As a matter of fact, there is. NASA admits that fuel vaporization could occur if the space probe in question were to burn up in the atmosphere. However, Cassini is already on its way to Saturn, via Venus and a near-Earth flyby in 1999. So far, everything has gone well, and there is only a very small chance that it could burn up in the atmosphere due to an error during its near-Earth flyby in 1999. But my concern is not just with the Cassini mission, but mostly with future missions in which RTG's are to be used.
Space exploration is important, and for so many reasons.
But, we need a dependable power source to supply the equipment we use to gather the data we want to collect. So far RTG's have been our only source for deep space exploration. Even if solar technology is improved drastically, there will still be a need for an alternate power source where solar technology simply cannot be used. The question still lingers. Exposure taken to bring out the very faint part of the Nebulae.