Good luck with that, I'm still waiting for my flying car.
Maybe we should go fishing on the moon to take our mind off-
War is an ugly thing, but not the ugliest of things: the decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth a war, is much worse...A man who has nothing which he is willing to fight for, nothing which he cares more about than he does about his personal safety, is a miserable creature who has no chance of being free, unless made and kept so by the exertions of better men than himself.
As has been mentioned the capture of German rocket technology is what really helped kick off the space race between the US and the USSR. Plus the US could already be considered a super power by 1947. Plus after 1947 the US didn't become an unstoppable technologcial powerhouse...unless aliens were helping the Russians as well.
But making the huge assumption that an alien did survive a crash and assuming we could communicate with it and assuming it was willing to teach us their techology (where's my space ship damn it!) it would also be under very close watch and guard with likely no way to communicate to the outside world. So how did the alien order the assination? How could it know what others wouldn't just keep going where Kenndy had left off?