Propulsion
The ISS orbits the Earth at an altitude of 217 to 285 miles (362 to 475 km). At this altitude, the Earth's atmosphere is extremely thin, but still thick enough to drag on the ISS and slow it down. As the ISS slows down, it loses altitude. In addition to atmospheric drag, solar flares also slow the station down and cause it to lose altitude. So the ISS will need to be boosted periodically to maintain its proper altitude. The command and service modules have rocket engines that can be used to boost the ISS in the early stages. However, the Progress supply ships will do most of the reboosting. Each reboosting event requires two rocket engine burns. During the burns, work on the ISS will be suspended. After the burns, station life will return to normal.
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