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
you don’t normally think of bouncing off water, but if you throw a rock at the right angle and velocity it bounces of the surface of the water. Air and water behave the same in this regard, it just requires greater velocity to achieve the “skip”.
The faster you encounter something, the “harder” it behaves.
For example, dropping into a swimming pool from a diving board is fun, but dropping into a swimming pool from 25,000 feet would be fatal. At that speed, the water can’t deform fast enough for a soft impact. It acts the same as if you’d fallen into steel.
The space craft itself still has wings to facilitate flight while in the atmosphere. In space it means nothing since there’s no air…
But as they start entering air they need to slow down – they use heat shields on the bottom of the craft and just hit the atmosphere to let drag slow them down. This means that their wings are now hitting air at an high angle of attack and that causes lift to happen which would push them up away from earth. This is the “bounce” effect. Gotta get the angle right for that.
The term “bounce” just refers to the fact that you will approach the planet, you will lose altitude, and then you will once again start gaining altitude. Contrary to the other posts in this thread, you don’t “bounce” the way most people think of bouncing. You aren’t skipping off the air like a stone off water or anything like that. Remember that unlike a stone meeting water, contact with the atmosphere is very gradual. The atmosphere doesn’t have a sudden boundary like at the top of a lake. Instead it just gets progressively thinner or thicker as you gain or lose altitude.
“Bouncing off the atmosphere” simply means your approach angle was too shallow and the thin upper atmosphere will not reduce your speed enough. Instead you will slow a *bit* as you encounter some atmosphere but not enough to cause reentry on that initial approach and the craft will eventually start gaining altitude again. At that point, at typical approach speeds, you will now be in a decaying orbit that will return you to the atmosphere and eventual reentry. Unfortunately this time reentry would be at an unplanned location, at an unplanned angle, and at an unplanned time, which are all very bad.
Edit: I should say, they are all very bad *if* you hadn’t intended this kind of approach. A spacecraft could use this kind of approach on purpose to help slow the vehicle, with the goal of making a second, planned approach that leads to reentry. I think NASA has even experimented with this.
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.)
Pretty much the same as when skipping a stone on water. If it moves too fast and at a shallow angle, it’ll bounce.
It’s a balance. Go in at a too shallow angle and you bounce, go at a too steep angle, and you’ll burn, or you’ll use more fuel to burn off speed. You want to go at a speed/angle where you ALMOST, but not quite, burn, there you’ll get the most effective reentry.
I think they release big amounts of energy strong enough to launch them into a speed faster than our planet’s escape velocity
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