This is a best-practice time-limit adopted by the world's space agencies for the removal of their equipment from orbit once it has completed its mission.

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They not only had to wait until the space station steered into the proper lane for the Soyuz, but they had to pick a time when the distance between the station and the Soyuz once the Soyuz reached orbit was at a minimum.

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What makes it Vulcanlike, though, is that it circles its star, Kepler-10, once every 20 hours, in an orbit a twentieth the size of Mercury's.

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Once the two GRAIL satellites are in orbit, their next step is to begin a five week phase in which they increase the speed at which they orbit the Moon.

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