While it's possible to inject the dragon capsule anywhere and get it's orbit in line with the ISS, its much more economical to inject it really close to the ISS so they dont need to do as much work to finish the trip.
They need to launch when the orbital plane of the station is directly overhead. The earth rotates surprisingly quickly, so even a minute or two later the plane of Dragon's orbit would be far enough off from the plane of the station that a couple expensive inclination change maneuvers would be necessary to line them up again.
trose|11 years ago
JshWright|11 years ago
rgbrenner|11 years ago
Not so much the earth as ISS.
The earth rotates at 24900mi/day or about 1040mph.
The ISS, to maintain its orbit, travels at 17150mph or about 5 miles per second.
So in two minutes, it travels about 600 miles.