Why is gravity different in space?

The acceleration a body produces is proportional to its mass and inversely proportional to the square of the distance from its centre. The Moon has far less mass than Earth, so its surface gravity is 1.62 m/s2 against 9.8 m/s2 here, close to one sixth. Mars, larger than the Moon but much lighter than Earth, gives 3.71 m/s2, a little over a third. Distance counts as much as mass, which is why the same rule that ranks the planets also explains why gravity weakens gradually as an orbit is raised.

The near-total weightlessness of a space station is not a weaker gravitational field. At about 400 km altitude Earth's pull is still roughly 90% of its sea-level strength; what has changed is that the station and everything in it are in permanent free fall, accelerating together, so nobody presses on a floor. That is why the honest term is microgravity: what remains are the tiny residual accelerations caused by atmospheric drag, thruster firings and people pushing off walls.

The difference shows up in the body. Without a load path through the legs, bone mineral and muscle are lost, fluids shift towards the head and the balance system recalibrates, which is why long-duration crews exercise for hours every day and why returning astronauts often need help walking. Partial gravity remains largely unexplored: nobody has spent more than about three days in the Moon's 1.62 m/s2, and no human being has ever experienced Martian gravity at all.