[ChuckMcM]

First stage re-entry after the retro burn, isn't going fast enough to burn up due to friction. Even the space Shuttle's or the Soyuz's boosters would simply fall back to earth and with some parachutes land intact.

As for keeping it clean, apparently they added a system after they realized its tendency to fog/ice up. I'd love the details on it too. The technique of using a rolling bit of plastic might work but given the weight penalty you probably wouldn't get more than a couple of lens "changes" in the tank.