Currently Rosetta is on a free fall collision course with 67P/Churyumov–Gerasimenko. If I understand correctly the probe will send back data and pictures during the descent, and once it has crashed onto the comet no communication will be possible anymore.
This raises the question, why not use some of the (presumably still available) fuel to have the space probe hover close to the comet so that there is enough time for measurements taken very close to the surface to be transmitted back? Or alternatively, if hovering is too complicated a maneuver while taking measurements, why not have multiple descents that get iteratively closer to the surface?
The reason why I am asking is that the impact speed that I found mentioned was 1m/s, and the current data transmission rate is of the order of dial-up speed [45kbps], which doesn't allow for much data to be sent back in the very last stages closest to the surface, which presumably is a region of great interest.
As far as the camera is concerned I understand that it can't focus on anything closer than 1km, but presumably it could be pointed sideways at surface features during the decent? And even if not, the other instruments might generate interesting data very close to the surface.
So as far as I can see there might be two answers to this:
- The above maneuvers cannot be performed or
- The data close to the comet wouldn't actually be all that interesting [e.g. due to the instruments not being built/optimized for that region].