Imagine waking up one morning to find a spacecraft tank in your backyard. For sure would be a very nice story to tell your friends and neighbours, but can it actually happen? In reality yes, many spacecraft and launcher stages have already been found completely intact on-ground, the most recent case in Spain last November 2015.
Tanks are one of the spacecraft equipment that are more difficult to demise during the atmospheric re-entry after End of Life. This is due to many factors: first of all the materials used, tanks are often made out of a monolithic shell of Titanium, a material with a very high melting point; second the design itself, at the End of Life tanks are empty shells that are large but quite light, thus when the get separated from the spacecraft during re-entry they will slow down very fast and therefore be exposed to low heat fluxes; finally their placement inside the satellites, they are normally close to the centre of mass and cover by several layers of structure which will lead to a very late exposure to the atmospheric flow and re-entry heat fluxes.
ESA is initiating the development of new tanks that are designed for demise, namely by changing the tank material to either a monolithic Aluminium alloys shell (a cheaper and less complex solution) or just an aluminium liner reinforced by a higher performance composite layer.
However, due to all of the factors mentioned above, changing the design of the tanks is only one side of the issue.
In the future the whole spacecraft will have to be designed to allow the internal equipment to demise more easily.