Phobos

Phobos was one of Mars' moons. Due to its ellipsoid shape and its proximity to Mars the gravity on the surface of Phobos altered dramaticly throughout its orbit around Mars. By the 26th century humans had established themselves on Phobos; such structures included Mars Luna Park an environment dome that maintained a fixed gravity within. Humans also visited Phobos for its range of "extreme sports" such as gravity-boarding and wormhole-jumping. (BFA: Phobos)