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View of the Gemini 8 heatshield sample.

Picture 2 of 3.

The irregular, black and white layer on the top is the ablated portion of the heatshield.

As the spacecraft reenters the Earth's atmosphere, it compresses the air ahead of it, heating it up. The heatshield material is designed to char, burn off, and erode in a controlled manner when subjected to high heat. As it does so, it releases gases which form a cooler boundary layer, and as the gas dissipates, it carries the heat with it. The black layer at the top is the result of this ablation (although I'm uncertain as to what the white surface is from).

The middle, white layer is virgin heatshield material which has not yet been affected by the heat of reentry. This consists of a metallic honeycomb filled with resin.

The darker layer below that is a metallic honeycomb which forms the base of the heatshield.