Dinosaur Thunder, by James F. DavidRead the other entries on the list.
The entire Thunder series concerns the modern world colliding with the Cretaceous in bizarre time distortions, but Dinosaur Thunder has a particularly memorable moment: astronauts find that a T-Rex made it to the moon before they did. Worse, there’s a certain asteroid on the way that might destroy present-day Earth. Dinosaurs, sci-fi, and a trip to the moon? Awesome.
--Marshal Zeringue