A nice feature of using the speed-of-sound as a measurement unit is that people know how difficult it is for aircraft to achieve it. So it makes it clear how much faster these things are going. We don’t have anything comparable between the speed-of-sound and the speed-of-light, do we? I suppose you could use escape-velocity, that isn’t something as many people know, but does I guess get you closer to the speeds in question.
There is nothing trivial about it. The only reason Voyagers are traveling so fast is we were very lucky at the time and got gravity boost from pretty much everything we could get gravity boost from.
But yeah, it is not comparable as the challenges for spacecraft and planes are completely different.
> The only reason Voyagers are traveling so fast is we were very lucky at the time
“Lucky”, only in the sense that (1) completing a large government project on time, and (2) not having some kind of disaster (particularly, at launch) screw up the mission require a certain degree of luck of luck on top of planning and execution (though, not relying completely on that luck is also why there were two Voyagers): we got all the gravity boosts because the mission was planned around an alignment that enabled it to do that and visiting each of the outer planets (which was really the main goal; the beyond the solar system part was gravy.)
"Lucky" because the planets LITERALLY aligned for this to work. This kind of alignment only happens very, very rarely.
The New Horizons probe was launched at much faster speed than Voyagers, actually beating the record of the absolute fastest launch in history, but because of not getting those gravity assists it will never overtake Voyagers.
