I have a feeling the key element here (which was unstated) would be the curvature of the planet.
The spacecraft needs to catch up to the alien, i.e. gain line of sight.
However, I don't see why any comparison of velocities would make any difference (aside from the alien merely being a...
You can probably find a rough estimate by doing the following:
-Get a rough estimate of the mass of the universe.
Assume all the mass is made up of the nuclei of hydrogen atoms. (a proton)
-Divide the mass of the universe by the mass of a proton. We now have a total # of protons in the...