I have little recollection when I read this, but it started with people observing indications of some special engine use many lightyears away, and it led them to 'reverse-engineer' the traces to create their own drive. Had the usual tropes of international crew-selection for first-voyage.
Somewhere in the travel, they encounter a sentient star or quasar.
When they get back hundreds/thousands of years later, they arrive to an uncaring Earth that no longer pursues space-travel.
