Apps similar to Ad Astra include Space Engine. It's paid, but Oolite is free. Between them they cover Windows, Linux and Mac.
Space Engine - a real-time 3D space simulator, procedural universe generator and visualiser. It lets you explore the universe in three dimensions, starting from planet Earth to the most distant galaxies.
Russia
Oolite is a space sim game, inspired by Elite. Among Oolite's several similarities to its inspirational source, the gaming experience is enhanced by the context set in Elite's original manual, and the accompanying novella, The Dark Wheel.
