[oliveshell]

It has to do with the interaction between solar wind and interstellar plasma.

There is a relatively sharp transition— the Heliopause— from solar wind dominating to interstellar “wind” (from other stars) dominating, which occurs where their respective radiation pressures are roughly equal:

https://en.wikipedia.org/wiki/Heliosphere#Additional_heliosp...

[daveguy]

Is there a prevailing wind in interstellar space from other stars? Seems like it would be chaotic eddies near net zero. But maybe due to solar system motion or something like that there is a prevailing direction. If so would that mean we could use solar sails to continue to accellerate a spacecraft (at a much reduced rate) outside the heliosphere as long as we want to go in a specific direction arc?

[oliveshell]

Yes, the apparent direction of the interstellar wind is due to the motion of our solar system through a 30-light-year-wide cloud of gas [1].

The specific dynamics of the cloud (e.g., its turbulence) are still largely unknown, though the fact that the direction of intersellar wind has shifted by several degrees over a few decades hints that it’s not totally calm.

I’m not sure about solar sails outside the heliosphere... that’s an interesting question. I’m not an expert, just a guy who likes reading about this stuff.

[1]: https://en.wikipedia.org/wiki/Local_Interstellar_Cloud