsis would be 200 AU. This would mean an orbital period of one thousand years. I can imagine ancient astronomers in this alternate-reality noting this magnitude -15 star (ten times brighter than the full moon) creep along the ecliptic at a rate of one degree every nine years. Its apparent brightness and apparent speed would gradually increase by a factor of 16 as it reaches the opposite side of the ecliptic. Our moon would take over a minute to occult the star at periapsis and one quarter this time at apoapsis. Would Kepler’s Laws be inferred at an earlier point in history? Early telescopes could have picked up terrestrial planets around the companion. Later, an Earth-twin’s moon would have allowed astronomers to pin down its mass. We would already have second generation optical space telescopes imaging continents. With the Earth-like planet as the ultimate goal, we may have walked on Mars within thirty years of the moon. Nuclear propulsion technology development would have received priority from congress and we perhaps would have already sent an armada of probes for flybys and landings.

  17. #17 Kim
    June 7, 2009

    Looks like you had better go non Newtonian .

  18. #18 Jim Thomerson
    June 7, 2009

    I think it is clear that we cannot go to the stars with physics as we know it today. We have had physics for a relatively short time, since Newton. Who knows what physics we will have in another hundred or thousand years?

    The two non-FTL methods that I recall from science fiction are the generation ship, which carries a self-sustaining population for perhaps tens or hundreds of generations; and the deep-sleep ship where the colonists are kept in suspended animation for many centuries.

    There was a story about the first sublight ship, a deep-sleep ship, sent to Alpha Centuri. It is intercepted by FTL ships from the long-established Alpha Centuri Colony. The astronauts are received and feted as heros. However, they come to realize that only a few scholars have any familiarity with the English language, and that the colonists think they have a really vile body odor.

  19. #19 Tim Gaede
    June 8, 2009

    I just realized that the moon would only occult (eclipse) the companion star when that star is very near one of two positions in its orbit…unless the plane of the orbit of the companion star very nearly coincides with the plane of the moon’s orbit (which is 5 degrees off the ecliptic). So there would be few opportunities to infer the angular size of the star by that method.

    Anyway, I was just wondering how the presence of a companion star would influence a civilization’s views on astronomy and space exploration.

  20. #20 Tim Gaede
    June 8, 2009

    Damn! I just found out that the moon’s period of precession of nodes is only 18.6 years. I wonder if this need be significantly different in the binary scenario. Anyway, if the period isn’t too long the moon COULD occult anywhere in the companion’s orbit.

    Please disregard these last two posts.

  21. #21 Paul Murray
    June 9, 2009

    Ringworld is perhaps more feasible than interstellar travel. All we need is a real long pice of string.

  22. #22 Saidziri
    August 20, 2009

    Would nano-technology be the answer ?

    Would nano-ships be able to ride the tide (cosmic rays) and once at the destination combine to form complex systems that, then would beam back data (including images) ?

  23. #23 John Hunt
    October 12, 2009

    There exists a plan for an interstellar mission which would overcome a lot of the difficulties listed above. It would eliminate the need for exotic propulsion by extending the mission duration about 2,000 years. Cosmic rays would be dealt with using very powerful superconducting magnets which need little power to maintain its magnetic field. Superconducting loops would decelerate against the interstellar medium. Life support would be unnecessary as embryos would remain viable via freezing. Habitat, life support, and power would be produced at destination through automation. Current research is already half way to conducting ectogenesis (i.e. fetal development apart from a mother). Child rearing would be achieved through today’s life-like androids and by developing an extensive database for interpersonal interaction.

    The purpose of this mission would be to establish a new human colony in a neighboring star system in case our solar system is rendered uninhabitable within this century through nanotech, biotech, AI, or chemical self-replicating technology.

    For a detailed write up visit:
    http://www.peregrinus-interstellar.net/index.php?option=com_content&task=view&id=135&Itemid=60

  24. #24 Emiliano
    November 1, 2009

    I would take a large asteroid, build a rotating colony (for gravity) inside it and apply a light sail to it, beaming lasers from the surface of the asteroid to the sail. The lasers would be powered by fusion reactors like the colony inside the asteroid. The light sail would slowly accelerate the asteroid to a fraction of the speed of light, while the asteroid rocky mass would shield the colony from the powerful impact of small particles and hydrogen atoms + protecting it from the cosmic rays + supplying the colony with minerals/water ice necessary to keep it running.

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