the probe is being launched atop a conventional chemical rocket, the plutonium is part of the radioisotope thermoelectric generator, or RTG. in very basic terms, the plutonium generates heat, the generator takes that heat and turns it into energy. the main reason for doing this is that pluto is really far out, so far that the normal solar panels would have little to no effect.
the so called "laws" of physics are really only theories, until the next theory comes out and breaks the old ones. in the late 1800's people thought that if you went faster than 60mph your skin would peel off. yet our astronauts are cruising at about 18,000 mph
