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Travel in space to Alpha Centuri

  1. Sep 23, 2009 #1
    I'm having a bit of a debate on travelling to AC. I was wondering if someone can post calculatons to show how long it would take travelling at 1g accelaration, and how much energy would be needed including any relativistic influences.

    Thanks
     
  2. jcsd
  3. Sep 23, 2009 #2

    DavidSnider

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    That depends, how long would we be accelerating? Also, what is the mass of the ship?
     
    Last edited: Sep 23, 2009
  4. Sep 23, 2009 #3

    JesseM

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    Also, 1G acceleration in the same direction the whole time so you pass Alpha Centauri at relativistic speed, or 1G acceleration towards AC for half the trip and then 1G acceleration in the opposite direction to "brake" so you arrive at low speed? In the latter case it would take 3.6 years of onboard time (though more time in the Earth's frame), according to the http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken] page. That page also mentions that if you used the most efficient possible engine that converted fuel mass directly to photons that shot out in the direction of travel, the ratio of fuel mass to payload mass to arrive at AC at low speeds would be 38:1, and unless I'm mistaken the energy expended in the Earth's frame would just be the rest mass energy of the fuel, calculated using E=Mc^2.
     
    Last edited by a moderator: May 4, 2017
  5. Sep 23, 2009 #4
    The size of the ship is an issue. It would include travelers and the food required to sustain them. Then their gear, and fuel. Closest I can think of would be a submarine for example. What I'm looking for is what the energy would be compared to a nuke reactor or oil equiv. The time of just 4 years is theoretical only (seems the CBR would cook them), what is the practical limit of speed attainable? Which then affects the time big time.
     
  6. Sep 23, 2009 #5

    Nabeshin

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    4 years is by no means difficult when considering the radiation of the CMB "cooking" any travelers. Assuming 1g acceleration half the trip and 1g deceleration the other half, you get an effective travel distance of about 12,000 ly before CMB radiation begins to kill off your travelers. If you want a time, this is a journey that takes approximately 18 years, ship time.
     
  7. Sep 23, 2009 #6
    Ok, then how much energy would be needed to make the trip?
     
  8. Sep 23, 2009 #7
    Using a Los Angeles class sub as an example, we have 7000Mt for the ship, with a crew of say 200, and 2kg food per person per day, comes to a total of just short 10,000Mt not including fuel. At a rate of 38:1 then we would need 380,000Mt just for fuel. And if I read that site right that's fuel that get's converted 100% matter to energy with E=MC^2, correct?
     
  9. Sep 23, 2009 #8

    Nabeshin

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    Obviously it depends on the propulsion method for such a ship.

    For your example, assuming 7000Mt ship mass (6*10^12 kg), and 38:1 fuel, I get an lower bound on total energy cost somewhere in the neighborhood of a yottajoule (10^24) for the entire trip.
     
  10. Sep 23, 2009 #9
    How many nuke plants?

    If I have that correct, that's 400,000,000kg load that needs to be accelarated for 18 years. Converting the fuel into pure energy would be 3.25x10^23 joules. Over 18 years that's a power of 5.6x10^16Watts. A 500kW nuke plant for comparison would mean 11 TRILLION nuke plants. Correct?

    Someone emailed me that they would accelarate for one year, getting to 1/10c, then coasting for 44 years and decelarating at AC. Thus 46 year travelling time. So can we calc the energy for that? Seems to me the number of equiv nuke plants would be more than a billion.
     
  11. Sep 23, 2009 #10

    Nabeshin

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    Yes. Cutting down on acceleration does reduce power consumption, to a certain point. However, you suffer diminishing returns as the voyage becomes exponentially longer as you begin to lose the benefits of time dilation. Reducing acceleration from 1g to .1g helps, and to .01g helps a bit more. Beyond this, there's almost no point (you're talking travel times in the centuries by then anyways).

    I'm not going to double check your specific numbers, but if you got 3*10^23 that's close enough to a yottajoule, we're in the same ballpark. Good.

    Yes, power consumption is a HUGE problem for this kind of travel. If you want more info, I have an article you might want to read.
     
  12. Sep 23, 2009 #11

    JesseM

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    calculating the energy in terms of nuke plants for a 38:1 fuel-to-payload mass ratio is kind of silly from a realistic point of view, because any civilization that can build photon drives that convert mass into photons with optimal efficiency (which would require something like a matter/antimatter drive) is probably going to have much better sources of power than fission-based nuclear power. If you want to assume technology not too far advanced beyond our own, you should probably assume some more feasible propulsion method like an ion drive or nuclear/electric propulsion...here's something I wrote about different propulsion methods on another thread:
     
  13. Sep 24, 2009 #12
    Yes please.
     
  14. Sep 24, 2009 #13
    My goal is to put the energy required into perspective, not an excersize to actually do it. An analogy, like what a trillion dollars looks like since that is so difficult to see. ($1,000 bills stacked 65 miles high). So to put that energy requirement into nuke plant eqivilent, 1000 for each person on the planet, kinda puts the huge huge massive number into perspective.

    Though your number above makes my number puny in comparison. So what's the message here?

    Is space travel by ANY civilzation actually physicaly possible? I think not, but that's this primative mind from this perspective. Drake was right.
     
  15. Sep 25, 2009 #14
    I would like to read it too, please; I've actually been wondering about that for a long time (the theoretical minimum fuel required to move an n kg. mass across a distance x at relativistic speeds with a 100% efficient engine).

    -- faye kane, idiot savant
     
  16. Sep 25, 2009 #15
    You might also find this web page of use which talks about accelerating at 1g to our nearest star using a 100% efficient propulsion system (i.e. matter-antimatter engine) and the fuel required-

    http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  17. Jan 21, 2011 #16
    I heard that there was some theoretical propulsion method that grabbed the material necessary for fuel from space as you went along. Did I understand that correctly? If so. and making the enormous assumption that such a propulsion system could actually work in practice, wouldn't that solve the fuel weight problem?
     
  18. Jan 21, 2011 #17

    jtbell

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  19. Jan 21, 2011 #18

    JesseM

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    But as noted in that article, later analysis led to the consensus that drag would outweigh forward propulsion, though this would work as a braking mechanism so you wouldn't need fuel to decelerate as you approached your destination.

    The article does mention that the idea could be modified by having fuel pellets (similar to the ones imagined for use in project Daedalus) laid out in advance on the path the ship would take, an idea which I remember was discussed in more detail in the book https://www.amazon.com/Centauri-Dreams-Imagining-Interstellar-Exploration/dp/038700436X (and also in this entry from the blog of the same name). Another possibility that would avoid the need for carrying huge amounts of fuel is some sort of beam-powered propulsion, often imagined to be a really huge laser aimed at some kind of solar sail type craft, but I've also read about the possibility of little pellets being accelerated to relativistic velocities in the Solar System (perhaps using a mass driver, or perhaps a laser with the pellets themselves being miniature solar sails, an idea discussed in the last part of this entry from the Centauri Dreams site) and aimed at a pusher plate on an interstellar trip--with some kind of basic nanotechnology the pellets could do a small amount of self-steering so the problem of the beam spreading out as it travels (and thus a lot of the energy missing the ship) could be avoided. This idea is also discussed in Centauri Dreams on p. 142-146, followed by the discussion of the idea I mentioned earlier of laying out fuel pellets on the track of a spacecraft in advance.
     
    Last edited by a moderator: May 5, 2017
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