Why is gravity in space?

Gravity weakens with the square of the distance from the centre of a body, not from its surface. Earth's radius is about 6,371 km, so a station orbiting 400 km up sits at roughly 6,771 km from the centre — only about six percent farther out. Squaring that ratio gives an acceleration near 8.7 m/s² instead of the familiar 9.81 m/s² at the ground. That is close to 90 percent of surface gravity, hardly the absence of weight that the word "zero-g" suggests.

The same reasoning explains the Moon. It sits some 384,400 km away on average and remains firmly bound to Earth, held by a pull that has thinned out but never stopped. Gravity is unlimited in range; it simply becomes faint.

What astronauts experience is free fall. A spacecraft in low Earth orbit travels sideways at roughly 7.7 km/s. Gravity bends its path downward exactly as fast as the curved surface of the planet falls away beneath it, so the vehicle keeps missing the ground. Everything inside accelerates identically — crew, tools, water droplets — and nothing pushes on anything else. Weight, in the sense of a floor pressing on your feet, disappears; mass and inertia do not.

Engineers prefer the term microgravity because the condition is imperfect. Atmospheric drag, tidal differences across the length of a large station and crew movement produce residual accelerations on the order of a millionth of Earth gravity, which matters for delicate materials and fluid-physics experiments.

Far from any large body, in interstellar space, the local gravitational field really does become negligible, though never precisely zero: the combined pull of distant galaxies still acts. Near a planet or a star, by contrast, gravity dominates every trajectory a spacecraft can follow, which is why mission designers spend their effort exploiting it rather than escaping it.