Acho que isso pode ser Arcturus Landing por Gordon R. Dickson.
O verão de Goodreads parece combinar decentemente com sua descrição.
Quarantine...Ever since Earth's first starship crept out on sublight drive a hundred years ago and was met by an emissary of the Galactic Federation, man has lived as a prisoner in the solar system. The Federation decreed that unless man could demonstrate his technological maturity and leave the solar system via a faster-than-light drive, he would not leave it at all. All Earth knows the test that man must pass to win Galactic Citizenship and the freedom of the stars; the FTL drive is every physicist's top priority. But what they cannot know is that they work against both an alien time-limit and human sabotage - and time is running out. Unless John Parent's star-drive project is successful, mankind will remain trapped in the solar system, forever
Nenhuma menção ao seu alienígena.
Foi originalmente publicado em 1956 (como "Alien from Arcturus"), em seguida, expandido e re-lançado em 1978, então ele também corresponde à sua estimativa de tempo.