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Space Travel

  1. Dec 14, 2009 #1
    Hi all,

    I'm writing myself a science fiction book. I'm also trying to keep the science and the fiction as separate as I can, space is just the setting, I'm focusing on the fiction but would like the science to be possible. So I've been reading a lot of stuff on wikipedia and all over the netscape trying to basically come up with scientific explanations for a lot of the technology that is/will be present in my book.

    Science and Physics were never my favorite subjects back in the days of the old schoolyard but I am interested in things that are interesting (such as science and physics). As such I understand more than what a lot of people probably do but a lot less than what I need. So I've come here looking for help.

    I would be highly appreciative of anyone who would be able to correct anything I've got wrong or anyone who could answer any of the questions below. If there's a decent book or excellent website that anyone knows about I would love to know about it.

    From my understanding of the physics of space, for something to move there needs to be a force greater than its mass to move it. So for example a spaceship that weighted 100kg would need a force of 101kg to move it (please correct me here if needed). This force would come in the form of a rocket or thruster propelling something out of it at the required force.

    So this means that a larger rocket/thruster would propel the craft faster and a larger craft would need a larger thruster to get it moving at the same rate as a smaller craft with a smaller thruster?

    If a larger craft requires a larger thrust to get it moving then to travel at the same rate as a smaller craft with a smaller thruster it would need to carry more propellant, meaning it would weigh more meaning it would need a bigger thruster etc etc? Would this mean that a smaller craft will always be able to accelerate faster than a larger craft (using the same thrusters with just size differences)?

    Ion Drives? They electrocize atoms and then use magnets to shoot them out at high speed creating enough force to propel the craft?

    Rocket Engines work much the same but it's heated gas that is expelled rather than ions?

    Fusion power. This is the process of smacking two atoms together to force them to become other atoms with the reaction giving off heat? It's the heat that would be used to generate electricity? Fusion is not proven yet?

    What is plasma? I know it has something to do with electricity but I've not been able to find much info on it's use in space travel (if even feasible).

    Thanks for any help.
  2. jcsd
  3. Dec 14, 2009 #2


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    Good luck with your writing. I'll see if I can offer some useful comments for you.

    This is quite wrong I'm afraid. Applying any force to a body, particularly in space where we don't have to worry about friction, will result in a change in the bodie's velocity, which is what acceleration is. Newtons second law describes the details:

    Force applied = mass of body X acceleration of the body

    Lets re-arrange this to see it more clearly

    acceleration of body = Force applied / mass of body

    So, if you have a given force, applied by a rocket or whatever, then the more massive the body (i.e the spaceship) is, the less acceleration will be impart. But not matter how small the force is or how massive the ship is any force applied will change the ships velocity.

    In the abscence of any forces being applied, the velocity will stay the same. You don't need to keep the rockets on to stay at a certain speed, if your rockets are on you will be getting faster and if you turn them off you'll maintain that speed until you either use rockets again or hit something...

    As above, you are mixing acceleration with velocity (or speed). Comparing big and small rockets used on the same ship, if you start from rest you'd need to fire the big rockets for less time to reach a certain speed compared to using smaller rockets, but either set could get you to that same speed. Once you reach that speed you have to turn the rockets off otherwise you will just keep getting faster and faster.

    That's correct.



    What do you mean by proven? We know that fusion is what powers stars and we also routinely perform fusion in the lab (and we also have fusion bombs which are even more powerful than the fission bombs used in Hiroshima and Nagasaki). We haven't yet mangaed to, in the lab, get more energy out from fusion than we put in to make the atoms fuse, so we can't yet use it as a usefull energy source. But the prospects for improvements in fusion technology are good, and hopefully this will be a viable power source in a few decades time. It's certainly a 'safe' technology to assume will exist in the future when you're writing sci-fi.

    Plasma is the so called 'fourth state' of matter (after solid, liquid and gas). A plasma is a hot gas that is ionised, meaning it is sufficiently hot that the atoms/molecules making up the gas have shed some of their outer electrons, such that the gas contains hot electrons and hot postive ions. Plasma is not a very 'exotic'. It is the cornerstone of many commonplace items available today. I'm not sure if it has much relevance for a space travel sci-fi story. Present fusion technology involves getting the fusing material into a plasma state, but as I say it's not really an exotic or futuristic technology.

    As a general note, I find that good sci-fi is much more about having a really good story and characters, rather than being about getting the science 'right'. There's obviously clangers you'd want to avoid (for instance you'd want to at least get Newtonian physics right), but personally I much prefer sci-fi that just states that a ship has 'a warp drive' that allows FTL travel, if that is required for the story, rather than trying to go into the nitty gritty about how such a drive works. Sci-fi is supposed to contain elements that are 'impossible' today, but trying to explain them in todays terms won't work, because by definition, it's impossible!

    If you really know your physics well, you can incorporate some speculative things into the story, but it is not essential and can muddy the water unneccesarily. Lawrence Krauss wrote a great book about the physics of Star Trek, but he's a professor of physics, so could get the details correct with much more ease than you will be able to.

    That's my view anyway, you may take it or leave it. Good luck with the writing!
  4. Dec 14, 2009 #3
    Thanks a lot man. You've been a big help. You are right about the fiction being more important but I had a feeling I was getting the science wrong.

    Just another quick question though. If I'm in a space suit outside a spaceship and I pushed against the spaceship am I right to now assume that the both of us would move away from each other rather than I would move away and the spaceship would remain stationary?
  5. Dec 14, 2009 #4
    Correct, but since most spaceships are much heavier than most humans, the ship would move a lot less than you. Were you to push off the Vanguard-1 satellite - the first US object in orbit, which was nicknamed the US space grapefruit by Khruschev due to its 1.47kg weight - you'd obviously push it much further than you. The relevant equations are related to the conservation of linear momentum, where, assuming initial velocity for the combined system is zero, Your Mass x Velocity = Spaceship Mass x Velocity. The two cancel each other out since velocity, as opposed to speed, is a vector quantity, meaning that its direction is very important. How fast either of you go depends on how much energy you impart - where the scalar quantity, kinetic energy, comes in.

    The simplicity of linear momentum assumes the force your muscles supply is directed in a line along both your centres of mass, yours and the spaceship. If you've offset your push then the conservation of angular momentum is invoked and things become more complicated, but still balanced.

    Whenever you jump, you push the Earth away a little bit. When you fall back down, you pull it toward you a little bit. Whenever you drive along the surface of the Earth, you spin it underneath you.
    Last edited: Dec 14, 2009
  6. Dec 14, 2009 #5


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    The acceleration deals with a = F/m (in the absence of an external force like gravity or drag), where F is thrust, but we have to look at all the forces.

    So a = (F - W - D)/m. If drag (D) is neglible, then a = (F-W)/m or a = athrust - g, where g is local acceleration due to gravity.

    At the surface of the earth, neglecting drag, a 101 kgf would accelerate 100 kgf at 0.01 m/s2, which would be pretty slow. Out in space, far away from any significant gravity, 101 kgf would accelerate the 100 kg at 1.01 m/s2, or about 0.103g.
  7. Dec 15, 2009 #6
    Thanks for your help guys. I'll probably post more questions at another time.
  8. Dec 15, 2009 #7
    15 minutes later and I've got another question. I just read that the weightlessness astronauts feel in orbit is actually free-fall weightlessness on account of zipping around in an orbit. (http://scienceray.com/physics/artificial-gravity/)

    So... Does that mean if I was in a spaceship smack bang in the middle of space, with no masses around me that I would no longer be weightless? Would I "stick" to the inside hull?
  9. Dec 15, 2009 #8


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    "weight" is a phenomenon we experience because the gravitational pull of the Earth is ever present in our lives. We feel weighty because gravity it always trying to pull us down, but the ground gets in the way and pushes against that. When you stand on the surface of the Earth what you feel is actually the ground pushing you up, rather than the gravity pulling you down.

    Therefore, the feeling we call 'weightlessness' is the abscence of that familiar upwards push. You could experience this in say an elevator that is falling very rapidly, if it falls at the same rate as you do you will not feel it push you upwards, hence you feel 'weightless'. Astronauts in orbit feel 'weightless' for the same reason; they are falling at the same rate as the ship, hence there is no push reaction force on them.

    To answer your question, if you are travelling on a ship in deep space, if the ship is not accelerating (it is maintainging a constant velocity) then everyone on board will feel 'weightless'. There is no gravity pushing them in any particular direction, and no walls pushing back against such a tug.

    If the ship accelerates, then it will feel, to the people in board, that there is suddenly a gravitational field pointed in the opposite direction to the acceleration. So if you accelerate forwards, you will feel like 'gravity' is trying to push you to the back of the ship. The walls parrallel to the back of the ship will preventing you doing this, so standing on a wall will feel like standing on the Earth's surface (the 'wall' now feels like it's oriented as the 'floor'). As soon as the acceleration stops, you'll feel 'weightless' again though.

    Most sci-fi either throws in a glib line about 'artifical inertia' (star trek) which somehow creates a gravity like feeling in a preferred direction, or else just ignores this issue entirely (such as star wars, Battlestar Galactica etc). I think space odessey 2001 treats this realistically, but I haven't seen this for many years so can't remember it clearly. Basically to film or describe a realistic situation would require that the familiar ways we walk around, sit on chairs etc be completely different. Therefore most sci-fi just ignores this for the sake of simplicity and just pretends things work the same as they do on Earth, despite the physics not making sense.
  10. Dec 15, 2009 #9
    You're always going to feel weight - the 75kg of your own body mass exerts an attractive force on itself, although it is almost nothing. Over time you will drift towards the combined centre of mass of you and the spacecraft, overshooting it, and leading to the world's most boring oscillations, and their eventual damping by air resistance.

    If it had a nuclear reactor, for example, which is typically surrounded by massive shielding, you might drift toward that. However, unless we're talking a moon-sized spacecraft (*ahem Death Star*) the attractive forces would be so miniscule you'd barely notice them, and any noticeable effect might very well be overwhelmed by your own actions, air currents, or even plain old thermal energy.

    One thing to note is that astronauts do not 'escape' Earth's gravity like I've heard people say over the years; like any mass its gravitational influence extends infinitely far. They may attain escape velocity, which means the Earth will exert enough attraction to pull them back, and they'll go on to infinity distance from the Earth. Rather, as Wallace describes, the perception of gravity goes.
  11. Dec 15, 2009 #10

    Force = mass x acceleration

    So, if you have a huge mass, but a tiny force, it will cause the speed to increase slightly. One thing that happens once you get outside of a gravity well is that the important thing is "impulse". One Newton of force spread out over 10000 seconds will cause the same velocity change as 1000 newtons of force over 1 second. This is important for things like ion rockets and solar sails. An ion rocket will only produce a few newtons of force, but it will do it over several months, and the same for solar sails. So you can use ion rockets and solar sails to get something moving very fast, by applying a small amount of force over a long period of time.

    Plasma is a form of matter in which electrons are split off from their nuclei. Fire is a plasma.
  12. Dec 15, 2009 #11
    So if the velocity is 0 and acceleration is 0 then I would still float around?
  13. Dec 15, 2009 #12


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    Yup. In fact, if you were on board the ship (and didn't have the benefit of looking out any windows) you would not be able to decide what speed you were going at! For all you could discern, it could be moving at 0m/s or .999999c.
  14. Dec 15, 2009 #13
    You'll float around unless your spacecraft is being accelerated by something other than gravity. If there's no mass nearby, and your rocket engines are turned off, you'll feel weightless; if you turn your thrusters on, then you'll feel weight. If the only thing accelerating your spacecraft is gravity (e.g. if you're in orbit or falling towards a planet), then you'll feel weightless. If something is blocking your fall, such as the ground or a rocket thrust, then you'll feel weight. Velocity has no effect on whether you feel weightless or not; it's an arbitrary value that depends on what you chose to measure yourself moving with respect to.
  15. Dec 16, 2009 #14
    This is very interesting. Without constant acceleration there's no gravity.

    I'm going to writing like crazy with this.
  16. Dec 16, 2009 #15


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    The link between gravity and acceleration is the heart of the Equivalence Principle, from which Einstein formulated General Relativity (at least in one version of history).
  17. Dec 18, 2009 #16
    Gravity is a fundamental property of matter - if there's "stuff" then there's gravity. We just feel it when something gets in the way of a free-fall, like the solid Earth or air resistance or the back of a spaceship wall. If all the atoms of a ship could feel the same force simultaneously then you wouldn't notice you were accelerating - you'd be free-falling. But rockets move by reaction forces - they throw stuff out one end and move in the opposite direction in reaction. Stuff shoving other stuff around, including the crew and passengers. Thus everyone feels the acceleration just like we feel the ground pushing against us when every atom in our body "wants" to fall freely. That's inertia - matter's resistance to changing course and being held against free-fall.

    What's the SF story about then?
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