How much gravity in outer space?

Gravity weakens with distance but never switches off. Double the distance from a body's centre and its pull drops to a quarter; move a thousand times further and it becomes a millionth, yet it remains. That is why the Moon stays with Earth, Earth with the Sun and the Sun with the Galaxy across distances that dwarf any spacecraft's journey. There is no boundary where gravity stops, and the phrase zero gravity has no physical meaning: even in the emptiest volume between galaxies, a test mass still feels the pull of everything around it.

When gravity is quoted as a single figure, it is usually surface gravity. On Earth it is 9.80 m/s², the speed an unimpeded object gains for each second of fall. On the Moon it is 1.62 m/s², about a sixth of Earth's, which is why the Apollo crews bounced rather than walked. On Mars it is 3.71 m/s², a little over a third. The same body weighs different amounts in each place while its mass never changes: weight is a force, mass is an amount of matter.

Low Earth orbit is not a low-gravity zone. A station a few hundred kilometres up is still well inside Earth's field, and the pull there is only modestly weaker than at sea level. Crews float because they and their vehicle fall together, so nothing presses them against a floor. Genuinely low gravity is found on the surfaces of small bodies: on an asteroid a few kilometres across, a careless jump can exceed escape velocity. The distinction is an engineering one as much as a physical one — a spacecraft in orbit is firmly held by gravity, a body on a tiny asteroid is barely held at all.