It wouldn't be impossible to build, just very difficult. We would have to find an area which is not plagued with too much seismic activity, and then the building would have to have a very large base, and yes the building would have to have new construction techniques to complete it, and yes it would have to be air tight at the top, but once it was finished we would have a docking port for space craft to be readily re-supplied, and re-fueled, and re-fitted. Other countries would either have to build one or more of their own to keep up with the USA or they would have to pay for use of ours. It keeps being mentioned that this building could come down, but other than the twin towers which were struck by planes how many other skyscrapers have fallen without planned demolition?