Does gravity change in space?

Gravity thins out with distance, following the square of the range from a body's centre — not from its surface. Earth's mean radius is about 6,371 km, so an orbit 400 km up sits only some 6% farther from the centre than the launch pad does, and the pull there is still roughly 90% of the 9.80 m/s² measured at sea level. The effect is a smooth curve, not a threshold: there is no altitude at which gravity switches off. To genuinely leave Earth's field behind you would have to travel far beyond the Moon, and even then the Sun's grip would simply take over.

Gravity also changes with what you are standing on. The Moon's surface gravity is 1.62 m/s², about one sixth of Earth's, which is why Apollo crews loped rather than walked. Mars gives 3.71 m/s², a little over a third — enough for dust to settle and liquids to pour normally, but probably too little to spare a long-stay crew the bone and muscle loss that dogs extended missions. Each value follows from mass and radius together, so a small dense world can out-pull a larger, lighter one.

Orbiting crews are weightless because they are in permanent free fall, not because gravity has vanished. The station and everything inside it fall around the planet at the same rate, so nothing presses against anything else. A parabolic aircraft flight produces exactly the same sensation for a few seconds a couple of kilometres above the ground, where the local pull is essentially identical to the runway's — proof that weightlessness is about motion, not about the strength of the field. The only places where gravity approaches zero are the deep voids between galaxies, and even there it never quite arrives.