Lasers powered by nuclear reactors would do the job. There are other alternatives too. Once in space. There are electric drives of various sorts, as well as using a reactor to heat reaction mass. Of course my favorite for the in space propulsion system is "Old Bang Bang", which would have used small fusion bombs (i.e thermonulcear devices) acting against a LARGE buffer plate to provide propulsion. See "Footfall", by Niven and Pournelle, for a description, and a rolling good story too.
I was re-reading some of Pournelle's non-fiction writings from the 70s, and we have made very darn little progress since then. Its 2002, 2001 is dead and gone, where is my Hilton space hotel? My Pan Am (I'll settle for American Airlines) shuttle, and my moon base? I'm so disappointed. Back when I first watched the movie (first run in theaters, had to travel 60 miles to see it) those were realistic expectation, more or less, if the politicians, starting with LBJ and Nixon, hadn't messed things up so badly.