"Lander" is maybe not a good job description for the Rosetta sub probe Philae. I understand that there is no practically useful gravity there. Philae will try to hook itself onto the comet surface in order to get stationary.
When it has completed its primary mission on one surface site, could it unhook itself and float (fall? hop?) to another location and do new science there? Within its battery life.
EDIT:Could Philae just detach itself and tumble but still make use of its instruments on another part of the comet?