Starbound Marsbound Series, Book 2.

Carmen Dula and her husband spent six years travelling to the distant home of the powerful race known as "The Others," in the hopes of forging a truce. But by the time Carmen returns to Earth, fifty years have passed-and the Earthlings have built a flotilla of warships to defend Earth against The Others. But The Others have more power than anyone could imagine-and they will brook no insolence from the upstart human race.