Space: February 18, 2003


The International Space Station (ISS) orbits at an altitude of 382-406 kilometers. That's a fairly low orbit, and the ISS drifts towards the earth by about 200 meters a day. To keep the station in orbit, the U.S. Space Shuttle has used it's rockets to push the ISS up another six kilometers every time the shuttle docks at the ISS. With the Space Shuttle grounded, Russian unmanned Progress M-47 resupply rockets are doing all the repositioning. Like the Space Shuttle, when the Progress rocket is docked at the ISS, the rockets are fired just enough to move the station back up. The Progress rockets do about 75 percent of these "repositioning missions," but will now have to do all of them until the Space Shuttle is operational again. Space satellites (especially reconnaissance birds) that use these low orbits, have built in rockets to allow them to move up (or down) or change orbits. It was not thought necessary to equip the ISS with such rockets because visiting Space Shuttles and resupply ships could do the repositioning with their rockets. 




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