How do you chase a comet—and once you get there, how do you stop? What if you have a craft the size of an SUV and want to use a force about the same as four one-liter water bottles? As NASA's Rosetta mission continues, Exploratorium Senior Scientist Paul Doherty explains its progress and how it will approach and land on its target, Comet 67P/Churyumov-Gerasimenko.