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The Earth’s gravity is constantly drawing the International Space Station back toward the surface of the Earth. This is called orbital decay, (90 meters/day for the ISS). Drag (air resistance) from the uppermost layers of the atmosphere slow down the ISS, which makes it more prone to orbital decay. Visiting supply spacecraft come equipped with booster rockets to push the ISS back into the desired orbit. The ISS also performs reboost maneuvers with rockets on the Zvezda module to stay on track.