love the note about Mach number being constant. For those who don't know, Mach number is a ratio between how fast you travel compared and the speed of sound. In lower-density places such as high altitudes or space the speed of sound is also slower making the Mach number higher. So it is possible to travel at a higher Mach number at say 3 miles above sea level than if you were 1 mile above sea level while still going the same velocity.