In conclusion: In the Early 1970s, Nixon saw the NERVA project as a gateway to a manned Mars station, which he also saw as ending up costing a LOT of money. Remember in the early 70s we were still in the Cold War with the Soviets, so anything we would try to do, the Soviets would (shortly thereafter, and pretending it was their idea all along) also do the same thing. So, Nixon didn't want to get into a spending match with the Soviets, to see who could blow the most money on their space program. It's too bad, too, because this could be seen as the end of manned exploration in space. I think XKCD said it best, "Ok, so Spirit and Opportunity are pretty awesome. And Kepler. And New Horizons, Cassini, Spirit and Opportunity, Curiosity, TiME, and Project M. But c'mon, if the Earth were a basketball, in 40 years no human's been more than half an inch from the surface."