The U.S. government wants to restart research on nuclear propulsion systems for space probes and buy radioactive plutonium generators to power the robots for long-duration missions where the sun doesn’t shine. “It’s been a long time since you could say the N-word in Washington,” said NASA’s space sciences chief Ed Weiler. “But this is the right thing to do.” With nuclear propulsion, even the farthest corners of the solar system, such as Pluto, which has never been explored, will be within reach, say scientists who have been pushing for a new research and development initiative for years. NASA has no plans to use nuclear systems to propel probes off the launch pad. Rather, the nuclear reactors, which like nuclear power plants on Earth would use enriched uranium, would not be started up until they were in orbit.