It was the SpaceHab science double module, which replaced SpaceLab.
Columbia was heavier than the other three shuttles by about two tons and flew very differently (each of the Orbiters had their own quirks, but Columbia was the outlier of the fleet). NASA didn’t fly her that often, and was holding back from using her on ISS missions due to weight restrictions. Although that was going to change with one of the other shuttles (Discovery I think) about to be pulled from the schedule for a major overhaul back at Palmdale.
Thanks. Right, Columbia was heavier than all the other orbiters because it was the first built. It also had a huge payload on that mission.
I was looking it up on Wiki, and a rescue mission would have been possible with Atlantis had management taken the problem seriously.
Had they dumped as much weight as possible and used a special reentry program they may have also landed in Australia where the atmosphere wouldn’t have been as dense.
I’ve also seen speculation that had the orbiter broken up at lower altitude they could have parachuted to safety. I doubt it though.