The end of the atmosphere is not like the horizon, where you can definitely say, “This is where the Earth ends and the atmosphere begins.” Rather, as one travels higher and higher, the atmosphere gets thinner and thinner. One can say, for practical purposes, that the upper atmosphere begins to be indistinguishable from outer space at about 435 miles (700 kilometers) altitude, but that is really just a random place to draw the borderline. The density of the atmosphere is getting very thin indeed at an elevation of 370 miles (about 600 kilometers). At this height, there are about six miles (10 kilometers) between each molecule (this gap is known as the “mean free path.” The air pressure here is, effectively, zero.