I see by the commentary about the Ansari X prize that 328,000 is generally considered to be the “edge of space.” This implies that there’s a fairly narrow band in which the atmosphere runs out of significant molecules, and where the environment is pretty much air-free. How rapidly does the atmosphere end and space begin? I guess I had thought that such a boundary was relatively broad, so that one would have to be at least 100 miles or so above the earth to be free of atmospheric material. What’s the straight dope on the way the air “ends” and space “begins”? xo C.
There is no sudden change in air density as you rise in altitude.
So, if you are correct and my math is correct, at some 330,000 feet, the atmosphere has been diminished to about one millionth of its sea level density. That’s a nice round number. But I’m still wondering how that height has been established as “the edge of space,” as it’s referred to in accounts of the X prize. Is there a physical reason to call it that? It doesn’t seem that it should be arbitrary.
From Slate.
The X-prize requirement is just a nice round number: 100 km. It only sounds meaningful and specific because the media convert it to feet.
The transition from sea level pressure (1000 mB) to “outer space” (0.000000001 mB) is a continuum. But if one selects a point arbitrarily in that where not only is the air not thick enough to be breathable but also too thin to be easily compressible to breathability, you’ve got a sensible defining line. There’s no clear dividing line on the spectrum between blue and green – you get peacock blue, turquoise, aquamarine, sea green… but at some point you move from a visual impression of blue-tinged-with-green to one of green-tinged-with blue, and that’s a legitimate dividing line on what’s by definition a spectrum. Same applies here – you set an arbitrary limit along a continuous change between clearly different end points as the “place where it stops being one and begins being the other.”
From here (bolding mine):
If the transition is a continuum, how is it possible to “bounce out” of the atmosphere? Doesn’t that require some kind of surface? I have always imagined the spaceship to behave like a stone on the water.
When a spacecraft re-enters the atmosphere, it’s traveling many times the speed of sound. As a result, the air (thin though it may be) in front of the craft cannot move out of the way fast enough, and thus pressure builds up. At some point, this pressure will become large–it is the source of the heating that causes objects to “burn up”, after all–and if the angle of re-entry is too shallow, it will be enough to push the craft back out of the atmosphere. Thus, the “bounce”. It doesn’t require a well-defined boundary where the atmosphere meets space, only a region where the pressure buildup on the leading surfaces of the craft become high enough to affect it’s trajectory. Where this happens depends on the geometry of the craft and its velocity.
The transition is a continuum, but at those speeds, the rate of change of the air density has steeper and shallower parts.
You’ve got three forces acting on your vehicle: gravity, air drag, and aerodynamic lift. Air drag acts in a direction directly opposed to your velocity, and increases with the square of velocity and linearly with density. Lift acts more-or-less perpendicular to your forward velocity, and in proportion to it. Gravity always pulls you toward the center of the earth. If you don’t balance your perpendicular and tangentual speeds, then air drag will only slow down your descent, not your tangential motion, and you achieve more lift than gravity can overcome. You rise back up out of the air that’s thick enough to create substantial lift or drag, and cover a huge horiztonal distance before gravity (the only force operating on you at that point) gives you another chance to come in through the atmosphere.
For purposes of tracking spacecraft reentering the Earth’s atmosphere, NASA sets “nominal reentry” at 400,000 feet. (~75 miles)