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Interstellar travel

  1. Dec 21, 2014 #1

    I am writing a piece which involves interstellar travel to nearby stars (Epsilon Indi being the one I will use in the following example.)

    It will be via a beam/ sail craft accelerated/ decelerated by solar-powered lasers (and lenses along the route). The craft will use constant acceleration/ deceleration at 1g to/ from a sizable percentage of LS (say 60%), and when drifting in interstellar space will rely upon centripetal forces to produce gravity.

    Travel times will be circa 15 years ship time and 25 years planet time. It will be in the future (although I will leave how far in the future ambiguous) so we will have some future technology, human lifespans will have increased by a reasonable margin (to 130 years or so) and there will be some sort of Induced Hibernation for large periods of the journey.

    It can rely upon some suspension of disbelief, but what I am interested in are the practical obstacles to achieving this; or any suggestions which might make this method of travel more practically feasible.

    Thanks in advance.

  2. jcsd
  3. Dec 21, 2014 #2

    Filip Larsen

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    Plugging acceleration and maximum speed into the equations for a relativistic rocket [1] with a coasting phase, the travel times are around the numbers you mention (16.7 years ship time vs 20.7 earth time) with most of the time spend coasting. So the purely kinematic part of your setup seems to make sense, even if having a huge solar sail traveling with 0.6 c through interstellar medium seems to suggest that a wealth of engineering problems has to solved first.

    Regarding the propulsion, obtaining 1 g acceleration using a light sail involves some pretty large mass-specific power levels, at the order of at least 1 to 2 GW/kg. At Earths distance from the Sun this means you need around 1 square kilometer of solar panels per kg of spaceship you want to accelerate, and that is assuming an unrealistic 100% energy efficiency converting solar energy to laser beam energy. For a 10 ton ship, this means you need a total laser beam energy of 20 TW, increasing to even more during the end of the acceleration phase due to relativistic dilation and distance. If you don't have a similar laser beam station at the destination star, you probably also need even more energy during the deceleration phase. Depending on how "hard" you want the science to be in your fiction you may have some trouble suspending disbelief of how such power levels are generated and transported over a distance of 11 light-years.

    For a realistic take on solar sail probes, I recommend you take a look at Friedman's book on the subject [2]. Perhaps others here can point you to a more recent or more detailed study of interstellar solar sail missions, if such exists.

    [1] http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]
    [2] Star Sailing: Solar Sails and Interstellar Flight, Louis Friedman, John Wiley & Sons Inc., 1988, ISBN 978-0-471-62593-3, 146 pgs.
    Last edited by a moderator: May 7, 2017
  4. Dec 21, 2014 #3
    Thanks Filip. The intention will be that the technology has been transmitted to us by a more advanced civilization in order to enable us to build the laser; there will also be an equivalent laser at the destination star.

    The size of the sails and the power requirements look like a bit of a show-stopper unless I do a lot of hand waving. Thanks for the figures, I will look into this in more detail.

    Also the Friedman book is one I wasn't even aware of; I will be jumping on Amazon in a few to order it.

    Thanks again.
  5. Dec 21, 2014 #4


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    The laser at the target star helps a lot. Power and distance between laser and ship are the two problematic things.

    Lenses: With a lense diameter of 1km (no idea how to make such a lense with the necessary precision), you need one lense every ~20 AU (give or take a factor of 4 depending on details). Clearly impractical for large distances (=the target laser is absolutely necessary). With your proposed acceleration profile, you would need 11000 AU to accelerate, or ~500 lenses. At that level, reflection at the lenses is a significant problem. Smaller or larger lenses do not change the total lense area.
    20 TW for 1km^2 would give 20MW/m^2, probably too much for the lenses (also for the primary mirror of the laser beam), so fewer larger lenses might be necessary.

    A faster acceleration would reduce the distance and therefore the number or size of lenses, but increase the required laser power even more.
  6. Dec 21, 2014 #5
    Thanks mfb.

    Even ignoring the engineering obstacles it will prove to be an incredibly lengthy process simply getting the infrastructure in place. An eleven year communication time and the manufacturer and placement of 500 separate lenses would take at least a century; and by the time you were halfway through the process your technology would have far surpassed your earlier work. Faster acceleration is sounding increasingly necessary and with some technology that would allow the mirror and the lens to operate. Perhaps a whole array of lasers may be the answer?

    Fortunately this is not much of an issue given that the events I am writing will occur a little after the initial building programme, but this period will be regularly referenced.
    Last edited: Dec 21, 2014
  7. Dec 21, 2014 #6


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    Hmm... you could re-use the lenses. If the amount of reflection and the frequency of their usage is right, the laser might even give the necessary force to keep them in place. Smaller shifts (to move them out of the way, and back into the beam once the spacecraft passed) could be possible with the laser beam as well, if you have smaller adjustable elements. Sounds interesting.

    More lasers allow more power, sure.
  8. Dec 24, 2014 #7
    10 tones? So the whole ship is smaller than Apollo Command and Control module?

    This miniaturization is making a great progress. ;)

    Anyway, I see one more obstacle:

    That needs 20 TW under perfect conditions?

    So roughly counting 20 big nuclear reactors that Hungarians are planning to build:

    In this size it sounds feasible. However it means freezing all those guys for whole voyage. As far as I remember ISS statistics one guy needs 2 tones of supplies/year. In order to cut costs I would sent this ship without landing module. If aliens ordered that, they have to pick it on their own from orbit.

    You want a bigger ship, because sending a few frozen guys even without presents for aliens don't make cool story? ;) Then remains subject of energy production on Earth.
    a) This mission is so crucial, it would be the greatest project for human history and building 100 big nuclear power plants each with dozen nuclear reactors within a decade seems as necessary sacrifice, even if it means rationing building materials for any other projects.
    b) Human civilization already reached such energy output that it can afford such stuff without problem. This however, means that astronauts came from a really rich place.
  9. Dec 24, 2014 #8
    One more idea, premises:
    a) you want to accelerate stuff near Earth
    b) all stuff is durable and guys are frozen, so really high g is not a problem
    c) building all those nuclear power plants & laser for one quick use would be somewhat wasteful
    d) more lasers and power plants are going to build in the meantime

    Then send the ship in modules, a few tones each, that would be automatically assembled on the way.

    The older stuff is going to be accelerated less than the old one, later there would be minor corrections until everything is assembled.

    Stop being creative? :D
  10. Dec 24, 2014 #9


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    1 Terawatt = 1000 Gigawatt. You would need 20 000 of those blocks.

    In addition, the laser has to be space-based as an atmosphere would ruin the beam parameters and all larger objects like the moon are rotating which is impractical. Huge arrays of solar cells look like the most promising option.
  11. Dec 24, 2014 #10
    Ooopsie. Yes, you are right. So for 10 tone ship you need fully utilize 20 times USA full electric energy capacity.

    Maybe we can just send those aliens an e-mail? :D
  12. Dec 29, 2014 #11


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    Also, what is your reflector made of? Consider that it will not be perfectly reflective. Some tiny fraction of the incident light energy will be absorbed. That will put a maximum on the incident light energy, because it will heat the mirror. I'm thinking an acceleration of 0.6 gee is pretty ambitious even if you have the light energy available. You could do rough estimates of this based on supposed efficiency, and the light energy per square meter. And you could assume black-body radiation to lose heat from the reflector.

    Don't forget that your cargo capsule is probably on the "bright side" of the reflector. To put it in front would mean you would have to make everything stiff and balanced, an increased engineering challenge. With the load behind it's only got to be strong enough to support the weight, and balanced by pull instead of against push. So it has to accept similar issues of heating. You won't be able to direct the light beam to avoid the capsule over light-year distances. You will be doing well to hit the ship at all.
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