The orange stuff is insulation, I believe, and it was Columbia that was lost due to a strike by ice/debris that dislodged a number of tiles. Challenger was lost on launch due to a failure of one or more of the booster’s O-rings. The damage to Columbia also occurred on launch, as I recall, but the loss of tiles didn’t result in catastrophe until they began reentry much later. I believe Challenger’s O-ring failure had to do with low air temps at the time of launch.

I recall seeing images of the underside of the shuttles taken by ground telescopes, inspecting the vehicle for tile loss. There was some threshold for tile-loss, beyond which it was considered unsafe to attempt reentry. The problem for Columbia was that while she’d not lost too many tiles, the strike she’d suffered on launch took out a number of tiles on a leading edge, at a point where heat/plasma would be maximized. This damage must have been undetected by the telescopic inspections and by the visual inspection done while docked at the space station.

Some forty-five years later, I would have thought we’d have a more elegant approach than a bunch of ceramic tiles glued to the underside of the vehicle. Seems to me that many of the problems that crippled the shuttle program will be replicated with Artemis. But maybe our glue is better now.