Some explorers have focused on alpine heights. Others on polar extremes. Megan McArthur is one of the elite few who can say she's piloted both submarines and spacecraft, exploring expanses from the ocean floor to low Earth orbit, looking down on the planet from 250 miles above. Now McArthur, 54, is retiring from NASA, where she has served for more than two decades as an astronaut and senior leader at the agency's Jet Propulsion Laboratory in Houston. Emily Carney, a space historian, described