Casey Dreier tells the winding tale of Plutonium 238, the radioactive isotope that is vital for exploration of the solar system. The US came dangerously close to exhausting its supply. Even with production restarted, there are major challenges facing explorers. Bill Nye reviews “Gravity,” while Emily Lakdawalla looks at what may be a budding space race between two ambitious Asian nations. Things turn spooky weird on this week’s Halloween edition of What’s Up.