Gravity. The thing has no engines now, Ankh. No propulsion what so ever. If you remember, it was the gravity of endor that pulled the Executor into the Death Star.
EDIT: And the Gravity of the Gas Giant Yavin that pulled in the Knight Hammer.
I don't think the entire thing would burn up upon entering the atmosphere, but you are going to loose a lot of armor plating from the bottom of the ship. The impact will also destroy more.. All in all I would have to say that, if this were real, it would take more effort to pull the hulk out and attempt to repair it than it would take to simply build a new one.