At age 43, America's space agency is having a midlife crisis. In its youth, NASA put men on the moon and inspired the nation's schoolchildren to study science, mathematics, and engineering. Today, like many middle-age Americans, NASA is struggling to get back in shape and find meaning in its life. The agency's main focus, the international space station, is mired in cost overruns that have driven the estimated price from $17.4 billion to more than $30 billion-and nobody believes even those numbers. Moreover, two of the past three missions to Mars have ended in disaster. In 1999 the Mars Polar Lander, which was supposed to gently touch down and search for ice, crashed into the planet's surface instead. That same year the Mars Climate Orbiter, intended to be a Martian weather satellite, was an even bigger embarrassment. It burned up in the atmosphere because of what seems a ludicrously careless mistake: The engineers neglected to convert navigation figures from English units to metric.