Twitter gardener friend has watched armageddon too many times. His reply has nothing to do with what mr astronaut said lol.
He is not wrong, though…
I guess technically, but Kelly is talking about an aircraft which implies the craft is still in atmosphere and therefore Mach can still be calculated.
The original thread was started by Neil deGrasse Tyson saying that Maverick would be "splattered" during ejection at mach 10. Scott Kelly responds saying he went on a space walk from the ISS at mach 25. The gardener is technically right that mach numbers don't really apply in a near vacuum. It's also kind of off topic because we know the Darkstar is in the atmosphere and not in space, and therefore there is no "re-entry". But Scott is right that the real issue would be heat and not getting splattered.
I also want to point out that we don't actually see Maverick eject, and it's likely that he would have been ejected in a capsule (like in several real aircraft like the B-58) rather than just thrown out of the plane in his pressure suit as Scott suggests.
The ISS does not travel in a full vaccum, so it still technically has a mach number
Do they actually calculate mach that way when they say a plane can go mach 3? Or do they just use the speed of sound at 1 atm?
I would hypothesize that the indicator for mach and air speed are decoupled from each other, as a true mach reading would likely give an indication of how hard a plane is working to push itself through the air.
Just a guess though.
It is calculated according to the speed of sound at the altitude you're in, and measured using a difference of pressure in the pitot tube.
