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Solar sail using radioactive decay

  1. Jun 28, 2011 #1
    Could you make a solar sail that derives it's energy from the decay of a radioactive element such as plutonium, uranium, or palonium? I understand that such elements radiate in all directions but if you had a 1 kg sphere of it attached to a boon which was attached to a reflective sail (e.g. aluminum) would you not be able to harness the total momentum generated by the alpha, beta, or gamma particles? If the alpha particles are absorbed rather than reflected then...couldn't you encase the radioactive element in a lead casing, allowing it to heat and emit it's energy via infrared to the solar sail? I understand that gamma will likely sail on through but most of the energy released in decay is from alpha particles which are easily blocked. My estimate is that you could get up to 1/1000th the speed of light from the energy that is released over a 100 day decay of palonium (140 W/gram). This wouldn't be worthwhile for interstellar travel but you could zip around the solar system quite nicely
     
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  3. Jun 28, 2011 #2

    Drakkith

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    Will the amount of thrust produced by this overcome any of the gravitational fields of the plantets and sun?
     
  4. Jun 29, 2011 #3
    The problem is that you've got the radioactive source chained to you.

    I'd like to see your calculations that you can get to 0.001c, since I don't see how that would work.
     
  5. Jun 29, 2011 #4

    Drakkith

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    Why would that be a problem? If the source is behind the sail then most of the particles either get emitted directly aft of the ship or get reflected off the sail.
     
  6. Jun 29, 2011 #5
    Newton three will stop you in your tracks - draw a diagram of the energy emitter with an arrow in the direction of the momentum change cause by the emission; then do the same for the reflector and see if you can get any net momentum.

    What you could conceivably do is have a shielded radioactive source with a hole at the back to let radiation out, but if that were practical it would have been done.

    Take a look at the Pioneer anomaly and see if you can work out what's going on.
     
  7. Jun 29, 2011 #6

    Drakkith

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    I don't see the difference in this and a standard rocket engine. Both involve expelling mass (or momentum) and channeling it in one direction.
     
  8. Jun 29, 2011 #7

    Chronos

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    If you mount a fan on a boat blowing into a sail on the boat, the boat will not be propelled. The vacuum pressure behind the fan will counteract the propulsive force of the air directed into the sail.
     
  9. Jun 29, 2011 #8
    I could be missing your point but the use of a fan on earth and the decay of a radioactive element are entirely different. The emission of radioactive particles in space doesnt create a vaccuum. I do agree that if you simply had a radioactive sphere sitting in space that it would not move because the omni-directional emission of the particles would cancel out. However, now imagine that a solar sail is attached as described. If the particles are absorbed (reflectivity = 1) then the net force would be in the direction of the sail. This means that of the total energy or momentum emitted you would retrieve half of it back. This really isn't that different than how a rocket works with mass being propelled away. Radioactive decay has been used as a form of energy for batteries in space for some time. Nasa is even testing an ion propulsion system that uses radioactive decay as a source of energy.

    There are 2 different ways I use to calculate the velocity (disregarding drag from planets and sun). If that causes problems of thrust then we could simply scale the amount of radioactive material up from 1kg to 1000kg or more. This wouldn't increase the velocity because you are adding mass but it would make the drag insignificant.

    The first way is to simply calculate how much energy is emitted by a radioactive element over it's life-time. With the understanding that it decreases over time. For 1kg of palonium this is roughly 1.0 * 10^12 Joules/kg (wiki palonium and calculation is straight forward). So you lose half of the energy and you are at 5.0 *10^11 Joules/kg. If you do a simply .5mv^2 equation you wind up with a 1 kg mass traveling at .003c. But of course nothing is entirely effecient and you have to add in the mass of the sale and boon etc so I figure you end up going about .001c.

    Another way to calculate it is to use the radiation pressure. The emission of alpha particles from a radioactive source is akin to the solar wind solar winds (which have rest mass). The average velocity of a solar wind particle is 500km/sec. The average velocity of a 5 MeV alpha particle is 15000km/sec. So using the radiation pressure equation of
    atomic mass * particle density * (Velocity^2)/2 yields a much higher speed of .05c. I don't know why, perhaps because this is more complicated with Curies and number of atoms etc.

    But I think if there is any weakness to it, it is in the way to calculate radiation pressure of mass particles (versus light).

    Or perhaps another way to ask the question- if you could attach a gigantic solar sail to the sun...how much could you make it move?
     
  10. Jun 29, 2011 #9

    russ_watters

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    Wind doesn't bounce off a sail, but em radiation does bounce off a solar sail, so they aren't the same issue[caveat: I don't know how well particle radiation or high energy em radiation bounces]. Similar to the OP's idea, a flashlight with a parabolic reflector is basically a light rocket.

    The OP's proposal would work, but I'd be shocked if it worked well.
     
    Last edited: Jun 29, 2011
  11. Jun 29, 2011 #10
    In the end all I am proposing is to increase the radiation pressure on a solar sail. The extra bonus (minus a slight increase in mass) is that the emitter gets to travel with you. This means that unlike the Sun the pressure remains high over time/distance. After fusion and fission, radioactive decay has the highest energy density gram for gram of any energy storage system. Also the emission process won't break or falter. I tend to think that the high velocity of the alpha particles would spald a reflector so it might be better to put a casing of lead (add another couple of kilograms) around the emitter and use the radiant light generated from the hot lead. With space being a near vaccuum you don't lose any significant energy as the alpha particles total kinetic energy is transferred into thermal energy which is then emitted via light.
     
  12. Jun 30, 2011 #11
    The reason solar sails work is that you can make them light since they aren't accelerating their own "fuel." If you chain the sail to a radioactive source then you add inertia to the spacecraft which is bad.

    The fact that you are carrying your fuel with you is a bad thing.
     
    Last edited: Jun 30, 2011
  13. Jun 30, 2011 #12
    Can some one show me why the math of using 1000 kg of Palonium-210 doesnt doesnt produce a solar sail (mass of 1010 kg) with a velocity near .001c? Or if it doesnt work well, why does it not work well? I just can't help but think that a radioactive element emitting 140,000 Watts/kg wouldn't be an ideal way to propel a solar sail. What am I missing?
     
  14. Jun 30, 2011 #13
    I'm running through the numbers, but I think the main the problem is that very little of the energy will get converted to momentum. The problem is not energy. It's momentum. If the energy gets scattered in different directions, it's not going to help you, and most of the energy in radioactive decay is going to be emitted as thermal radiation.

    My suspicion is that if you had 1000 kg of Polonium, that there are more efficient ways of converting energy into momentum, than solar sails, and you could use it to drive an ion engine.

    It's really not. The thing about solar sails is that you can make them arbitararily big and arbritrarily light.
     
    Last edited: Jun 30, 2011
  15. Jun 30, 2011 #14

    Drakkith

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    Nothing in your post makes any sense to me josh.
     
  16. Jun 30, 2011 #15
    Whereas udtsith is asking some very good questions.

    I *think* the problem is that radioactive decay produces a lot of energy, but there is no obvious way of turning that decay into momentum. In particular, the problem with simply "sailing" with the block of polonium, is that most of the radioactive particles are going to get reabsorbed by the polonium so they turn into useless heat before hitting the sail.
     
  17. Jun 30, 2011 #16
    No, but once you get into interplanetary space, it doesn't matter. Once you get out of the gravity well, then your enemy is inertial and not gravity. So you can end up with rocket engines that produce small amounts of thrust for a long time.
     
  18. Jun 30, 2011 #17
    Thank you all for your posts and insights.

    About the heat that would be generated from the alpha particles smashing into other palonium atoms. First, because the palonium is in space (a near vaccuum), any thermal energy generated (heat) would be reradiated back.

    I thought what was so telling about the gold foil experiment was that any alpha particles actually came back to the detector at all (were reflected). This is because most of matter is in the nucleus and speeding alpha particles are bowling balls. Now if the particle is highly charged (e.g. 5 MeV) I would expect it to smash into a "reflector" the result is that some momentum will be transferred by the collision but most will go to heating the reflector up. If you drive a billiard ball into a scattering of billiard balls, most of the kinetic energy of the first ball will go into scattering the balls in all direction (heat) rather than forward momentum of the entire mass of balls. This is why I think it would be better to have a casing of some metal like lead. Now all the energy is thermal and you can use the radiant pressure equation (stephan boltzman * T^4)/c. The energy doesn't disappear it simply is radiated back out as light. Since space is a near vaccuum very little heat will be transferred by conduction to it's surrounding environment. This radiant energy (from infrared to x-ray) will then be used in the normal fashion for the solar sail. So I think you still recover your total energy.

    Or...you could make the palonium into a sheet and that way the "inner" palonium decay wouldn't be blocked by the radius of spherical/block shaped palonium. But you still have the problem of heating the reflector versus transferring momentum.

    Palonium might be too hard to work with (obtain/assemble/lift off). Plutonium has a much longer half life, history of being used aboard satellites, and wouldn't cause as much problem transferring it's heat. For greater speeds maybe you could use palonium as well as uranium. Once the palonium decays significantly (about a year) you let it go and get the energy from uranium which contains more energy than palonium but has a much longer half life (approx. 80 years).
     
  19. Jun 30, 2011 #18
    There are a few problems with using alpha decay. First, since decay is isotropic, you can only harness half the momentum (the half in the direction of the sail that reflects off the sail). Actually, you can harness even less. Decays that are normal to the surface of the sail provide forward momentum, but since momentum is a vector quantity, those decays at very oblique angles to the normal provide very little momentum.

    Second, increasing the half life means you have to increase the amount of radioactive material in order to get the same activity.

    Third, Po-210 is very difficult to both synthesize and isolate. Due to its radiotoxicity, it is also extremely difficult to handle. Only 1 microgram killed Litvenenko. Imagine if the launch vehicle exploded, spreading 1 kilograms of this over an area (thats 9 orders of magnitude!), especially a population center or water source.

    Finally, 14kW/s (assuming you could trasnfer all the energy into kinetic) isn't a lot when trying to accelerate something that massive. True, if you want to do it slowly, once it's out of the influence of earth's gravity it might work. But getting it to that point is a pain.
     
  20. Jul 1, 2011 #19

    Chronos

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    udtsith, you fail to comprehend the action-reaction principle of newtonian physics.
     
  21. Jul 1, 2011 #20
    You can do such and such a concept has been seriously examined by space propulsion researchers. What's needed are alpha and beta particle emitting radioactives - gamma rays would find the sail too transparent. Standard fission fuels aren't radioactive enough, so you need the fast decaying isotopes, probably transuranic elements.

    A related concept is the fission fragment rocket which minimizes waste heat and maximizes exhaust velocity by venting fission products directly. Does require some very tricky design and I am not sure all the bugs have been eliminated from the concept, but it does hold promise as an interstellar propulsion system.
     
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