If you live on the coast, the air is thick. It doesn’t feel like that, but at sea level you’re at the bottom of a sea of air. The gases of the atmosphere there are under the weight of the maximum column of overlying air. Thus they are pressed together more densely than at higher elevation. This barometric pressure is even higher at dry places below sea level—about 5 percent higher at the Dead Sea—which is just a more extreme case in point. As you walk inland from the beach and up into the foothills, that air body gets less dense until you reach mountain peaks where you wish you had some bottled oxygen because the air is so thin. Above that, it just gets more so, but even after traveling miles straight up, you’re still in Earth’s atmosphere. If you are exploring the upper echelons of the atmosphere in an airplane, you come to a place a few tens of miles above the earth where you have to go very, very fast to maintain altitude. At the point where you just can’t get enough gas molecules to provide lift, you’ll start sinking back into the atmospheric sea. If you have enough firepower, you can push yourself to 18,000 mph, a speed called orbital velocity, and start winging your way around the planet as a satellite in space. But that requires a significant grunt. It takes twenty times as much energy to get into actual orbit as it does simply to get to the needed elevation.
That magical level where you can fall back toward Earth or kick out the jams and go to orbit in space is called the Karman Line. It was defined by Theodore von Kármán (apologies if those accents didn’t translate on your computer), a Hungarian American research engineer. He calculated that it was at 100 km (or 62 mi) above Earth that atmospheric conditions were as has just been described. Apparently, if you can achieve something in excess of 18,000 mph, you can keep yourself flying at the uppermost level, but hey, why not go into orbit? This was first achieved by a German V2 rocket in 1944, and the Karman Line is now crossed almost as often as the International Date Line—or at least with about as much fanfare. Despite its consequential altitude, NASA and the US military don’t even use the Karman Line to define space. If you pilot a craft to a mere 50 miles in altitude, you get anointed as an astronaut.
Still, other than rocket scientists and astronauts, who really cares at what altitude you run out of atmosphere for all practical reasons and transition into space? Governments do, apparently. Way back in the 1950s, President Dwight D. Eisenhower tried to get other governments to see outer space as akin to the open ocean, an area of free movement by all, for peaceful purposes. Like the briny deep, he argued, outer space ought to be controlled by international agreement (although the high seas are often lawless). This came about because nations are bounded not only by land borders and certain offshore boundaries, but also airspace, into which other countries’ aircraft are not supposed to fly uninvited. Russia saw Eisenhower’s proposal as a way to allow American spy satellites to fly unrestricted over Soviet territory, and the Russkies weren’t far off the mark. In hopes of denying that interpretation, the US was planning to send up a scientific satellite for the International Geophysical Year of 1957. As a first effort, Eisenhower hoped it would act as a non-confrontational introduction to satellite flight paths. But the Soviets beat the US into space by sending up Sputnik, and the little beeping orb sort of set the tone—it’s OK to fly over anywhere on Earth if you’re in space. As to how far up the sovereign airspace goes, there is still no international agreement or definition, but the Karman Line defines one practical gateway to space, so it can be used as a rule of thumb. Now that boundary may be closer to Earth than we had thought.
The 50-mile altitude that confers astronautical achievement recently took on even more significance. Mr. Karman was quite a whiz with numbers, aeronautics, and theory, but expiring as he did in 1963, he didn’t have a lot of hard practical data upon which to base the height of the Karman Line. By 2018, we have more data than we know what to do with regarding the flight patterns of spacecraft. By sifting through the altitude statistics of 43,000 satellites (most of which were way beyond the Karman Line), astrophysicist Jonathan McDowell isolated those that came down low enough to be of interest. These were satellites that re-entered the atmosphere and burned up. Of those, 50 made at least two full orbits of Earth once below the accepted 62-mile-high Karman Line. That meant the line had to be lower. Modeling the data mathematically, he found the line to be somewhere between 41 and 55 miles out. Overall, though, once below 50 miles, the satellites were doomed to plunge inexorably toward Earth. So the new Karman Line looks to be at that 50-mile height. That doesn’t change anything about how NASA will launch rockets, but it’s a new insight into the physics operating at the high ranges of our atmosphere, and maybe means a lower ceiling over earthly domains.
Interestingly, though you’re in space when beyond the Karman Line, the atmosphere doesn’t completely disappear there. Dribs and drabs of atmospheric material stretch far, far beyond the line, more than 6,000 miles into space. However, the molecules of oxygen and nitrogen and the other elements of air cease to behave as a gas at these distances and so don’t really play much of a role. Think of those molecules as the first greeters to aliens coming back to Earth to check up on all their big stone structures. And who knows what kind of atmosphere those guys are used to?
You may not get a chance to cross the Karman Line and go into space yourself, but you could have a beautiful National Geographic map of the Solar System on your wall! It’s available from Maps.com with a quick click.
caption: Our thin skin—the precious blanket of gas that we live in doesn’t extend far, compared to the size of Earth.
source: NASA: Atlantis Space Shuttle crew (Public domain)
caption: The Space Shuttle displayed against the troposphere (orange), the stratosphere (white) and the mesosphere (blue). The Karman Line sits at the top of the mesosphere.
source: Wikimedia Commons: NASA/ISS Expedition 22 crew (Public domain)
caption: The layers of the atmosphere. The Karman Line now appears to be lower than shown here—new research puts it at about 50 miles above Earth, where the mesosphere meets the thermosphere.
source: NOAA: NOAA (Public domain)
caption: A NASA weather satellite launch heads for orbit. The curved path allows it to have maximum horizontal speed as it crosses the Karman Line and enters space.
source: NASA: NASA/Bill Ingalls (Public domain)
caption: Going the other way. Meteors, like these from a Perseid shower, usually burn out somewhere between 43 and 62 miles up, as they encounter the gradually thickening atmosphere.
source: NASA: NASA/JPL (Public domain)