What are some alternatives?
When comparing Universe Sandbox and Space Engine, you can also consider the following products
Celestia - Celestia - real-time 3D visualization of space.
Stellarium - Stellarium is a free open source planetarium for your computer.
Kerbal Space Program - Build spacecraft, fly them, and try to help the Kerbals to fulfill their ultimate mission of conquering space.
Gaia Sky - 3D visualization software for ESA's (European Space Agency) Gaia mission to chart about 1...
Solar System Scope - 3D model of the Solar System and night sky in real time
EVE Online - Player-created empires, player-driven markets, and endless ways to embark on your personal sci-fi adventure.