Some other reports suggest that the piece of foam may have become saturated with water and hit the wing not as a light bit of foam, but as a great lump of ice.
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At launch the Columbia’s left wing was hit by a piece of foam insulation from the main fuel tank, at a speed of around 480 MPH. Urgent analysis done while the craft was still in orbit concluded that the likelihood of serious damage was slight. However, the video recording of this impact shows it to be very hard indeed with an explosive spray of what looks to be more than just foam. It almost certainly was, probably containing a large amount of ice. The piece was estimated to be around 51 cm x 41cm x 15 cm and would have weighed around 1.2kg if only foam. However, if it were saturated with water (ice) it would have weighed around 29kg, a far more dangerous scenario. The Columbia had been sitting for weeks on the launch pad, out in the freezing weather. It seems incredible that some sort of shelter is not provided to prevent water freezing on and in the Shuttles’ with the attendant risk of mechanical damage due to the expansion of the ice. This is probably what caused the foam to come loose.
Recent test impacts with foam fired at a shuttle wing at 768 ft/sec or 523 MPH showed that not only was the surface deformed by the impact, but also a main rib in the wing failed with a clearly visible five-inch crack through the carbon reinforced material. The crack extended across the rib lock and onto the panel surface. This panel is a high-carbon heat shield. A crack or hole in it means almost certain destruction of the shuttle upon re-entry.