If you remember the old comic books about Buck Rogers you will have noted that the events depicted in the storyline took place in the 25th century. The problem is that NASA wants to do it now before the necessary technology has evolved. They want to do in the 21st century what should be happening over the next 400 years. To make those necessary advancements in the next few years will cost the American taxpayer a lot of money and they can’t guarantee that the reliability will be such as to make the mission failure proof.