The difficulty of interstellar travel for humans has an article reviewing the results of a survey they conducted with their readers asking when the first human might leave the solar system.  The leading answer was after the year 2100, which make sense given our current level of progress just getting humans back out of low Earth orbit.  But I think the prospects for human interstellar exploration are far more difficult than most people realize.

First, when pondering this question, we can forget about the traditional sci-fi answers of hyperspace, warp drive, or similar notions.  These concepts are fictional plot devices with little or no basis in science.  Even concepts that have been considered by scientists, such as wormholes and Alcubierre warp drives, are extremely speculative, requiring that certain speculative and unproven aspects of physics be true.

Getting anything like these types of technology will require a new physics.  Undoubtedly, we will learn new things about physics in the decades and centuries to come, but the probability that what we learn will enable interstellar travel to function like sea travel (the preoccupation of most space opera stories) is an infinitesimal slice of the possibilities.

The only way to speculate scientifically on this kind of thing is to take the science that we currently have and try to extrapolate from it.  When we do that, the obstacles to human interstellar flight seem pretty daunting.

Worrying about the speed of light limit, which current physics tells us is the ultimate speed limit in the universe, is sour grapes to a large extent.  Even getting to an appreciable percentage of the speed of light turns out to require astounding amounts of energy.  Doing it with our current chemical rocket technology is a lost cause.  According to Paul Gilster, in his book Centauri Dreams, Imagining and Planning Interstellar Exploration (which I recommend for anyone interested in this subject, as well as his blog), it would take more mass than exists in the visible universe to propel a chemical rocket to a substantial fraction of the speed of light.

An artist’s concept of nuclear pulse propulsion.
Image credit: NASA

Of course, there are better, more efficient propulsion options that might eventually be available.  For purposes of this post, let’s leap to the most efficient and plausible near term option, nuclear pulse propulsion.  This is a refined version of an original idea that involved lobbing nuclear bombs behind a spacecraft to push it forward.

Giltster, in his book, notes that a nuclear pulse propulsion spacecraft, to reach 10% of light speed, would need a mass ratio of 100:1.  This means that for every kilogram of the spacecraft with only its payload, you’d need 100 kilograms of fuel.   Initially, that doesn’t sound too bad since the Apollo missions had an overall mass ratio of 600:1.  But that was for the entire mission, and all we’ve considered so far is the mass ratio to accelerate to 10% of light speed.  We haven’t talked about slowing down at the destination.

In space, given inertia and no air friction, slowing down takes just as much energy as speeding up.  And the kicker is that you have to accelerate the fuel you’ll later need to decelerate.  So slowing down doesn’t just double the mass ratio from 100:1 to 200:1.  The deceleration fuel has to be on the “1” side in the initial acceleration ratio.  That means the ratio for the overall mission (and we’re only talking about a one way mission here), has to be squared, taking it from 100:1 to 10,000:1.

Traveling at 10% lightspeed gets us to Proxima Centauri, the nearest star to the sun, in about 43 years.  When you consider what kind of living space a human crew would need for that time span, and multiply it out by 10,000, an interstellar mission starts to look like the most expensive thing human civilization might ever attempt.  It gets worse if we try to shorten the time span.  Increasing the speed to 20% of light speed raises the ratio to 100,000,000:1.

Imagining antimatter technology might improve the mass ratio substantially.  But it adds new difficulties.  Producing antimatter itself takes tremendous amounts of energy.  It would have to be manufactured and stored in deep space, since any containment failure of any substantial amount would likely result in a gigaton level explosion.  We might save on the mass ratio of the spacecraft, but only at the expense of vast resources dedicated to generating and storing the fuel.  And human crews would likely have to be heavily shielded from the gamma rays generated by antimatter reactions, increasing mass throughout the overall craft.

No discussion of this type is complete without at least mentioning the Bussard ramjet, the idea of a spacecraft with an immense ram scoop to take in interstellar dust to use as fuel.  There was a lot of excitement for this concept in the 60s and 70s, but further study has shown that the interstellar medium isn’t nearly as thick as the initial design hoped for, and many think the ram scoop would generate as much friction as thrust.

Other options are to forego rocketry altogether and go for something like light sails.  Robert Forward, decades ago, put forth a design where a gigantic laser on Mercury would send out a beam to an interstellar light sail spacecraft, steadily accelerating it.  At some point, the craft would separate its sail into two components, one of which would be hit by the laser and reflect it back to the remaining component attached to the craft, decelerating it.  Forward’s design is ingenious, but it would still require titanic amounts of energy, and precise coordination across centuries and light years to work.

Things get a lot easier if we just think about sending uncrewed probes.  That’s the current direction of the Starshot Breakthrough initiative.  The idea is to propel a small, perhaps gram sized probe, to 20% of light speed using Earth based lasers.  The probes would reach Proxima Centauri in about 22 years, taking pictures and readings as they fly through the system, and transmitting the information back to Earth.  There are still major technological hurdles to overcome with this idea, but they all seem achievable within reasonable time periods and with reasonable amounts of energy.

The big drawback to the Starshot design is that it doesn’t have any way to slow the probe down, so everything would have to be learned in the few hours available as it sped through the target system.  An alternate design has been proposed, using the unique topology of the Alpha Centauri / Proxima Centauri system to slow down the probe, but at the cost of increasing the travel time to over a century.

But once we give up the idea of crewed missions, the rocket solutions actually become more plausible.  A 10,000:1 ratio doesn’t seem problematic if the ultimate payload is a one gram probe.  Even the 100,000,000:1 ratio associated with a 20% light speed mission starts to look conceivably manageable.

And when we consider the ongoing improvements in artificial intelligence and the idea of probes building their own daughter probes to explore the destination system, and perhaps even to eventually launch toward systems further out, the possibilities start to look endless.

All of which is to say, that it’s much easier to conduct interstellar exploration with robots, particularly very small ones, than with humans.  It seems likely that we’re going to be exploring the stars with robots for a long time before humans get there, if they ever do.

Unless of course I’m missing something?

26 thoughts on “The difficulty of interstellar travel for humans

  1. Sounds about right to me. I’d guess that by 2100, we’ll probably be on Mars. After that, there’s still a whole lot of Solar System for us to explore and colonize, so I don’t think there’d be much rush to go interstellar until the very, very far distant future.

    Liked by 1 person

    1. I can see that timeline for exploration. I’m reasonably optimistic we’ll have scientific stations on Mars, and many of the moons of the outer planets in the centuries ahead.

      I’m less sure about colonization though. It would be extremely expensive to colonize the solar system, both to establish and to maintain. The cost of living in a space colony would be orders of magnitude higher than any locale on Earth (including deserts, Antarctica, the ocean floors, or underground). It seems like there would have to be a major economic impetus for it to happen, one that would require the presence of humans, not just machines. I’m not sure what that might be.

      Liked by 1 person

      1. I’m a little bit more optimistic about colonizing the Solar System, but only a little. When I say we’ll be on Mars by 2100, I mean we’ll have at best a small outpost.

        As for colonization, I recently read a paper by Elon Musk about how he plans to bring the costs of space travel down. His goal is to make the cost of a ticket to Mars roughly equivalent to buying a house in the U.S.

        I think Musk’s timetable for doing that is unrealistic. He’s talking about decades when I think he should be talking about centuries, but his cost-cutting ideas seemed plausible to me. I think it could happen eventually.

        Liked by 1 person

  2. Thanks for the interesting post as this is a particularly favorite topic of mine. The information about Paul Glister is especially appreciated. I agree that tiny robots is the best solution to interstellar travel.

    Are you aware of the organization 100 Year Starship a government created organization whose purpose is to explore the technology required for human travel beyond the solar system. They will be having a symposium in September 14 -17, in Santa Monica, CA. My paper on the subject, “Microbots—The Seeds of Interstellar Civilization” has been nominated for the 100YSS Canopus award for interstellar writing.

    Liked by 3 people

    1. Thanks Ry! I have heard of the 100 Year Starship organization, but have to confess my knowledge of it is pretty limited.

      Wow. Congratulations on the nomination! Is your paper online anywhere? It sounds interesting.


    2. Depressing as hell realizing everyone here will be dust by the time our faraway future relatives will have the technology to travel interstellar. All we have today is Sci-fi books and movies, and dreams. Bummer!


  3. Sounds good to me.

    I think the best bet would be a combination of Nuclear Pulse and Laser/Microwave Propulsion. You’d use the latter to accelerate the spaceship up to 10% of the speed of light, then use the former to slow down at the destination. You’d also heavily use staging, so that you’re only decelerating the lowest amount of mass possible (hopefully aiming the still high-speed components on a trajectory that takes them away from any stars and planets). Eventually you’d get down to a ship that’s finishing decelerating using solar sails and maybe nuclear-electric propulsion, so it can then simply travel around in the destination system at interplanetary speeds.

    At least theoretically, you’d only have to do that once with a particular destination. If you can build a laser at the destination system to decelerate any follow-up starships, then travel to that system becomes less complicated and mass heavy. Then you would basically “island hop” between nearby stars, doing the same thing until you’ve got a network of inhabited solar systems connected by laser propulsion systems.

    . . . Assuming of course we can build either the laser/microwave array, or the nuclear pulse starship carrying its own nuclear weapons factory.

    Liked by 1 person

    1. I like the sound of that. Staging often doesn’t get enough attention in these discussions. Science fiction has always seemed like it wanted staging to just go away. But for any mission where energy is a major constraint, it would almost certainly be used. It reduces the mass ratio for any relevant scenario.

      What I’d wonder about your framework is how powerful the laser would have to be and how big the sail would have to be for the initial acceleration to carry all the nuclear pulse stages and fuel to be used in the deceleration phase.

      Another possibility that’s occurred to me for the initial acceleration is some kind of gigantic electromagnetic accelerator tube, but its size and power requirements strike me as probably being more costly than just adding more stages for the initial acceleration stage.

      Your framework reminds of me of another one that I didn’t mention, electric propulsion powered by a beam from the home system. It’s a case where the beam isn’t itself propulsion, but a power feed. It wouldn’t be a fast mission, and the coordination across light years would still be daunting, but it’s another method to make it happen.


  4. Another question is: for what purpose should we try to leave the solar system. It looks like it is tremendously expensive to do so, in terms of resources required. What could we get in return for that investment? I don’t find it a plausible scenario at all. I don’t think it is going to happen.
    What reasons do people give as possible justifications for trying it? I think it cannot be that we need another planet because we are destroying the one we have. The technical challanges to reach another habitable planet seem to be several orders of magnitude larger than the challenges to turn our civilization into something sustainable (which, I think, it currently is not). Once it is sustainable, there is no good reason to go somewhere else, for at least a couple of million years. So space opera romanticism aside, what would it be good for?

    Liked by 1 person

    1. That’s a good question. Myself, I think it’s unlikely that biological humans will ever go to the stars, or at least not in any great number. A very rich future society might send some as a grand symbolic gesture. But given the resources and distance involved, plus the fact that we’re unlikely to find another biosphere enough like Earth’s for us to live directly in it, it’s hard to see it happening as more than a token effort. At least unless some new physics discovery dramatically lowers the cost.

      I do think there’s a lot of value in sending scientific probes. As artificial intelligence improves, the ability of those probes to explore will provide us with as much information as we’d have been able to glean by sending humans. If mind copying of some kind eventually becomes possible, it might enable us to experience those locations directly by being transmitted through an interstellar internet. If not, then the universe will belong to our AI heirs.


    2. May I add it can be for the pure excitement of exploring new worlds. Like why does a cliff diver jump off cliffs? Just to experience it.
      As far as it being expensive and the economics of it, I’m sure if (and when) we can manage such a task as interstellar travel we can also come up with better economic paradigms (?resource based economy).


      1. O.K., but jumping over a cliff or climbing a mountain or anything like that does not take much resources. Here we are talking about something that is more expensive, in terms of energy and other resources, than what we might ever be able to achieve, as a civilization. And how much is a civilization willing to invest to produce such an experience? And how exciting would it actually be, if one looks at it realistically? Can it be so exciting that we would invest a large part of our resources so that a few of us can have such an excitement? And how exciting would it actually be? My impression is that there is a lot of romanticism as well as ideology in the whole space exploration topic. For example, how exciting would it be to live on mars? Would it not be like living in a prision in the middle of a desert? What would be the difference between interstellar space flight and lifelong imprisonment? I remain sceptic.

        Liked by 1 person

        1. Good points. I agree there is a lot of romanticism, but I kind of like that. Perhaps living on Mars is akin to a desert but I still feel attracted to the idea (as I’m sure others do), because the atmosphere will be different and there would be awe inspiring sights which aren’t afforded on earth (I know a few on earth as well, you can only experience such stuff not describe it). But when going interstellar in fact, we may be looking at other earth like planets, or even unimaginable planets. Which seems very attractive indeed.

          I do agree with you on the allotment of resources and prioritization of tasks, this one certainly isn’t a priority by any means. We first need to take care of our home before venturing on adventures. But for me, it isn’t out of the question to have such dreams because I believe it to be within our capacity eventually.


    1. A black hole engine, if possible, might keep the size of the starship down, which is helpful. But like the antimatter starship, it still can’t escape the overall energy cost. The energy that would likely have to be expended to create it would be astronomical. And it would likely have the same shielding problems as antimatter reactions.

      I’ve never been clear on how you would turn a black hole engine on or off. It seems like attempting to contain the Hawking radiation by closing off the parabolic reflector would just cause the reflector (and overall starship) to melt.

      Still, if there is any way to do it, it will likely be something along these lines.


        1. Actually it occurs to me that the ship could be designed to radiate its energy out the back or front. When it wanted to thrust forward, it would close off the front with all the thrust going out the back. When it wanted to go back, it would close off the back with all the thrust going out the front. When it wanted to remain stationary, it could open both ends with the thrust cancelling each other out. It’d be an interesting site when it was stationary, particularly as the singularity was in the final stages of burning out.

          Liked by 1 person

  5. Couple thoughts.

    Why would we go? Because we can. Obviously not everyone will want to go, but there will almost certainly be enough people unhappy on earth who will jump at the chance to start a colony out there of “just us”. Look at how many people signed up for the almost-certain-early death of being the first to try to live on Mars. And then of course, there’s Google. How long before they start grabbing asteroids? What happens when they want to dismantle Chiron to build the solar internet? If they get to Proxima Centauri first, can they charge late comers for supplies?
    I don’t think the very first interstellar travelers will have to worry too much about slowing down at the end. The way I expect it will play out, first we’ll send robots. When the robots get there, they will build more robots, start work on terraforming if appropriate, build habitats, build catch mechanisms for travelers who want to stop there, build resupply mechanisms for travelers who want to travel farther, send robots out to do similar things to nearby star systems, etc. In any case, the very first travelers are likely to get passed by later travelers with better tech.

    (I suspect a lot of y’all really don’t get the singularity)

    Liked by 1 person

    1. James,
      I don’t doubt a lot of people would want to go, just as a lot of people do want to go to Mars today. But the people who want to go to Mars haven’t gone. Why? Because it’s horrendously expensive. An interstellar mission will be several orders of magnitude more expensive, and not necessarily in a way that economic growth might cover. It would be expensive in terms of solar system resources, which might seem unlimited right now, but might not to a future solar system economy.

      But, as I noted to nannus, who knows what a sufficiently rich future society might do? They might turn it into a society wide project to send a group of people, similar to the Egyptians building the pyramids. But it’s pretty clear that everyone who wanted a pyramid didn’t get one, and there aren’t that many of them.

      But to whatever extent current biological humans do go to the stars, I agree that they’ll be going to places long settled and prepared by the robots.


  6. Somewhat far fetched, but how about mind uploading + sel replicating nanorobots + quantum entaglement communication. Send nuclear pulse drive ship at 20-30% light speed.withw just a few microscopic.self replicating nanobots. Nanobots build robot body at destination and uploaded consciousness is transferred via quantum entaglement

    Liked by 1 person

    1. I’m onboard with the mind uploading. It’s speculative, but no more speculative than black hole or antimatter engines. Same with self replicating nanobots.

      Unfortunately, quantum entanglement can’t be used for communication, which is a bummer.

      Uploaded or AI minds would have to be transmitted at the speed of light, but doing so would be far faster and cheaper than trying to physically send the information.


      1. Damn. The laws of physics always seem to foil us. Whenever we find some phenomenon that seems like it will change everything there always ends up being some caveat that prevents it from being exploited. I.e. quantum entaglement, vacuum energy, etc.

        Liked by 1 person

  7. First things first, you have got a cool blog, commenting on this post,
    There are reasons why we do not put humans into space, well you can answer that yourself by looking at the success rate of our space ships : Of all the 5,038 rockets launched to date, some 4,621 accomplished their mission, with a success rate of 92%. . 417 rockets have failed it, but our situation is improving day by day, by our understanding of physics.
    Remember the time humans thought TV’s achieving sound speed was impossible, we have achieved it now. We have to know and discuss the problems but still focus more on solutions of everything. p.s i followed 🙂

    Liked by 1 person

    1. Thanks. And good point about the rockets. It pays to remember that a rocket is a payload riding on a controlled explosion, where if anything goes wrong, the controlled explosion can easily turn into a straight out explosion. The first step of crewed space exploration is the crew riding a missile into orbit.

      Liked by 1 person

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