When I started this book, I was sure I'd read something like it -- where clones on the Moon kept going back to Earth to try and make it habitable again -- but it wasn't entirely like this one. I thought maybe it was a smaller part I read somewhere, but it's not noted in the front of the book, and I didn't find it in ISFDB. Any ideas?
This book starts on Earth where a man is trying to set up a colony on the Moon. They have three big rocks hit the Earth, the last one big enough to kill everybody, so his specialists, and a man who points a gun at them, makes it onto the spaceplane and up to the station. The robots make clones whenever they think something can be done to help Earth. At first the clones are very stupid and get killed, but there's history of what happens and later clones are not as bad. They meet aliens, humans who don't believe who they are, a colony made by earlier clones that now has slavery, etc.
A set of clones wake up with Uncle Pen, who is smaller, has internal sex organs, and brown fur on his head. The Moon had a rock impact that covered over the station. He dug it up and made a big project for his world government. The clones don't believe that they were made just to be a reminder of how the station was, and three of the guys follow Pen back to Earth. It's not at all what they think, but their differences from the new "humans" allows them to solve problems.