tl;dr: What is the density of "space dust" or micrometeorites in interstellar space, and how likely is it that a tiny interstellar craft could avoid hitting any?

In a future time we will likely figure out how to accelerate a usable space probe to 0.1c and shoot it to Alpha Centuri, considering there is already serious work being done on this topic.

If such a craft hits a particle, even something smaller than a grain of sand, it will probably be game-over for it (0.5*0.67mg*(0.1c)^2 = 300,000,000 joules, which is about a ton of TNT).

Considering that there are a lot of micrometeorites, at least around earth, and navigation at 0.1c is fairly difficult, what would be the chance of a space probe making it through interstellar space?

  • $\begingroup$ How should we know the density of interstellar dust? It is not measurable using telescopes. There is too few data from spaceprobes. By the way, a ton of TNT is equivalent to 4.184 · 10^9 J or 4,184,000,000 joules. So your particle should be a lot heavier. $\endgroup$ – Uwe Apr 1 '19 at 15:53
  • $\begingroup$ Isaac Arthur did a YouTube video on this. I forget which one (an early one). The chances depend on the size of the particle (it decreases with increase in size). But I think the chance of not hitting something is basically zero. You need frickin' lasers! $\endgroup$ – rghome Apr 3 '19 at 14:34

According to the Ohio State University's Astronomy department

Composition: carbon, metals, silicates, and ice
Size of grains: 500 nanometers or less in diameter (1 nanometer = 1 billionth of a meter)
Number density of grains: 1 per million cubic meters 

Of course, that small isn't likely to cause a huge issue, but 100% any spacecraft will hit many 500 nm particles.

Grains of sand are around 50 μm, so about 100 times larger. I don't know the distribution of larger particles, but it seems quite likely that there will be a fairly large number of such particles.

From this paper, the larger particles seem to be quite a bit rarer, but it seems still pretty likely that one will have to manage such sizes of particles, because space is really big.

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