Once the shuttle is retired this year, the only way for the United States to get astronauts into orbit will be on Russian Soyuz rockets.
VOA: special.2010.04.28
This is a best-practice time-limit adopted by the world's space agencies for the removal of their equipment from orbitonce it has completed its mission.
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.