# Bounce off the atmosphere at reentry?

I am just watching Space Race part 3 and the cosmonauts just got their first view of Wostok. The scientist who designed the retrorockets told the soon-to-be spacemen that the rockets would have to be fired precisely in order to not:

1. reenter too steep (hence to fast) and crash or
2. reenter too shallow and bounce off the atmosphere into a higher orbit

The first point is, of course, well taken (even though one has to wonder why the rockets should be designed with too much power in the first place), but the second point seems dubious to me: After playing KSP, I would suspect that the Wostok's orbit would decay nonetheless after even a minimal braking. How could you enter a higher orbit by touching the atmosphere? That would seem like a great way to get to the moon, after all.

• A capsule can actually skip off of the atmosphere the same way a stone skips off the surface of a pond. See this question for more explanation. Dec 7 '16 at 5:05
• It is not necessary for the rockets to be overpowered in order to effect a re-entry that is too steep. Just aim closer to the center of the cosmological object (presumably, Earth) than an ideal reentry point as you approach, and you'll succeed in destroying your spacecraft. In fact, the more overpowered the rockets are, the better your chances of surviving in a too-steep re-entry; you can use the rockets to shed speed before your heat shield burns away, and try to slow to a speed that won't rip out your craft's parachutes (or blow your impact cushions, or whatever) as you reach the surface. Dec 7 '16 at 5:23
• So, it's another power-to-weight tradeoff problem, best solved by coming into the descent just steeply enough to ensure the atmospheric drag slows your craft to the point where it is not going fast enough to re-emerge from the atmosphere, and not much more than that. Dec 7 '16 at 5:25
• @MilesBudnek I was thinking a lot about the lifting body effect, lift could obviously alter the trajectory in a way that drag alone couldn't, the question is if lift could in any sense of the word "raise" the orbit, I get the feeling that an encounter with the atmosphere strictly lowers apoapsis. Lift would have a significant effect on how hard the capsule "bites" into the atmosphere, as depending on angle of attack lift effects can exert an upward or downwards force on the capsule, pushing it deeper into or lifting it out of the atmosphere and thus altering areobraking efficacy. Dec 7 '16 at 9:20

Yes, a capsule cannot literally bounce off the atmosphere and its kinetic energy must be reduced by an encounter with the atmosphere, rather it would just pass through the atmosphere and back into space, having failed to lose enough velocity to stay in the atmosphere. After going partially around the planet it will reenter the atmosphere, that is actually where the real problem lies, the capsule will come down at the wrong location at the wrong time. There may also be problems with power / oxygen reserves running out or heat shielding failure, for example the design may require jettisoning the heat shield to get rid of the the accumulated heat, or rely on convection in the atmosphere to cool the vessel. So there are lots of things that would go wrong if the vessel goes back into space instead of deeper into the atmosphere.

With that said, there is something of a lack of precise, every day terminology to describe the problem in a pithy way and ordinary people do not understand the distinction between "in orbit" and "high up" anyway, the capsule would literally go back high up into space, and with the right framing (i.e. a graph of altitude vs time) the trajectory would kind of look like a bounce.

Energy conservation is valid for orbits too. You need energy to get from a lower orbit to a higher one.

But "bouncing" of the atmosphere does not add energy, some energy is lost to air friction. Finally you will leave the orbit anyway when entering the upper atmosphere even for short time due to an elliptical orbit. If not at the first time, then at the next times at the lowest point of the elliptical orbit. Each short pass of the atmosphere will take some of the orbit energy again. Removing some orbit energy results in a lower orbit. If the orbit is too low finally, reentry is unavoidable. But a delayed reentry may be dangerous for the crew, the reserves for electrical energy and oxygen for the capsule are very limited.

• While your answer was first, Blake's is more clear. No votes in a couple months you might want to delete this one. Mar 23 '17 at 15:15
• +1 For me at least, this is a better explanation of what the often-heard phrase "bounding off of the atmosphere" really means.
– uhoh
Feb 23 '18 at 1:03
• "is lost to air friction" as this happens somewhere around atmosphere border, and this question is not limited to Earth, when you write "air" it makes the answer very misleading. Nov 23 '18 at 10:06