In October 1997, a Titan rocket streaked across the sky and shot a spacecraft called Cassini toward Saturn. The road trip, minus roads, was long, and Cassini didn’t arrive until 2004. But it stayed there till its mission ended on Sept. 15, 2017 — with a bang, and a good deal of whimpering from Earth. Early that morning, engineers at NASA’s Jet Propulsion Laboratory sent Cassini down to meet the planet it had spent 13 years studying. The greeting was fatal for Cassini, which …