A generation ship was sent out a long time ago, to colonize a far away planet. However the ship failed to reach its goal due to navigational issues (which also means they didn't have any clue where in the galaxy they actually were, and how to return to earth). They continued the journey in the hope to eventually either find a colonizeable planet or find back to earth.

It is now travelling for much longer than its originally planned travel time, indeed, a multiple of that. This means that obviously any fixed energy supply the ship may originally have been equipped with is long consumed. So the ship somehow needs to harvest energy from the universe. I don't want it to simply go into orbit around a star and remain there, but it shall still be on its interstellar journey.

This of course poses the problem where the ship gets its energy from. I see the following possibilities:

  • It harvests hydrogen from space and uses it for nuclear fusion.
  • From time to time it goes close to a star and refreshes its energy supplies from solar (or rather, stellar) power, which it then uses until the next visit of a stellar system.

However given the small density of hydrogen in space, and the long time travelling between star systems (remember, this is a generation ship, so you'd not expect extraordinary high speeds), both options don't seem too convincing to me.

Does anyone have a better solution (or alternatively, can convince me that the two solutions are not as bad as they seem)?

Note that I don't need (actually, don't want) an abundance of energy, but it should be sufficient to keep the spaceship operating and the people on it alive.

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    $\begingroup$ A Bussard ramjet should ensure unlimited energy supply. The low density isn't an issue as long as the ram scoop is large enough and the ship is travelling at 0.x c $\endgroup$
    – March Ho
    Jan 11, 2015 at 17:11
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    $\begingroup$ I find the premise really hard to believe. A generation ship implies mSTL travel (much slower than light), Project Orion assumed a low percentage of c as the speed. So having "no clue where they are" seems highly unlikely cos you'll be always in the close neighborhood of earth. $\endgroup$
    – Ghanima
    Jan 11, 2015 at 22:33
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    $\begingroup$ Another hard to believe detail is having a generation ship ran out of energy. Nuclear fuels have very high energy densities, it would not really be an issue to have lots of extra over the planned need. And a generation ship would also have lots of spare water and probably other compounds with hydrogen. I think the thing they'd ran out of first would be propellant. Although with nuclear pulse propulsion nuclear fuel and propellant would be somewhat interchangeable. Anyway, replenishing supplies would require stopping by on a solar system. Mining a Kuiper Belt Object might be best solution. $\endgroup$ Jan 11, 2015 at 22:55
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    $\begingroup$ If you are past the suns magnetic field (it has ventured past the heliopause), protons (hydrogen) flying at near speed of light might be a potential energy source as well. $\endgroup$
    – Twelfth
    Jan 21, 2015 at 22:51
  • $\begingroup$ Another "I find the premise really hard to believe" comment, though four years late, because we've taken a lot of pictures of the stars. If we can do this (youtu.be/37inzFez9vA) now, imagine what we can do by the time we launch the generation ship. $\endgroup$
    – RonJohn
    May 28, 2019 at 1:47

6 Answers 6


The Bussard ramjet idea could very well work - if you plan your route correctly.

You're worried about the low number density of hydrogen atoms/ions in outer space. This table shows that this is indeed quite a big problem. There is a solution, though, which is to travel nearly exclusively - when possible - through molecular clouds. These are swaths of space filled with hydrogen and helium where stars are born. They also have a very atomic/ionic number density - roughly $10^2-10^6$ atoms per cubic centimeter (which translates to $10^8-10^{12}$ atoms per cubic meter). Wikipedia says that ~$\text{91%}$ of the interstellar medium is hydrogen; molecular clouds could have an even higher percentage. I'll go with that, and assume a density of $10^{10}$ atoms per cubic meter.

You're on a generation ship, so it's most likely large - after all, it needs to have enough supplies (not just food) to last for many decades. So attaching a large scoop to the front of it shouldn't cause a substantial structural problem. Let's say that the scoop is circular, with a one-kilometer radius. That translates to an area of $1,000,000 \pi$ square meters. Assuming you're traveling at a nice clip - say, $20$ kilometers per second. That means that each second, your scoop travels through $20,000,000,000 \pi=2 \pi \times 10^{10}$ cubic meters of space. With $10^{10}$ atoms per cubic meter, you've scooped up $2 \pi \times 10^{20}$ atoms in that second. If ~$\text{91%}$ of those are hydrogen atoms, you've picked up $5.718 \times 10^{20}$ hydrogen atoms. With one hydrogen atom having a mass of one atomic mass unit ($1.661 \times 10^{-27}$ kilograms); you pick up $8.601 \times 10^{-7}$ kilograms of hydrogen. So it will take you about $115$ days to collect one kilogram of hydrogen.

Fortunately, most Bussard ramjet ideas have much larger scoops. To collect one kilogram of atomic hydrogen per day, you'd only need a scoop with a radius of $10.7$ kilometers. In an answer here, user23660 said that approximately $\text{0.0037681%}$ of the mass is converted to energy. So you have $0.0037681$ kilograms of hydrogen. Via $E=mc^2$, that translates to $3.387 \times 10^{14}$ Joules (using $c=299,792,458$) produced each day, or a power output of $3.920 \times 10^{10}$ Joules per second.

In that one second, $3.920 \times 10^{10}$ Joules of energy have been consumed. The velocity I gave before was $20$ kilometers per second, so the ship has gone $20,000$ meters. Work is defined as $\text{Force} \times \text{distance}$; setting the energy used as work, I get a force of about $195,984$ Newtons. Given that the mass of the ship is $\text{something very large}$, you're not going to have a huge acceleration. You can always increase the size of the scoop: multiplying the radius by $x$ will multiply the area by $x^2$, and thus multiply the amount of hydrogen collected by $x^2$, so long as $x$ is in meters.

Of course, there are ways around this. For example, maybe your ship is propelled by a solar sail. A solar sail takes a while to get to a decent cruising speed, but it needs no fuel. Plus, it will be about the same size as the scoop, so the scoop shouldn't cause any additional drag. This means that all the energy can go towards keeping the ship's inhabitants alive.

Molecular clouds aren't easy to come by, though. Wikipedia (the molecular cloud article) says that they only compose one percent of the total interstellar medium. Ouch. However, it does say this:

The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy.

So you can think of that ring as an express lane for Bussard ramjets. I haven't been able to find an image showing the location, but this (paywalled) paper seems to be a good start. In summary: choose the right locations (and take a solar sail along for the ride) at it seems like you'll be okay with a Bussard Ramjet.

I just realized that there's a loophole to this (and I feel stupid for not realizing it before!).

The Swiss plane Solar Impulse aims to circumnavigate the globe. One of the problems, though, is that it must fly during the night, where there's no solar energy to be gathered. Solar Impulse solves this problem by gathering extra energy during the day, storing it in batteries, and using it slowly at night, while drifting down a little. The ramjet could do this, too, collecting excess hydrogen and storing it in tanks. The fuel would add extra mass, but it would enable the craft to go to new places along new routes.

  • $\begingroup$ I saw a paper somewhere, that said BR wouldn't work, I think they drive away the H? But paper was behind a paywall, so only got someone else's synopsis. $\endgroup$
    – user3082
    Jan 21, 2015 at 16:28
  • $\begingroup$ I like the point that you cannot go anywhere you want with this method. This gives a nice alternative to the reason why return to earth is not possible: The navigational issues moved them out of the molecular clouds, and thus they have no substantial thrust; if the collected hydrogen was also the ship's main energy source it also explains neatly why now energy is scarce. $\endgroup$
    – celtschk
    Feb 14, 2015 at 15:50
  • $\begingroup$ @celtschk I realized that they could probably go more places if they stored fuel before going to those places. There are some solar-powered planes that do something similar, getting extra solar energy during the day and using it at night. $\endgroup$
    – HDE 226868
    Feb 14, 2015 at 15:53
  • $\begingroup$ @user3082 , I'm not completely caught up on this work but read the same thing. Essentially it looks like except under special circumstances, a BR generates more drag than thrust, this is at the very least due to the extremely slow reaction rate of $4H \rightarrow He_4$ fusion. If you use very optimistic assumptions and favorable interstellar medium, I think the papers conclude that the concept still could work though. $\endgroup$
    – Jim2B
    May 24, 2015 at 22:34

My favorite idea for a reactionless drive is based on ZPE. That's Zero Point Energy. How does it work? The simple answer is that "empty space" is anything but empty, instead is a boiling cauldron of energy, with quantum uncertainty generating sets of particle-antiparticle combinations (with zero net energy) all the time. To use a loose metaphor, you can think of "real" particles as a thin icy film on top of a very, very deep ocean.

How do you harness this? One current way of doing so is by generating a pressure differential via the Casimir Effect. Two plates are placed so close together, that virtual particles with a certain wavelength cannot form inside, but freely form outside. This generates a pressure that pushes the two plates together. If a more advanced version of this can be vectorized, you don't even need to bother with getting reaction mass on board.

vaccum perturbations

  • $\begingroup$ Definitely the most reliable and long lasting solution. $\endgroup$
    – MER
    Feb 14, 2015 at 22:03
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    $\begingroup$ This doesn't answer the question, as this is not an energy source, but a handwavy concept for a reactionmassless drive. $\endgroup$
    – M.Herzkamp
    Aug 26, 2016 at 10:25

You could always have the ship go planet to planet, stripping the planet of whatever resources it has to continue operation. It would require relatively long stops to refuel, but then the ship could continue on its way for a long time. Also many star systems have multiple planets, meaning that within a relatively small region, there would be multiple opportunities to gather resources. Likely lacking the more efficient technology of the world it was launched from, this task would be difficult, and the ship wouldn't find itself with an abundance of energy.

  • $\begingroup$ Decelerating and accelerating a huge generation ship takes a lot of energy on its own. What you are proposing is basically colonizing star systems to get energy there. This is no small feat in itself. The original question has exactly this at its core. $\endgroup$
    – orkoden
    Feb 15, 2015 at 22:26

Heinlein, iirc, had one of his generation ships use direct matter to energy conversion... of course, since they were off-course and long past their estimated trip time, the degenerate descendants were slowly stripping the ship (and their wastes and life-support) to feed the energy converters...


The ship could be powered by Dark Energy (which Wikipedia tells me is 68.3% of the universe). Theoretically, exists in great abundance everywhere, but there's no reason that your ship's collectors/generators need to be any more efficient than the plot demands. And, of course, there's no telling when the ship will find itself in a region of particularly low energy for some reason.

  • $\begingroup$ This is, unfortunately incorrect - though a creative idea. There is no way to harness dark energy. $\endgroup$
    – HDE 226868
    Jan 11, 2015 at 17:50
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    $\begingroup$ @jdlugosz The cable would have to be thousands of light-years long, because dark energy only works at those scales. That would be completely unfeasible for a generation ship. $\endgroup$
    – HDE 226868
    Jan 14, 2015 at 16:14
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    $\begingroup$ @HDE226868 you know how theorists tease experimentalists: that's just an engineering issue. I know it's impractical and not useful even to a Type 3 civilization. As a gandanken experiment, it illustrates a diatinction between impossible and unimplementable. In this case, a profound point concerning the infinite potential energy of gravity. Check out A universe from nothing by Lawrence Krauss. $\endgroup$
    – JDługosz
    Jan 14, 2015 at 17:45
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    $\begingroup$ @jdlugosz My favorite version is a hollow sphere that is so large that its inner surface has Earth normal gravity. A habitable surface area counted in square parsecs just appeals to me. $\endgroup$ Jan 14, 2015 at 20:49
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    $\begingroup$ @VilleNiemi a hollow sphere of uniform construction will have no net gravity near the inside surface. The spot below your feet is small and close, but the entire hemisphere over your head is pulling the other way. Area increases in the same manner as diatance in any bearing, so cancel out. $\endgroup$
    – JDługosz
    Jan 14, 2015 at 21:58

An highly energized nuclear reactor.

EXTREMELY efficient solar?

Harnessed cosmic radiation?

outer structure made of superconductive coils, a magnetic center spun to induce current. balanced freely in the middle and spun in a vacuum to avoid friction. It would also produce artificial gravity by spinning. Anyone who says the magnetic field would cause it STOP spinning on its own... Check out the dynaflux alternator: https://www.youtube.com/watch?v=HK3JOlY0V8Y&list=LLPZOJw60jGMI9oKNJQVQ6pg&index=6

Cold Fusion?

A zero-point-module from an ancient's device. (Stargate anyone?)

A portable black hole ? Feeding it rips matter apart producing powerful x-rays. Magnetic containment may be a problem though! It worked for the Romulans.


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