An object is said to enter the earth’s atmosphere when it crosses what’s known as the Kármán line, 100 kilometers above the surface of the earth where the atmosphere becomes dense enough to support traditional air flight. But where the atmosphere is dense enough to support planes it is too dense for space craft to be traveling at extreme speeds. Take our beloved, retired space shuttle. When it began its decent back to the earth’s surface it entered the earth’s atmosphere at a face-melting 7700 meters per second. And that was after it had slowed down considerably.
At this kind of speed transition from the relative emptiness of space to our nice thick atmosphere is like running into a wall of air. The air doesn’t have time to move out of the way of the spacecraft, so it just compresses. And the more it gets compressed the hotter it gets, up to 1477 degrees Celsius in the space shuttle’s case.
Any craft not specifically designed to withstand that kind of heat would burn up entirely. But that compression isn’t the only problem, there’s also gravity. Upon re-entry space craft begin to decelerate very rapidly. We’re talking going from 7700 meters per second to a full stop on the runway in about half an hour.
The force that this deceleration exerts on the craft, and everyone inside of it, is perceived as weight. This is called “G-force”, with one G equaling the force exerted on an object at earth’s surface. The human body can withstand a maximum of about 12 G’s but after just about 4 G’s your vision starts to blur and beyond 5 G’s you’re just moments away from passing out. So the shuttle usually maintains a max of around 3 G’s during re-entry.
So why don’t the space pilots simply slow their craft way down and avoid the G’s? Well, because the atmosphere is just too dense to penetrate without going really fast. Traveling at less than 7700 meters per second, that’s about 28,000 kilometers an hour, would just result in your craft bouncing right off the atmosphere.
Likewise, the angle that the spacecraft enters the atmosphere is incredibly important. If you try to enter the atmosphere at too shallow an angle it’ll simply skip off the surface like a rock skipping off the surface of a lake. But if you go in too steep the craft will plunge through the atmosphere too quickly causing huge spikes in G-forces and heat that astronauts might not survive.
This is what happened to a Russian Soyuz vessel in 2008. Because of a malfunction, the craft came in too steep and went through what’s known as a ballistic re-entry. Plummeting through the atmosphere and subjecting cosmonauts to as much as 10 G’s which, thankfully, they survived.
So the lesson here is that the angle of approach has to be somewhere between these two extremes. And exactly what angle you need depends on the size and shape of your craft. A streamlined craft can more easily penetrate at a shallow angle while a blunt shape can more easily survive the heat. Thus, each spacecraft has its own specific angle of attack. But generally that’s about 40 degrees. Steep enough that it can penetrate the Kármán line but shallow enough that, ya know, everybody survives.
The precise angle and a specific point of re-entry are together called the “re-entry corridor”. And its location depends on where the craft is supposed to land. Surviving re-entry, as you might expect, relies on a lotta math. It is rocket science, after all.
But of course you also need a craft that is made from materials that can shield it and its contents from the extreme forces of re-entry. And that is what we’re gonna explore next week on SciShow Space.
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