In movies at least, astronauts in desperate times have mentioned that unless they get to a certain angle or re-entry window, they would bounce off the atmosphere into space. What’s the science behind that? What conditions have to be perfect for re-entry and why?
In: Physics
It doesn’t actually bounce, per se. But a spacecraft that enters the atmosphere CAN rise back out of it again for reasons other than starting up a rocket motor.
It helps to remember that, in general, a thing that is in orbit around a planet (like Earth) moves along an ellipse, where the planet is at one of the two foci of the ellipse. When you’re talking about an airless body, like the Moon, then the orbiter will swoop down close to the body at one end of the ellipse and then farther away at the other end. In other words, the altitude of the orbiter decreases, slows down, stops, then rises again as it heads out to the other end of the orbit.
The same thing happens with an atmospheric body. The only difference is that if the low end of the orbit is in the atmosphere, then the orbiter is running into a lot of air molecules while its at the low end. That slows down the orbiter, which reduces the height it will reach at the OTHER end (the high end) of the orbit. A spacecraft that is trying to come home aim to hit a fair bit of air, to slow down enough to stay in the atmosphere and eventually land. That’s the “perfect angle” it’s looking for. Too shallow, and it won’t slow down enough; instead, it’ll just rise right back out to the other side of the orbit. Too steep, and it’ll hit a LOT of air and heat up to the point of melting or exploding.
(And, of course, that explanation is vastly oversimplified—orbits can also be parabolas and hyperbolas if craft are moving faster, and I haven’t even attempted to get into the effects of spherical asymmetry or general relativity. But that’s the basics in a nutshell.)
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