I was using some graphics software I have to boost try to get a better look at the images of the debris hitting the shuttle wing during launch, and I wasn't able to resolve a clearer outline of the "foam" debris.

However it occurred to me as I fiddled with the gamma, contrast, brightness, RGB, level, edge detection, etc. that the debris seemed to be too bright, it has high albedo, in fact it seems to be the same color as the shuttle! That's odd. Because the main external tank is definitely not the same color as the shuttle (white).

So I looked at another photo of the shuttle during launch, and it occurred to me that the solid rocket boosters ARE the same color, and at supersonic speeds the standing shock on the nosecone of the SRB might pull debris outwards enough, and straight down to hit the shuttle wing, which overhangs the lower portion of the SRB:

Now those SRBs were recovered, right?