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Futuristic space travel between star systems

  1. Feb 20, 2012 #1
    I am a writer of science fiction and I need to know some basics about space travel. I've read that it would take a ship 40 years to travel 1 parsec (3.26 light years) if the ship moved at 10% the speed of light. Does that sound right? Given that a futuristic ship could travel at this speed, how many kilometers would the ship travel in one hour? Should a ship slow down when cruising through a solar system or approaching an asteroid where, in this case, it needs to land at a port. This said asteroid is traveling in an orbit about 3.26 light years out of the Centuari system.
  2. jcsd
  3. Feb 20, 2012 #2


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    The speed of light is 299,792,458 m/s. I'm going to round that to 300,000,000 m/s for simplicity. 10% of that is 30,000,000 m/s. There are 60 seconds in a minute so at that speed you would go (60)(30,000,000)= 1,800,000,000 m/min. There are then 60 minutes in an hour so that would be (60)(1,800,000,000)= 108,000,000,000= 1.08 x 1011 m/h or 108 x 108 km per hour.

    A parsec is 3.1 x 1016 m. At 108,000,000,000 m/h, it would take 3.1 x 1016/1.08 x 1011= (3.1/1.08) x1016-11= 2.87x 105= 287,000 which is approximately 287,000/24= 11958 days or 11958/365.25= 39.75 years (39 years 9 months). Yes, that's pretty close to 40 years.

    If the ship wants to land, yes, I would recommend it slow down! Otherwise, there is going to be a really big kaboom!
  4. Feb 20, 2012 #3
    yeah, which is why, if you want the ship to accelerate and decelerate so that it actually starts and stops at your two places, then it's going to take a bit more than 40 years.

    If you want the ship to accelerate at a comfortable rate like 1.0g, which is to say 9.8 m/s2, then we'd do this:

    10% of c is 30,000,000 m/s, v = at, so (30,000,000 m/s) = (9.8 m/s2)t

    so t = (30,000,000 m/s)/(9.8 m/s2) = 3061224.5 seconds, or about 35 and a half days.

    Deceleration would take the same amount of time.
  5. Feb 21, 2012 #4
    Okay, thanks for the reply. Now, let me ask this. Say my space ship left the Centauri system, needs to land on an asteroid that is in the ships direct path. After 40 years of traveling at the said speed (10% light year), HOW would the ship slow down in space in order to achieve the decelerated velocity it needs to land? How do objects slow down in a vacuum? And how long would it take, in other words, from that 40 year period of traveling that fast from the point where it could dock. Looking for ideas, since I am a molecular biologist and the theories of this for my story should be based on fact.
  6. Feb 21, 2012 #5
  7. Feb 21, 2012 #6


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    How did it get to 10% of the speed of light in the first place? Some propulsion unit presumably. To accelerate, it turns it propulsion on and comes up to speed.

    To decelerate, it turns around, pointing its propulsion straight forward, and turns its propulsion on until its velocity is zero again.
  8. Feb 21, 2012 #7
    Theoretically it could use solar sails (maybe powered by a laser), magnetic sails or even aerobraking but I think at 0.1c rocket motors would be the best choice.
  9. Feb 22, 2012 #8
    If you want to write SF, maybe you would find this one interesting also.

    Well I would add gravity assist to the list, to use the gravity and orbit of planets and stars to your advantage, to reduce fuel costs.
    I also read a solution, where magnetic sails were driven by giant particle beams, although it is doubtful, that you dont need independant engines for interstellar travel.
    But again, reducing fuel costs is good. To build a realistic interstellar ship... that would a bit like to a tiny artifical planet.
  10. Feb 22, 2012 #9


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    When we're talking relativistic velocities and interstellar distances, planetary gravity assist is not a contender. It's tantamount to embarking on a RV journey across the country and asking your kid to give the RV a push as you pull out of the driveway.

    Stellar gravity assist won't work because the gravitational body needs to be in motion relative to you to give you an assist. We start off stationary wrt the solar system, so no boost.
  11. Feb 22, 2012 #10


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    Frankly, if you do not even know how rockets work, you should not be writing science fiction about rockets. Why not science fiction about molecular biology? That would be cool!
  12. Feb 22, 2012 #11
    This is quite true, if you are wanting to write "hard sci fi" then you will need to do a lot of research. If you are writing soft sci fi then it does not really matter with regards to the technological specifics. What is important, IMO, is how the existent technology affects the story.

    With regards to long space voyages, propulsion may be one of the easier hurdles to overcome!
  13. Feb 22, 2012 #12


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    Objects moving in a vacuum stay moving until another force acts upon them. To slow down the ship would probably use the same mechanism it used to speed it, the easiest way to do that is to flip over and fire up the rockets again. As for how long it will take that depends on the acceleration; if you are accelerating at 1g then every second you get 9.82mps faster (also if means that everyone in your ship will be treated to the same "gravity" as Earth). To get to 10% of the speed of light at 1G would take roughly 1 month. In that time the ship would have travelled between 1 and 2 lightdays (I'm doing this in my head hence the roughness).

    You may want to check out this website
    It's designed to help people write realistic science fiction. Check out the index for all the articles covered.
    Everything about biology is cool :wink:
  14. Feb 22, 2012 #13


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    I once read a "sci fi" novel in which the propulsion was described in detail: heavy weights were moved to the stern at high speed, then returned back to the front slowly slowly. To illustrate why this would work, they gave the example of a sled on ice. If you lean forward quickly, the sled moves slightly backward. If you straighten up slowly, you do not move back to front!

  15. Feb 23, 2012 #14
    I correct myself.

    So after you decelerated to a lower speed, than you can use gravity assist for further deceleration, to reach low orbit of an alien planet.
    I guess such a giant ship wont be able to land, they will have to use many shuttles, maybe space elevators, skyhooks.

    Otherwise it would be interesting to read about the artifical biosphere of an interstellar ship. :)
  16. Feb 23, 2012 #15


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    1] Gravity assist will only work using planets other than the one you trying to land on. Which puts you dozens of millions of miles from your target, meaning you've got months of in-system transit ahead of you.

    2] If you have technology advanced enough for interstellar travel, why try to save gas on maneuvering in the driveway?

    Well, space elevators and skyhooks need to be built first. You'd still have to shuttle down to set up a small city to build the ground-base.
  17. Feb 28, 2012 #16
    I see.
    Well it can be a dramatic plot, that you (almost) run out of fuel, and you have to return to such outdated concept.

    Otherwise, i wondered on the following since Middle School : a ship reaches relativistic speeds. It's mass is growing. The fuel's mass also growing. In case of antimatter, full mass is turned into energy.
    How can it affect the ship's acceleration?

    Or just maybe mass growing is because electromagnetic interactions become weaker and weaker, that is why acceleration becomes harder?
    That idea was given to me by this article :

    Bookmark : Simple inference of time dilation due to relative velocity

    I dont know if it is right, but sure made me understand, how to derive Lorentz factor.
    Last edited: Feb 28, 2012
  18. Feb 28, 2012 #17
    It doesn't. From the point of view of the ship at least. Which as far as the ship is concerned is all that matters. Both ship and propellant are at the same level of mass increase, thus there is no gain.

    It's more complicated than that. Do a search of this Forum and you might discover exactly how.

    Understanding is the goal, but beware of pushing the analogy too far.
  19. Feb 28, 2012 #18
    I also wondered on the following thing : a hungarian scientist had the idea, that IF we could somehow control gravity, we could build galaxy railways that accelerate with gravity, we could reach big acceleration as we would freefall.

    Can we reach ANY speed we want in this way? I mean, in case of black holes acceleration of gravity even stronger than light, am I wrong?
  20. Feb 28, 2012 #19


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    There's no substance to this. Yes, if we had godlike powers we could build godlike devices. That goes without saying.

    Can we reach ANY speed we want in this way? I mean, in case of black holes acceleration of gravity even stronger than light, am I wrong?[/QUOTE]
    No. Gravity cannot accelerate any massive object to or past the speed of light.
  21. Feb 28, 2012 #20
    "No. Gravity cannot accelerate any massive object to or past the speed of light. "

    Can it accelerate light to a different speed? (If it can bend it, and hold it beyond an event horizont.)

    But return to the more realistic ship. So, it could maintain a constant acceleration until c, or acc. would drop by a factor of 1-v/c or something like that? Or it would still drop by Lorentz factor?
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