> in March, Nasa asked Congress for funding to start development of a "space tug" that might be needed to perform the task – a spacecraft that can push the station back into the atmosphere. Kathy Leuders, head of Nasa's human spaceflight programme, later revealed it was estimated the tug vehicle would cost just shy of $1bn (£800m).

This part sounds insane. Use a docked Progress, Soyuz or Dragon to give it the right calculated nudges at a fraction of the cost. There is complexity, angular momentum and rotation in there I'm sure but NASA is good at rocket science, why develop a new vehicle.