What are some alternatives?
When comparing Space Engine and Universe Sandbox, you can also consider the following products
Stellarium - Stellarium is a free open source planetarium for your computer.
Celestia - Celestia - real-time 3D visualization of space.
Gaia Sky - 3D visualization software for ESA's (European Space Agency) Gaia mission to chart about 1...
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
KStars - KStars is a Desktop Planetarium for KDE.