Ground based astronomical observatories are designed for a generation-long lifetime in order to maximise their scientific output. The structure of the European Space Agency supports "experiment-like" missions with a significant shorter lifetime in an optimal way whereas long-lifetime missions become challenging in several aspects. The talk will motivate the need for long lifetime space missions form a conceptual and scientific perspective. The specific challenges of long-lifetime missions will be pointed out. The talk will further report about the specific experiences gained with the increasing lifetime of XMM-Newton mission.