B Expected Earth-relative velocity of interstellar particles

[Grinkle]

TL;DR Summary

Do we expect interstellar particles to be co-moving with us?

I recently saw (on a pop-sci video) an opinion that the engineering challenges involved in interstellar travel are insurmountable.

One of the challenges discussed was how to protect a ship moving at, say 0.2c, from being disintegrated over time by collisions with interstellar particles. That made me wonder why the hypothesized velocity of the vessel matters.

Are most interstellar particles expected to be more or less co-moving with the earth so that it matters if a vessel launched from Earth is noticeably NOT co-moving with Earth? I'd have thought that interstellar particles will have lots of different velocities relative to Earth, a distribution of velocities that is distributed like white noise, not even Gaussian (relative to Earth's frame) and the velocity of a vessel relative to Earth is not relevant in predicting the average velocity of a potential collision with an interstellar particle.

 

[PeterDonis]

Are most interstellar particles expected to be more or less co-moving with the earth

No, but they are expected to be more or less comoving with the center of mass of the solar system, at least in the vicinity of the solar system. Or, to put it in a way more directly relevant to this discussion, their speeds relative to the center of mass of the solar system are expected to be very small compared to the speed of light, so an interstellar ship moving at 0.2c relative to the solar system will be moving at a speed reasonably close to that relative to interstellar particles.

 

[PeterDonis]

I'd have thought that interstellar particles will have lots of different velocities relative to Earth, a distribution of velocities that is distributed like white noise

Why would you think that?

 

[Ibix]

Inside the galaxy, I would expect that interstellar dust is mostly doing more-or-less the same more-or-less galactic orbital velocity the Sun is doing (of order $0.001c$). For anything that isn't, collisions and other interactions would force it into step in relatively short order, unless it's just going so fast it escapes

So yes, I would expect collision risk to be a serious issue for a 0.2c vehicle.

 

[Grinkle]

Why would you think that?

I was thinking that the process of a particle gradually gaining velocity via collisions with other particles would be pretty random with respect to the resultant velocity vector of the particle. If I had thought more I might have considered that most particles would tend to be, as @Ibix noted, in orbit around the Galactic center of mass, and this is the dominant input into their relative velocity, collisions and other perturbations that would cause a rogue particle going very fast in an arbitrary direction being much less common.

 

[PeterDonis]

I was thinking that the process of a particle gradually gaining velocity via collisions with other particles

Why would you expect one particle to gradually gain velocity from collisions?

 

[Ibix]

I was thinking that the process of a particle gradually gaining velocity via collisions with other particles would be pretty random with respect to the resultant velocity vector of the particle. If I had thought more I might have considered that most particles would tend to be, as @Ibix noted, in orbit around the Galactic center of mass, and this is the dominant input into their relative velocity, collisions and other perturbations that would cause a rogue particle going very fast in an arbitrary direction being much less common.

You can see the interstellar medium as a very low pressure gas. If you inject a random high velocity particle it'll either escape, or dump its energy into the other particles of the gas and cool to their typical speeds.

 

[Grinkle]

@PeterDonis @Ibix Thanks, both - I see the connection between posts 6 and 7.