A couple of things come to mind, though I'm speculating. Operations: Rosetta and Philae are a pair of robots 300 million kilometers from earth, one on a comet

A couple of things come to mind, though I'm speculating. Operations: Rosetta and Philae are a pair of robots 300 million kilometers from earth, one on a comet and the other orbiting it. Rosetta's orbits require a bit of work, if I remember correctly it's not a stable orbit and Rosetta needs to be within communication range of Philae while Philae is in the sunlight. Science: Interpreting data is not easy and probably takes months in many cases. Once they've come to their conclusions they have to write and publish their papers which takes time. Planning: They have to plan what they will do with Philae's future awake time which is filled with unknowns and opportunities that may only happen once.