Just a general question: how does NASA fund extended missions? Is there a special source for this? does it need to go past congress again? Is it extra money or will it be subtracted from other missions?

just wondering...

I am just speculating, I have no special knowledge;

* Unused budget from programs that didn't use all their $ (hard to imagine in any governmental endeavor, I know)

* After 40 some years of surprises, I hope there is actually a budget item for extended missions.

* Probably a 'prudent reserve' factor in most departments, tap into that for unusually long lived missions.

* If I was 'in charge' I might be tempted to hold off a couple of smaller projects if there was an intriguing rib cage or leg bone sticking out of a distant sand dune on the last frame sent down in 2009 by the still functioning, but congressionaly mandated abandoned Mars Rover.

* Run a deficit.

Probably ... The yearly NASA budget takes of this ( sum beispiel: MER increased yearly ) ?