Check out our huuuuge(!) list of space game assets for Unity3D: Unity3D Space Game Assets.
In order to help not only authors, but also user, we decided to feature thematically sorted asset lists.
Space Game Assets List for Unity3D
Our first list is about space games. It features all different kinds of assets you will need to build a space game: Asteroids, Space Ships, Planets, UI, FX, Sounds etc. etc.
Browse thru the bazillion of assets, which we pre-evaluated, selected and categorized for you. Here is the link: Unity3D Space Game Assets.