Testing tiles
After the Columbia disaster in 2003, the Enterprise was used to determine the reason behind the tragedy. Here's a section of the craft that was laid with the tiles used on the Columbia; the tiles covering the underside of the rest of the Enterprise are "simulated" and weren't built to withstand re-entry.
The new tiled section of the craft was bombarded with foam blocks (like what fell from the Challenger at launch) to test the tiles' ability to withstand impacts. At the top left, you can see the damage caused.
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