Americium tends to radiate mostly as alpha particles. It would seem to me that as particles irradiated away, they would transfer momentum and the object would accelerate in the opposite direction. If a substantial amount of the object were this radioactive substance, could it be strategically placed and focused and used as a propulsion method for spacecraft?

Edit: https://en.wikipedia.org/wiki/Fission_sail


These aren't the best sources, but at least it's a topic that's been thought about.

  • $\begingroup$ what about a bulb vs a laser? $\endgroup$
    – lineage
    Nov 24, 2019 at 1:59
  • $\begingroup$ What do you mean? In that light can carry momentum too? $\endgroup$
    – Bowtie
    Nov 24, 2019 at 2:02
  • $\begingroup$ Thermal radiation has actually pushed some of NASA's probes slightly off course. $\endgroup$ Nov 24, 2019 at 2:03
  • $\begingroup$ radiation does carry away momentum energy so the substance emitting it does feel a reaction...unfortunately a macroscopic radioactive substance would in general radiate isotropically. Also the reaction generates heat instead of motion. This is analogous to bulb radiating in all directions. Unlike a collimated laser $\endgroup$
    – lineage
    Nov 24, 2019 at 2:05
  • $\begingroup$ @lineage Sure, a lump of americium (or other alpha emitter) will radiate isotropically, but Bowtie mentions placing it strategically and focusing the alpha particles, which sounds feasible. Alpha particles are fairly easy to focus and deflect electrically, they're just fast-moving helium ions. The tricky part is to minimize the number of particles that don't manage to get sent in the right direction and just waste their KE producing heat. $\endgroup$
    – PM 2Ring
    Nov 24, 2019 at 2:55

1 Answer 1


Yes, it would work fine. Then it mostly comes down to practical considerations. Momentum is mv (mass times velocity) and you always get the best specific impulse if you achieve that with as much v as possible compared to m, because you have to carry that m with you all the way until you use it. Alpha particles are a bit slow and have a high rest mass, and you'd get a better specific impulse from gammas. But specific impulse isn't everything and there might be other reasons to use an alpha emitter, I don't know.


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