It’s not a perfect vacuum; there are particles — so there is some speed at which pressure waves propagate through the medium. That’s the speed of sound. Speed

It’s not a perfect vacuum; there are particles — so there is some speed at which pressure waves propagate through the medium. That’s the speed of sound. Speeds faster than that speed are "supersonic”. http://en.wikipedia.org/wiki/Heliosphere#Termination_shock http://en.wikipedia.org/wiki/Heliosphere#Termination_shock expands on this somewhat.