Considering the speed at which the Earth moves, even a short mission away from Earth would result in the planet being millions of miles away from the point of origin at the time you left. Nasa calculations to reach a simple point like the Moon require enormous calculations and pin point accuracy. Any small error left uncompensated would result in Astronauts missing the Earth on return.

Imagine for a moment that you have a faster than light or warp capable star ship...

Why is this of importance? For any discussion on the posibility or the probabiliy of space flight dealing with interstellar flight, it is necessary to have this data.