Awesome in its power and beauty, the planet Saturn has gripped the collective imagination of humankind from the day of its discovery, but its extraordinary remoteness has made it inaccessible to telescopes on Earth. Two decades ago, a team led by the Jet Propulsion Laboratory in Pasadena, California, started planning the mission that changed that. 7 years and 2.2 billion miles in transit to Saturn, the spacecraft Cassini arrived in July 2004 for a...