How does gravity in space work?

The governing relationship is simple: attraction grows with the masses involved and falls with the square of the separation, measured from centre to centre. Doubling the distance quarters the pull. Critically, the rule never returns zero. Earth's field reaches the Moon 384,400 km away and beyond; the Sun's holds Neptune at 4.5 billion km. Gravity cannot be blocked, absorbed or cancelled by any material, which is why spacecraft designers treat it as the permanent background condition of every trajectory.

That is also why orbital altitude changes less than people expect. Measured from Earth's centre, a spacecraft 400 km up is only about six percent farther out than someone standing on the ground, so it still experiences roughly 90 percent of surface gravity.

An orbit is a permanent fall that never lands. Give an object enough horizontal velocity and gravity's downward curvature of its path matches the curvature of the planet beneath. In low Earth orbit that velocity is around 7.7 km/s, completing a circuit in roughly 90 minutes. Everything aboard falls at the identical rate, so nothing presses against anything else and the crew float — free fall, not absent gravity.

Mission design exploits the field rather than fighting it. A gravity assist steals a sliver of a planet's orbital momentum to accelerate a probe without propellant; Voyager 2 used successive encounters with Jupiter, Saturn, Uranus and Neptune to cross the outer Solar System. Lagrange points, where the combined fields of two bodies and the orbital motion balance, park observatories such as the James Webb Space Telescope in stable stations 1.5 million km from Earth.