> Well, technically, 15km/s IS "more than 10x the speed of sound".

Technically, the speed of sound depends on the medium, and 15km/s is much slower than the speed of sound in interstellar space. (Which the sources I can find give at ~100km/s.)

honestly I was just shooting for easy round numbers. "More than 43x the speed of sound" doesn't have the same ring to it. And besides, as we all know "technically correct is the best kind of correct!" :)

So what was wrong about "40 times the speed of sound"?

Also, I don't particularly like the speed of sound for this comparison. Most people think of speed of sound as speed of sound at about sea level pressure, in gas composed of around 78% nitrogen and 21% oxygen and at roughly 25C temperatures. But the speed of sound is highly dependant on the medium and its temperature and pressure. There actually can be sound waves in space (pressure waves in interstellar gas resulting from various astronomic phenomena) and they propagate at very wide range of speeds, typically somewhere between 10 and 100km/s.

The main reason to use "speed of sound" is because important things change when objects travel at little below or above speed of sound in the medium they are in. But this is only useful in relation to the actual medium the object travels through.

One place where it trips people up is when they are talking high altitude airplanes or rocketry. They are talking about something traveling at "X Mach", or "X times the speed of sound" and then I try to figure out if they mean X in relation to the speed of sound up there or the speed of sound at sea level. Just a nightmare trying to use it to convey speeds even within confines of our atmosphere.

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.

> 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.

But it's not aircraft ? It's trivial for spacecraft to achieve it

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.

Trivial seems the wrong word here. Picking your nose is trivial. Space travel is exceptional.

Well, getting to space is the hardest part, once you're there breaking speed of sound is trivial

I think Mach numbers are always given for the situation the aircraft is in at the time

That's the idea. But quick survey of people in my vicinity confirmed most people think about Mach numbers as just another unit for speed of sound.