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
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.
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