Comment Re:To make room (Score 5, Interesting) 25
Pirs, the module that was discarded, was the 3rd oldest module from the ROS (Russian Orbital Section). It was used as (among other things) an airlock, and above and beyond the base age of the module, the pressure cycles from nominal to vaacum take a toll on the structure and equipment of the module. So, age was a factor.
The new module that is coming up, Nauka, has all the capabilities (and more) of Pirs. Therefore, there was no technical reason to retain the old module.
There are some physical constraints to consider as well. The ISS orbits at an altitude that's not far enough up to be entirely free of the effects of atmospheric drag, so it requires periodic boosts to raise its orbit. This is accomplished using thrusters, which consume fuel. The more massive the station is, the more fuel it takes to do so. Because of the technical capability overlap mentioned above, in this regard Pirs was unecessary excess mass.
The mass distribution of the station matters as well. If all the mass were evenly appllied around the fore-and-aft axis of the station, then applying thrust in the forward direction from the aft of the station would put the thrust vector exactly through the center of mass, which would make controlling said thrust vector easy. However, the mass is not thus equally distributed - which the station control systems can deal with, but the further away the center of mass is from the thrust vector, the harder it is to control (and at some point, it would become impossible). There are only so many places for modules to connect (ports) along the fore-and-aft axis of the station. Thus, to keep the mass distribution closer to this axis, the less massive Pirs.had to at least be moved out of the way to make room for the more massive Nauka. And, since there was no reason to retain it, per the above, it could be removed from the station entirely.