Eros

From Orson Scott Card Wiki

An asteroid (24km long) where I.F. Command (FleetCom) is located. Eros is commonly thought to have been reduced to radioactive ash by the Formics. It was a roughly spindle-shaped rock only six and a half kilometers thick at its narrowest point. Since the surface of the planetoid was entirely devoted to absorbing sunlight and converting it to energy, everyone lived in the smooth-walled rooms linked by tunnels that laced the interior of the asteroid.