After a 30-year hiatus, government rocket scientists want to resurrect efforts to design a nuclear-powered propulsion system, a controversial concept crucial to any program for human exploration of the solar system. A $125-million initiative for developing the technology, which has frustrated scientists and engineers since the 1950s, was quietly inserted into the proposed fiscal 2003 NASA budget, which was unveiled this week. The space agency’s plan calls for a $1-billion program over five years, a project that would include significant roles for the Jet Propulsion Laboratory in Pasadena, Glenn Research Center in Cleveland and Marshall Space Flight Center in Huntsville, Alabama. Using nuclear technology would in theory slash a trip to Mars and back by more than half from about two years to less than a year, for instance, and alleviate lingering concerns with the health effects of long-duration space travel, NASA officials said. Astronauts who occupied the former Soviet operated Mir space station for months at a time suffered from muscle atrophy, bone loss and other crippling effects of prolonged exposure to micro gravity.