The mission is currently set to end in December 2015, after which Rosetta could simply be switched off as it continues to orbit the comet, and the mission team disperse to work on new projects. But for several months now a plan has been quietly hatched to see the craft go out with a bang by being brought down to a collision with 67P.

Source: Rosetta team propose ending mission by landing on comet – Sen.com

It would see the spacecraft brought gradually closer to the comet in a slowly spiralling orbit that would allow its cameras and instruments to gain ever more detailed views and measurements of the twin-lobed icy body. Then eventually—probably in September 2016—it would collide with the comet, bringing the mission to an end.