They can see it. Galileo and Ulysses were launched aboard a space shuttle, Aftergood noted, and in the event of an explosion the spacecraft itself, mounted alongside the booster rockets, would have been in the line of fire for pieces of shrapnel, and the plutonium generators could have been severely damaged. With the Titan IV, the spacecraft is at the very top of the rocket and unlikely to be hit by any flying debris even in the worst-case scenario. But if humans want to explore the outer planets, they have no real alternative to the use of plutonium-based power sources, NASA officials say. These Radioisotope Thermal Generators, or RTGs, are very simple devices with no moving parts that produce electricity directly from the heat of the plutonium's radioactive decay.
Some scientists question that claim, however.
A report by a NASA scientist in the early 1980s suggested that with sufficient research, solar cells could have been produced to power Galileo. And one scientist from the European Space Agency said that given five years, a solar power source could be developed for Cassini, according to a report last year in the Florida Today newspaper. They raise questions that deserve answers.