Space? No: Stratosphere!

Reading articles about space tourism, there are a couple of terms that are thrown around – more as marketing slogans than for anything else. "Near space", "upper atmosphere" and "high altitude", just to name a few. But what do they usually mean? I would like to attempt to put them into perspective, so that we can see them for what they really are.

First of all, space (meaning "outer space"):

Where space begins is a matter of definition. The usual definition is everything not within the earth’s atmosphere. Of course we could get particular and exclude the surface and atmospheres of other planets, but from our perspective from "down here", we usually just throw the other planets and celestial objects into the grab bag of "space". This will probably start changing once it’s normal for people to be flying around up there and once leaving space for them means to stop off on any old planet/settlement, not only earth.

At any rate, leaving the earth’s surface might be the start to a journey into space, but the journey is only completed once the earth’s atmosphere is left. This means that we first of all need to define the term "atmosphere". Generally speaking, this is "where there’s air". The only problem is that this is quite a gradual condition, so that we have to then define how much air we’re talking about.

For practical purposes, a consensus in the scientific community has been built at around the 100-kilometer mark. Or, English, 60 miles. But since scientists don’t use miles, we’ll keep to the metric system. The only problem is that this "break" is in a part of the atmosphere called the ionosphere. There is an ionic atmosphere (atoms and electrons stripped of their connection) both above and below this point, it just decreases as one moves farther and farther away from earth’s surface. A satellite orbiting at 110km will fall from that position within days, simply because there is too much friction, too much air floating around. Even at 300km, for instance, a satellite will gradually experience too much drag to stay up there forever.

Meaning, most space junk will take care of itself within years, perhaps decades – simply because there’s "stray" atmosphere up there!

Of course, you have to draw the line somewhere, so I really have no problem with the 100-kilometer mark.

Now – back to the marketing. If you look up high-altitude flight, you’ll sooner or later realize that they’re talking about anything which flies above international airliners, meaning above 12,000 meters. High-altitude flight, however, doesn’t even reach up to 20,000m, or only a fifth of the way up in the official atmosphere and only a bit more than twice as high up as Mt. Everest. I mean, I agree that this is not exactly "low altitude", but do you really want to call it high? It’s true that we don’t have jet engines that can fly in the thin air up there. NASA is, of course, working on Ram- and Scram-Jets to compress the air a bunch before it’s used for ignition.

Then, of course, there’s near space. This usually means reaching levels even higher than "high-altitude" – anything between 20km to 100km, or 4/5ths of the height of the official atmosphere!

Now I’m not going to say that there isn’t a reason for this. For someone hovering around in "near space", it’s hardly any different from outer space. The view is similar and one certainly needs a space suit to stay alive. Gravity, on the other hand, is like being on the earth’s surface – for only when one is in orbit is there weightlessness. Just to demonstrate the similarity of the stratosphere and of space, here is a picture:

 

So, which is it? Space, or the stratosphere? The only give away is that (at least in this non-panorama view) there is no curvature on the horizon. This picture was, as far as I know, below 30,000 meters altitude, or around three times as high as Mt. Everest. I would not mind looking out of my capsule window at this view and at the stars (which you can’t see in this picture), that you can see even in the middle of the day.

Now, how about the real thing:

 

 

There you go. Recognize the curvature? The same clouds, but less atmosphere on the horizon? I'll admit that the resolution is much higher for the second picture.. But if you didn’t put the two pictures side by side, you could think of both of these as space, right? In this picture you can’t see the stars either, even though this is the view that one would have from the orbit of the Hubble Telescope – meaning that here there is absolutely nothing between you and the stars.

And there’s another difference. When looking down from the vantage in the first picture, the ground doesn’t change – meaning you’re not moving. Of course there’s day and night, cloudy and clear weather, but the landmarks remain the same. In the second picture, you can see the same spot on the ground every hour and a half. In the first picture there is absolutely no motion while in the second one, the surface of the earth speeds by at 36,000km/h!

Now, if you really want to go to near-space, you would at least have to go above the ozone layer, at about 33 kilometers, where ultra-violet light is filtered out. You would be, however, still "only" in the middle of the stratosphere. A purist, on the other hand, would have to go up twice that high, into the middle of the mesosphere – or, "middle sphere" - meaning, the sphere between earth and space. Well, actually, between atmosphere and exosphere, that awful thing that puts drag on our orbitting satellites.

At the top of the mesosphere (or even in the middle of it), for all intents and purposes, you really are at the "edge of space", but only if you don’t listen to the marketing. For them, the edge is in the lower stratosphere, at least 50km lower.

Now, I know this might be a bit tedious, but it does help sometimes to differentiate!