Deep space probes beyond Jupiter can't use solar power because they're too far from the sun. So they rely on a certain type of plutonium, plutonium-238. It powers these spacecraft with the heat of its natural decay.
NASA is having problems now since the end of the cold war we've stopped producing any plutonium.
There are those who are concerned about blasting nuclear fuel into space on rockets that have a dismal safety track record (2 of the last 125 shuttle missions blew up - not a good ratio - imagine if your car did that!)
Curt runs a resource management software company in Texas