Gravity pulls a satellite towards Earth's centre at every moment, bending what would be a straight-line path into a curve around the planet.
Gravity does not switch off in space. It is the very thing that keeps a satellite in orbit. At every instant, Earth pulls the satellite towards its centre.
If gravity vanished, a satellite would simply carry on in a straight line and drift away from Earth. NASA describes it this way: the motion of a satellite around Earth is mostly controlled by Earth's gravity.
Many people believe there is no gravity where the International Space Station flies. That is wrong. At the station's height, Earth's gravity is about 90 percent of its strength on the ground, according to NASA.
Gravity does get weaker with distance, but the distance that matters is from Earth's centre, not from the ground. Earth's mean radius is about 6,371 km. Flying 400 km up adds only about 6 percent to that distance, so the pull barely drops.
Gravity weakens with the square of the distance from Earth's centre. At the surface, NASA gives the mean pull as 9.82 m/s².
Even far out, gravity is weak but never zero. That small pull is still enough to hold a slow, distant satellite in its path.
Picture a satellite moving sideways, parallel to the ground. Gravity pulls it downward, across its direction of travel. A pull across the direction of motion does not slow the satellite; it turns it. With the right sideways speed, the turning never stops and the path closes into a loop: an orbit.
Because gravity is weaker higher up, a high satellite needs less sideways speed to stay in its loop. NASA notes that the higher a satellite's orbit, the slower it moves.
Check the Altitude card: the station holds about 400 km up, where Earth's gravity is still roughly 90 percent as strong as on the ground. Open the ISS tracker
Newton's law of gravitation gives the acceleration of a small body at distance r from Earth's centre:
g(r) = GM / r²
For Earth, GM ≈ 398,600 km³/s² (NASA Earth Fact Sheet). Using GM directly avoids needing G and Earth's mass separately.
Check at the surface, with mean radius r = 6,371 km: r² = 40,589,641 km². g = 398,600 ÷ 40,589,641 = 0.009820 km/s² = 9.82 m/s². This matches the fact sheet's mean surface gravity.
Now the table for satellites:
What gravity does to the path follows from Newton's second law. For a circular orbit, the acceleration needed to keep turning is v²/r. Setting v²/r = GM/r² gives v = √(GM/r), the circular orbital speed. Gravity supplies exactly the inward acceleration a circle demands; there is no separate outward force acting on the satellite. The ratio g(r)/g(surface) = (R/r)² is why the pull falls slowly at first: a 6 percent increase in r gives about a 12 percent drop in g.
Yes. At the height of the ISS, Earth's gravity is about 90 percent as strong as on the ground. Gravity is what keeps satellites in orbit.
It falls to one quarter, because gravity weakens with the square of the distance from the centre.
No. In a circular orbit gravity pulls at right angles to the motion, so it only changes the satellite's direction, keeping it on a curved path.