Gravity

Gravity is the 2,712th installment of the webcomic xkcd, released to promote creator Randall Munroe's book What If? 2. It's a rocket simulation that starts on the ground, with the player launching a small craft into space. The player controls the speed and direction of the spaceship using the arrow keys, guiding it around the vastness of space to discover various planetary bodies. Each planet or other body has its own gravity well, which affects the steering of the ship. To successfully steer the craft the player must apply their knowledge of how orbital mechanics work.

The ship can visit Earth, the Sun and Europa, as well as fictional locations like Milliways (from The Hitchhiker's Guide to the Galaxy) and the Starship Enterprise (from Star Trek). Each planetary body is filled with stick figures and jokes, many of which are references to previous installments of xkcd and Munroe's books. These jokes are presented as mini-comics with no animation or sound.