Outer Space
Published on May 24, 2006 at 6:27 pm.
2 Comments.
Filed under Earth, space exploration.
So, how high is outer space? How high do you have to be until you are in space? Like a lot of the questions that I’ve tackled, this one seems simple at first, but more complicated when you look at it closely.
The dictionary definition of outer space is that space that is beyond the atmosphere. Well, that seems simple enough. However, when you look at it, it is a bit more complicated. The atmosphere is a system of gasses. Gasses don’t have a sharp or defined edge as do liquids or solids. An atmosphere, thus, just fades away into space. The air is held to the surface of a world by the world’s gravity. It is thicker and under higher pressure the closer you are to the surface of the world. As you get higher, the atmosphere gets thinner and thinner, until you are in space. But there is no sharp edge.
Let’s take Earth’s atmosphere, for example. Most of the atmosphere is confined to the lowest 10 miles. Once you get above this altitude, the air is very thin indeed. In fact, if you are more than a four or five miles up, say where many commercial airliners fly, then the air is so thin that you can’t even breathe. You’d pass out and eventually die of lack of oxygen. Higher than about 20 miles or so, then the air is so thin that air passing over aerodynamic surfaces on an aircraft is unable to provide enough lift to fly and air passing over an aircraft’s control surfaces does not provide enough force to properly maneuver the aircraft. At least this is more or less true for conventional aircraft. You might argue that the Space Shuttle can maneuver at these altitudes. Yes, but while the Space Shuttle might look like an airplane, and my behave sort of like one at low speeds and altitudes, it most definitely does not use the same sort of forces when it is at those altitudes. But, I digress.
The Earth’s atmosphere is composed of several parts. The lowest part is the Troposphere. It extends up to about 7 miles (12km) or so, give or take a mile. The troposphere is marked by a decrease in pressure and temperature with altitude. However, when the air is thin enough, ultraviolet light can break apart oxygen molecules, which then can recombine into ozone. This ozone then absorbs UV light, warming it. So, in such thin air, the temperature will actually increase with altitude. The part of the atmosphere where this happens in the Stratosphere. The stratosphere extends up until about 50km (31 miles). The next layer of the atmosphere is very thin and tenuous. Here, aircraft can not fly as aircraft because the air is so thin. It is called the Mesosphere. In the Mesosphere the temperature again drops with altitude. The Mesosphere extends up to about 80km (about 50 miles). Above the mesosphere, the air is so thin that individual atoms and molecules pick up energy and seldom run into one another to share that energy. The more energetic atoms and molecules move farther against gravity, and so higher. This means that the temperature increases with altitude. We call this the Thermosphere, and it extends to about 300 – 400 km (185 – 250 miles). Above this is a tenuous region of individual atoms and particles just flying around Earth. This is the Exosphere, and it goes out for thousands, or even tens of thousands of kilometers. That means the Space Shuttle, Hubble Space Telescope, and International Space Station are actually flying through the top of the thermosphere! This means that they will bump against atoms and molecules from time to time, slowing them down. Granted it is a tiny effect, but it adds up over time. Slowing down causes them to fall to a slightly lower orbit, there they run into more things, slowing them more. Unless the space station or space telescope are boosted to a higher orbit now and then, they will fall back to Earth. All low Earth orbits have this problem. We call it orbital decay, and it results from the fact that, though in space, these bodies are technically still in Earth’s atmosphere!Â
So, how do we define being in space? As you see, the atmosphere has no sharp edge. Rather, it just fades away. So, we arbitrarily make up some altitude and say that is the top of the atmosphere. Beyond that is “outer space.â€Â
So, what is this magic altitude? In the United States, NASA has traditionally declared an altitude of 50 miles to be the beginning of space. Astronauts are really just members of the astronaut core until they first fly past 50 miles. Then, they get their official astronaut wings. 50 miles works as well as any other defining number in the US, since we still use imperial units. However, for the rest of the world, using metric units, 50 miles is 80km, and that is not such a nice round number. So, the FAI (Fédération Aéronautique Internationale), which is an international sanctioning body for keeping track of aeronautical records, defines outer space as being above 100km (62 miles). To win the Ansari prize, SpaceShip One had to carry a pilot above 100km twice within a specified time frame. For those of us stuck on the ground or in commercial airliners, the distinction hardly matters!
However, this does raise an interesting point. We think of spacecraft as launching into space on a rocket lifting off from a launch pad. However, what about flying into space? NASA, for a long time, had a dream of a Spaceplane that would fly into space and then land again as an aircraft. Could this really be done? Well, the X-15 experimental rocket planes did very much this sort of thing. In fact, these planes flew so high that they could no longer be controlled by their aerodynamic surfaces. At those altitudes, they were controlled with thrusters, just like on a spacecraft. Furthermore, 13 of the X-15 flights, ranging from July, 1962, to August, 1968, actually flew higher than the 50 mile altitude used in the United States to designate spaceflight! Two flights, both by Joe Walker, on July 19, 1963, and August 22, 1963, were even higher than 100km, the FAI mark for spaceflight! So, one would be justified to think of the X-15 as a spacecraft, and as a spaceplane.Â
-Astroprof
Juliette Harrington on November 6, 2006 at 2:53 pm: 1
Fascinating article, thanks very much. I also thought that either Boeing or Mcdonnell douglas had made experimental designs of an aircraft that would effectively “bounce” along the earths outer atmosphere before reentering the earths atmosphere proper. I believe the advantage was to significantly reduce flying time for particularly long flights. Is this true and if so what else do you know about this?
Thanks, Juliette,
Astroprof on November 6, 2006 at 5:45 pm: 2
As a matter of fact, yes Boeing was working on a Space Plane. The idea didn’t start with them, though. It goes back to the Germans during World War II, when some of their scientists suggested using this strategy for ultra-long range intercontinental bombers. The Air Force experimented with the idea on a project called DynaSoar. Every now and then, the idea comes back in a new form, but always money is not available to see the project through. I posted a bit about this here: http://astroprofspage.com/archives/120