right or left therefore rotates the Solar System around the ship instead. Each planet or moon has a mass, and therefore a gravitational pull, though they do not affect one another and only the single strongest pull affects the player's ship. This sometimes results in odd behavior; for example, the gravitational effect of Mars is much stronger than that of its moon Phobos. This means that a player attempting to land on Phobos needs to allow the ship to fall below the moon's surface until it is close enough to Phobos's center that Phobos's pull becomes the dominant force, at which point the ship snaps back to be landed on the surface. The name of the planet or moon with the current strongest pull is displayed on the screen. Players are able to edit the program to change the conditions; popular variations by the original players were increasing the gravity level and thus the difficulty, or an adjustment to the coordinate display system so that, rather than the ship staying in the center of the screen and the planets moving relative to it, the current dominant planet would always be at the bottom of the screen, with the ship moving relative to it. Development In 1969, programmer Ken Thompson worked for Bell Labs on the Multics operating system on a GE 645 mainframe. During his work, Thompson developed Space Travel on the system. Multics was a collaborative project between several institutions for an interactive, multi-user operating system that provided convenient access for programming. It was also growing very large and complex, and Bell Labs decided to exit the project in 1969. Thompson then ported the Space Travel code to Fortran so that it could run on the GECOS operating system already present on the GE 645. Thompson and other Bell Labs employees, such as Ravi Sethi and Dennis Ritchie, played the game on the system. GECOS, however, used an "interactive batch" model, meaning that while several computer terminals were attached to the central computer, each terminal's program jobs had to be submitted in a queue, resulting in long pauses in the game while the GE 645 worked on jobs for other terminals. Additionally, the system required the user to type in commands rather than press buttons, resulting in the ship being difficult to control. That same year Bell's internal computer departments were split, with the teams working on development put