Quote Originally Posted by 7cscb View Post
And, regardless, we will likely expel much mass in our interstellar travels, which is not Closed.
Mass expelled from an interstellar vessel will be propellant, not from the life supporting biosphere. A separate system entirely. We don't use the breathing oxygen inside an airplane cabin to run the jets.

Now, any sealed chamber is imperfect. There will be some material losses from inside the starship biosphere, that's inevitable, and replacement volatiles will have to be carried. But the more closure the builders can achieve, the better for life supporting purposes. So they'll aim high.

For an orbital habitat replenishment is easier, because there's asteroids and comets to mine, but high degrees of closure of the internal material cycles is still a significant goal, especially as the habitats are being established.