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