NASA is considering the use of nuclear-powered spacecraft in the future to explore Mars — an idea certain to ignite a firestorm of protest on Earth. But the plan to use nuclear power at Mars, as was done with the 1970s-era Viking mission, is “not off the table,” said Ed Weiler, NASA’s associate administrator for space science. “For better or worse, Mars is a faraway planet and using these things makes a lot of sense when you’re trying to build a robust program there.”