As I noted previously, campaigns in space require a bit of additional focus to make them sufficient in scope and scale to handle at the table. This is why I'm using Knights of Sidonia as an example. In this case, you have a space-faring campaign centered around a generation ship and the militia tasked with guarding it against the ship's alien opposition. (You want more? Watch the series.)
In RIFTS, this is easily done. One focusing on the solar system can work from Mutants In Orbit (you can do the orbital end of Mobile Suit Gundam and its successors easily) or Scrapers, and elsewhere using the Phase World books (selectively, of course). If you decide to use the Aliens Unlimited sourcebooks for Heroes Unlimited for additional material, you will go far. Yes, you will have to brew your own content, but there is so much archetypical examples to work with that doing it won't be hard.