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Why can't space shuttle enter without using heat shield?

I know the space shuttle enters the atmosphere at very fast speeds and thus reaches EXTREME temperatures and so the need for the heat shield. My question, assuming fuel resources, why can't the space shuttle slow down sufficiently to enter the atmosphere slowly? Someone mentioned to me that at slow velocities they would bounce off. WHY???? It is not a hard object, so why can't they just gradually push through??? Confused!!!! Thanks!!!
As for the pond example, but the stone does finally enter the pond. So, why can't the shuttle find someway to enter slowly?
Albert, I understand the phases it goes through. That is NOT my question. Why CAN'T it go through slowly????

Basically, the shuttle travels at orbital speed to stay aloft. Slowing from that kind of velocity requires incredible force that can harm the crew. The slowing is accomplished safely by turning the shuttle bottom forward so that the force of the atmosphere pushes against the bottom of the shuttle. This creates a tremendous amount of heat, which is why the heat shield is required.

The shuttle can't enter the atmosphere more slowly because there is no way to exert the forces necessary to do so without harming the crew.

A similar question was asked on a NASA Q&A site:

"Shuttle re-entry from space-
How come when the space shuttle re-enters the atmosphere it does not enter more gradually to avoid heating up a lot? I have wondered about that for a while. Hope you can clear that up for me. Thanks."

"The answer is, such gradual re-entry will not work.

Generally, a craft suspended above the ground (excluding balloons) stays up in one of two ways. Either it has enough velocity (looking at it one way) for the curvature of its fall to match the curvature of the round Earth. Or else, it gets lift from the air its motion encounters, the way airplanes do.

At orbital speed, at least 8 km/sec, the shuttle keeps its height the first way. However, once it enters the atmosphere and slows down, its fall no longer matches the curvature of the Earth, and instead it gets lower and lower. It could have entered more gradually if it could have used the atmosphere to keep its height, the way an airplane does. But at 15-20 times the speed of sound, wings create more resistance than lift, and anyway, presenting a wing edge-forward as an airplane does would concentrate too much heating and pressure on its front.

The shuttle starts re-entry with appreciable altitude and a lot of forward speed, so its fall will take some minutes. The trick is now to lose speed and kinetic energy safely in the time allowed by this fall, and it turns out possible to do so--just barely--without deceleration forces getting too high for the crew. To lose its energy, the shuttle turns its bottom (covered with heat resisting tiles) to face forward, creating a wide shock front in which most of the heating occurs, sparing the shuttle itself. Only at the last stage of its descend does the shuttle actually "fly." By that time, only a small fraction of its energy remains."

Search the NASA website to learn more.

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