Fuel was expensive and physics was free, so the most economical way to get a lot of people from here to there was to depend heavily on gravity assists, which meant waiting years for the planets to align themselves in a way that facilitated a slingshot. Missed your flight? No problem, there'd be another in eight years.

— The Vela: a novel by Becky Chambers, Yoon Ha Lee, Rivers Solomon, and 1 other