## solar sail using radioactive decay

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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 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.

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## solar sail using radioactive decay

 Quote by twofish-quant 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.
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.
 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.
 Recognitions: Gold Member 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.
 Recognitions: Gold Member Science Advisor 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.

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 Quote by Chronos 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.
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.
 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.

 Quote by Drakkith 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.
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.
 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?

 Quote by udtsith 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?
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.

 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?
It's really not. The thing about solar sails is that you can make them arbitararily big and arbritrarily light.
 Recognitions: Gold Member Nothing in your post makes any sense to me josh.

 Quote by Drakkith Nothing in your post makes any sense to me josh.
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.

 Quote by Drakkith Will the amount of thrust produced by this overcome any of the gravitational fields of the plantets and sun?
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.