This question is inspired by two existing spacecraft propulsion concepts.

  • Beam-powered propulsion involves a spacecraft being propelled by a beam of some kind, and one potential type of beam would consist of particles moving at high speed (e.g. the MagBeam concept). This means that the spacecraft would not have to carry its own reaction mass or power supply, it would just need a magnetic sail to reflect the particle beam, making it faster and/or increasing its payload. A downside to using a particle beam is that the beam-producing facility would experience recoil and also need to be refuelled.
  • Powdered regolith propulsion is similar to ion propulsion, except that it involves accelerating solid powder instead of ions. The isp is lower than ion propulsion but the thrust is higher. And the powder would be made by grinding up lunar or asteroidal regolith, making it a widely available resource. However, spacecraft using powdered regolith propulsion would still have to carry their own reaction mass plus an engine and power supply.

It seems to me that these two concepts could be merged into one: "beamed regolith propulsion". Basically, you would have facilities on moons or large asteroids that process regolith into powder and beam it towards spacecraft. The spacecraft have much better performance than if they had to carry their own reaction mass, engines and power supplies. The beam-producing facilities have access to effectively infinite supplies of powder, and the moon/asteroid absorbs the recoil of their beams.

Would this be a useful method of propelling interplanetary spacecraft? And on a related note, would it be viable for launching spacecraft off the surface of airless bodies like Earth's Moon?

  • $\begingroup$ Are you suggesting propelling a spacecraft through high-speed impacts of small particles? $\endgroup$ – GdD Nov 20 '20 at 11:22
  • $\begingroup$ In most cases when wishes for one's power beam to have vastly less momentum than one's thruster exhaust. If this is not the case navigation becomes awkward $\endgroup$ – ikrase Nov 20 '20 at 11:37
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    $\begingroup$ Depending on direction, and given some sort of deflector (acting like a mirror, just for solid particles) there would be some freedom in directing the propulsion. It would still be limited by the speed of the 'beam' (unlike a rocket), and require own propulsion to stay in the stream, but seems like a viable 'second stage' for interplanetary travel for use shortly after a lunar launch, a "departure burn" replacement. $\endgroup$ – SF. Nov 20 '20 at 13:07

Such a propulsion method would have extraordinary challenges with aiming the beam over interplanetary distances. Let us ignore the issue of beam dispersion (which, let us assume, has been solved by 0xDBFB7's space-charge neutralization method). The issue we would now face is solar radiation pressure and the unpredictable solar magnetic field.

For radiation pressure, there are two distinct effects that we can consider, but they appear to be dependent on the dust grains' size. Presuming that the dust grains are of uniform size, then the grains will be off course in one of two ways: if they are large enough, they will lose orbital momentum, and if they are small enough they will gain it.

Let us now presume that the dust grains aren't travelling relativistically. They are basically tiny impactors, so they can't be moving that fast or else you'll just obliterate your spacecraft. If they are moving so fast, then you'd need a massive ablative plate attached to the back, at which point much better options exist that don't require mass drivers.

If they are moving non-relativistically and at a low enough velocity relative to the spacecraft to avoid blowing it up, then they are essentially orbiting the sun. Their orbits are probably hyperbolically but honestly I don't know what speed would be required to maximize energy while minimizing destruction of the spacecraft. It has been shown that dust grains interact with solar radiation pressure quite a bit, and that this interaction changes their trajectory.

Now, the sun is not a constant bulb of light. As a result, it is practically impossible to predict the exact radiation pressure acting on the dust beam at all points in its travel and therefore it is impossible to aim it. Although the actual change in trajectory would be absurdly small, so would the spacecraft, so if we are off by even a few meters we at least lose propulsive efficiency.

However, radiation pressure isn't the only issue. If the dust particles remain charged, and I do not see why they wouldn't, then they would be moving charged particles in a magnetic field. The fields of planets and the sun are fantastically complex and dynamic, and as such cannot be predicted for all points on the dust's orbits. The force acting on the dust is given by the Lorentz force, and it would likely be fairly small (I don't really feel like running the numbers, so if someone wants to edit this answer feel free).

This effect alone would disperse the dust, or at least make it so hard to aim that your propulsion method would only be feasible at short distances.

So ultimately you'd be unable to hit the spacecraft with any accuracy. Compared to laser propulsion methods, which are being studied and which don't have this issue, the idea doesn't sound like it will work very well.

  • $\begingroup$ Thanks for the detailed response. I will point out, though, that the dust in this idea would be hitting a magnetic sail attached to the spacecraft, not the main body of the spacecraft. $\endgroup$ – Pitto Nov 20 '20 at 21:49
  • $\begingroup$ I don't think it's viable on interplanetary distances but could be a reasonable method to use on distances up to ~10,000km - essentially the initial push, "departure burn". $\endgroup$ – SF. Nov 20 '20 at 22:42
  • $\begingroup$ @piito yes... But if you aim even a little bit wrong you're hitting the spacecraft $\endgroup$ – Michael Stachowsky Nov 21 '20 at 0:32

How do you keep the beam coherent? To accelerate the particles they have to be charged. Particles end up all with positive charge, so repel each other once they get free of drive field. I think the beam would fan out rapidly.

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    $\begingroup$ Good point. There's a technique called space-charge neutralization (or 'compensation'), where an electron beam of equal charge is injected into the ion beam. $\endgroup$ – 0xDBFB7 Nov 20 '20 at 15:43

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