I wish I could remember more of this book. The overall premise was that there were humans living within a comet, possibly even Halley's Comet. I don't remember how they got there, but it was not something like a lost offshoot of the race. They were the result of a mission to the comet or they were refugees.
They had tunnels running throughout the comet and had adapted to the difficult living situations and I think they had even evolved. In the story, I think the comet was making its first approach toward the Sun after many years (I think so many that it likely wasn't Halley's Comet, but it might have been).
My memory is very sketchy, but I think they had to deal with not only the issues of the destabilization of their world as it approached the Sun, but also with the rest of humans on the Earth either wanting to land on the comet or bring it in or something else -- I don't remember what, but there was either some kind of conflict with Earth or Earth was totally disinterested in helping them as they swung so close to the Sun.
It was definitely a hard science book and I think the author was well known, but I don't remember for sure on that last point.