[gyaniv]

The problem is that since there's always a switch between Republican and Democrats, each president has a different perspective on the overall NASA mission (plus each one wants to leave a big mark), so they keep changing.

I think what the people actually working there try to do (as it can be seen here) is find a way to fit what they have already done, with what the new mission statement is, so that they won't have to scrap everything they have done, only the limited amount. That's good is some senses (less wasted work), but the result is then usually not the most fitting solution.

[lukeschlather]

I don't think it's accurate to say there are any partisan components to the NASA mission. I guess the exception is the Republican party is trying to cut Earth observation satellites because the data coming back is politically inconvenient. However that's a pretty small part of NASA's budget.