Let assume a metal tube open on one side having alpha emitter material on the other blocked side. Since alpha particles are blocked by the metal they will be emitted only through the open side and make thrust by the momentum of the alpha particles. Very small one, but continually accelerate the satellite. Partially depends on other question - how alpha particles are emitted from in depth of a chunk of material, aren't they blocked by the outer layer of this chunk?

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    $\begingroup$ Only the alpha decays close to the surface will be able to emit ions with enough energy en.wikipedia.org/wiki/Alpha_decay , IMO such a device would not generate enough thrust to be measured . You might be interested in this en.wikipedia.org/wiki/Ion_thruster $\endgroup$
    – anna v
    Commented Jul 18, 2023 at 9:15
  • $\begingroup$ You know the mass of 1 alpha , so please do yourself the calculation how many alphas you would need going in one direction to get say 1kg accelerated. This material would have to be very active so it would have a very short half life. $\endgroup$
    – trula
    Commented Jul 18, 2023 at 10:14
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    $\begingroup$ "Jet engine" is a term of art in the aerospace industry. Jet engines react fuel with oxygen from the surrounding air to produce the jet of exhaust that propels the aircraft. The aerospace term of art for an engine that can produce a similar jet of exhaust without using atmospheric oxygen is "rocket engine." $\endgroup$ Commented Jul 18, 2023 at 14:11

2 Answers 2


Side note:

Several people have told you that the thrust would be negligible. But, the thrust actually would be zero unless your spacecraft also were fitted with some means of emitting electrons or negatively charged ions in the same direction as the alpha particles. For best efficiency, the negatively charged particles should be emitted at the same average speed W.R.T. the spacecraft as the alpha particles have.

Each alpha particle carries a +2 charge. If you don't provide some means of sending -2 charge along with each alpha, the negative charge will build up on the spacecraft to the point where it pulls all of the alphas back to itself, and there is no net thrust.


The thrust would be around 9.8936e-11 N e.g. for Americium-241, Specific activity of : 3.7 x 10^13 Bq/g Molar mass of Americium-241: 241 g/mole Avogadro's number: 6.022 x 10^23 particles/mole

Number of particles emitted per second = (Specific activity * Molar mass) / Avogadro's number Number of particles emitted per second = (3.7 x 10^13 Bq/g) * (241 g/mole) / (6.022 x 10^23 particles/mole)

Number of particles emitted per second ≈ 1.49 x 10^9 particles/second

Mass of the alpha particle (m): Approximately 6.64 x 10^-27 kg (twice the mass of a proton and neutron). Velocity of the alpha particle (v): Approximately 10^7 m/s (a typical velocity for alpha particles).

Using the equation for momentum (p = mv), we can calculate the momentum of the alpha particle: p = (6.64 x 10^-27 kg) * (10^7 m/s) p ≈ 6.64 x 10^-20 kg·m/s

Since we assume the alpha particle is expelled in one direction, the change in momentum (Δp) is equal to the momentum carried by the particle.

The thrust (T) produced by the alpha particle can be estimated by assuming a small time interval (Δt) over which the momentum change occurs. The thrust is then given by the change in momentum per unit time (T = Δp/Δt).

Using Δt = 1 second, we can estimate the thrust produced by the alpha particle: T = (6.64 x 10^-20 kg·m/s) / (1 second) T ≈ 6.64 x 10^-20 N

multiplying this result with above one gives ~9.8936e-11 N Compared to the thrust of e.g. Ion thruster: in the mN range up to several N

  • $\begingroup$ Technically it is a nigthmare, small, shielding, $\endgroup$
    – Samson
    Commented Jul 18, 2023 at 12:07

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