Eon by Greg Bear (1985)
This is an older book (published in 1985), and is set in the near future (even from today’s standards). A massive asteroid suddenly appearing in earth’s orbit? Ruins inside the asteroid? More space inside the asteroid than outside? Yes to all of these.
This book takes you on a journey inside the asteroid, with awe-inspiring hard-science fiction. It is also the first book of a 3-part series, so if you enjoy it, there is more!