In 17 hours, NASA's probe to the planet Pluto, the New Horizons mission, will begin its seven-year journey to the Solar System's ninth planet. At the far reaches of the solar system, there is too little sunlight for solar panels to make electricity, so the New Horizons probe is powered by 24 pounds of plutonium.
Needless to say, placing 24 pounds of radioactive material on a rocket is not done without some controversy. The claim is that dangerous radioactive material could be spread across a large area if the rocket explodes during liftoff, endangering large numbers of people.
Personally, I am confident that the launch will proceed safely and that there is little danger of radioactive release even if the rocket explodes. There is a phenomenal amount of engineering and study that goes into the design and launch of a space probe, and the amount of work increases dramatically when environmental and public safety is an issue. Given the potential risks and repercussions of a poorly-engineered launch, I feel that NASA has studied the issue.
However, I do not dismiss these protesters out of hand. They have a valid point. Radioactive material has been released from spacecraft. One of the most famous examples is the Soviet Cosmos 954 satellite, which crashed into Canada in 1978. As we explore space, we must not become careless, as two shuttle disasters have shown. Putting a heavy chunk of metal into space takes a LOT of energy (rocket fuel), and if quality control is not maintained, there can be disasterous consequences.