On September 15, 2017, the Cassini spacecraft will dive into Saturn, ending a 13-year tour of the ringed planet and its strange moons. Cassini arrived at Saturn in 2004, after a seven-year journey through the solar system. Its first port of call was Titan, Saturn’s largest moon. A frigid world of nitrogen smog and dark hydrocarbon lakes. Cassini released the Huygens probe to land on Titan. Parachuting through the moon’s smoggy atmosphere, Huygens sent back images of alien river beds carved out of methane and water ice. Our first touchdown on an alien moon. Cassini returned to Titan over a hundred times, using the moon’s gravity as a slingshot to shift its orbit and weave a three-dimensional pattern through space. Over hundreds of flybys, Cassini’s cameras dissolved Saturn’s majestic rings into grooves and gaps, bands and braids. For 13 years, Cassini joined the dance of Saturn’s 62 moons. Scuffed marbles chasing each other around a golden ring. The flattened moonlet Pan clears a narrow track through the rings. Potato-shaped Prometheus carves ripples in Saturn’s thin F ring. Sponge-like Hyperion tumbles chaotically through the void. And pale Iapetus sweeps its orbit clean. A ball of ice dusted with black and ridged with mountains. But the most surprising moon of all was Enceladus, glistening with fresh snow. Its crinkled shell hides an ocean of water that might be hospitable to life. Geysers of salt water shoot from stretch marks near its south pole. Cassini flew through these plumes several times. It’s sensors detected promising molecules but they were not designed to look for life. Are alien microbes hitching a ride in the briny spray? It will take a future spacecraft to find that answer. Cassini arrived at Saturn in the depths of northern winter, with the north pole in darkness. As the planet tipped downward, Saturn’s seasons slowly changed. Perfect lighting to study the north polar hurricane. A six-sided storm that could swallow four Earths. Some of Cassini’s orbits took it behind Saturn. An alien sunset before hours of darkness. Looking back past Saturn’s rings, Cassini even saw the distant Earth, a pinprick of blue light. In April, the spacecraft swung close by Titan for the last time, letting the moon’s gravity pull it inward. For the first of 22 dives inside Saturn’s rings. The “Grand Finale,” 22 chances to peer at Saturn’s cloud tops, study the pole and look out at the rings from the inside. But Cassini’s fuel is almost gone. Its watch is ending after 20 years in space. To keep the lakes of Titan and the snows of Enceladus untouched by any earthly microbes, the spacecraft must be destroyed. On September 15, Cassini will make its final dive, piercing Saturn’s clouds at over 70,000 miles an hour. Straining to remain upright as it sends its final data back to Earth. Saturn’s butterscotch clouds will burn and scatter it into a wisp of alien atoms, leaving nary a ruffle nor a burp to show for it. Just a brief meteor flash. A streak of light that no eyes that we know of may ever see.