Orbit gravity is about 90% of surface gravity; floating is free fall.
Precise claimAt the space station's altitude of roughly 400 km, Earth's gravitational acceleration is still about 88–90% of its surface value — roughly 8.6-8.7 m/s². Astronauts float because they, the station, and everything aboard are in continuous free fall around Earth, not because gravity has vanished.
Applies
Low Earth orbit at the station's nominal altitude, computed with a spherical-Earth inverse-square model and consistent with NASA's rounded figures. Free fall, not altitude, explains the floating seen aboard.
Does not prove
The model ignores Earth's oblateness, orbital eccentricity, residual atmospheric drag, and gravity-gradient effects. It does not describe gravity far from Earth, and 'microgravity' means apparent weightlessness, not a measured zero field.
Portable rule
If something floats or feels weightless, then check whether it is falling freely before concluding that gravity is absent.
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Is There Really No Gravity on the Space Station?
Estimate how strong gravity really is at the space station's altitude, then change only the sideways speed and watch falling turn into orbiting.
The International Space Station orbits roughly 400 km above Earth's surface, and the astronauts aboard float. Before you see a single number: how strong is Earth's gravity up there, as a percentage of its strength at the surface?
0% — no gravity at all100% — same as the surface
Slide to your estimate, then lock it. The reveal stays hidden until you commit.
The reveal
At the space station's altitude of roughly 400 km, Earth's gravitational acceleration is still about 88–90% of its surface value — roughly 8.6-8.7 m/s². Astronauts float because they, the station, and everything aboard are in continuous free fall around Earth, not because gravity has vanished.
Computed live in this page from g(h) = g0·(R/(R+h))² with R = 6,371 km and g0 = 9.81 m/s² — nothing prerecorded:
At 420 km, gravity computes to8.63 m/s² —88.0% of its surface strength.
The slider stops at 300 and 600 km. Across that whole low-orbit range gravity stays between83.5% and 91.2% — a single-digit-percent change. At the station's roughly 400–420 km altitude that is 8.63–8.69 m/s². Distance did not turn gravity off.
computed 88.0%you
0%surface strength · 100%
You estimated —; the computed value at 420 km is 88.0%.
So if gravity up there is still about 88% of surface strength, why do astronauts float? Because the station, the crew, and everything on board are falling toward Earth together, without pause. A freely falling elevator floats its passengers the same way. Orbit is that same fall — with enough sideways speed (about 7.7 km/s, circling Earth roughly every 93 minutes) that the ground curves away as fast as the craft falls toward it.
The rebuilt model
You assumed:
Astronauts float because gravity at the space station's altitude is nearly zero — up there, Earth has run out of pull.
The actual model:
Gravity falls off with the square of distance from Earth's center, and 400 km adds only about 6% to that distance, so about 90% of the pull remains. Orbit is free fall with enough sideways speed — about 7.7 km/s — to keep missing the ground; take the speed away and the same gravity brings the craft straight down.
The variable that failed you:
Tangential velocity — the sideways speed that turns falling under about 90% gravity into falling around Earth.
Change one variable
Hold the altitude — and therefore gravity — fixed at 420 km (8.63 m/s², 88.0% of surface strength, exactly as in the reveal). Change exactly one thing: the craft's sideways (tangential) speed at launch.
0–10 km/s. Altitude stays fixed at 420 km; gravity stays fixed at 88.0% of surface. Only the speed changes.
Circular-orbit speed at 420 km computes to 7.66 km/s. Keep the altitude fixed, slide only the speed, and launch again — well below it the craft falls back, near it the fall keeps missing the ground.
Fixed in this rerun: Earth's mass and radius, the inverse-square law, the altitude, and gravity at 88.0% of surface. Changed: tangential velocity only. Spherical, drag-free model — the boundary is stated with the evidence below.
ISS altitude of 200-250 miles with Earth's gravity there about 90 percent of surface strength; astronauts float because they are in free fall; station speed of 17,500 miles per hour; free fall reproducible in parabolic flights and drop towers.
Astronauts and the ISS fall together around Earth so astronauts float, even though gravity at that altitude is only about 10% less than at the surface.
Station travels at about five miles per second and orbits Earth about every 90 minutes, making 16 orbits per day — corroborating the computed orbital speed and period.
official-doc — checked 2026-07-20.
Scope: Low Earth orbit at the station's nominal altitude, computed with a spherical-Earth inverse-square model and consistent with NASA's rounded figures. Free fall, not altitude, explains the floating seen aboard.
Does not prove: The model ignores Earth's oblateness, orbital eccentricity, residual atmospheric drag, and gravity-gradient effects. It does not describe gravity far from Earth, and 'microgravity' means apparent weightlessness, not a measured zero field.
If something floats or feels weightless, then check whether it is falling freely before concluding that gravity is absent.
Science news
A 'zero gravity' experiment headline usually means a free-fall setup — a drop tower, a parabolic flight, or an orbit — not a place beyond Earth's pull.
Explaining to others
The floating sensation in a dipping elevator or at a roller-coaster crest is the same free-fall mechanism as orbit; no altitude is required.
Science fiction review
A scene where a ship's engines cut and gravity 'returns' misstates the physics; weightlessness ends when a floor or thrust pushes back, not when gravity switches on.