What are some alternatives?
When comparing Universe Sandbox and Gaia Sky, 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.
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.
KStars - KStars is a Desktop Planetarium for KDE.
EVE Online - Player-created empires, player-driven markets, and endless ways to embark on your personal sci-fi adventure.