Stars - the number of stars that a project has on GitHub. Growth - month over month growth in stars.
Activity is a relative number indicating how actively a project is being developed. Recent commits have higher weight than older ones.
For example, an activity of 9.0 indicates that a project is amongst the top 10% of the most actively developed projects that we are tracking.
What are some alternatives?
Ferram-Aerospace-Research - Aerodynamics model for Kerbal Space Program
RealSolarSystem - Changes KSP's solar system to make it like the real one.
RSS-Textures - Real Solar System Textures
RealFuels - Modular fuel tanks and engines, with real fuels and realistic performance.