Kerbal Space Program is an indie game about building rockets that get the cute little Kerbals into space (and hopefully back, but probably not).
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Pro You don't get bored with it, and you will learn a lot
Build rockets, space planes, space stations, rovers, landers, probes. Explore planets, moons, capture asteroids. With the addition of mods you can even test warp drive, set up a communications network, colonize a planet, mine for resources and much much more. It can even simulate our solar system (the in-game one is very similar, although smaller to encourage new players.
Learn orbital mechanics. Inspire yourself to learn about space and space travel history of human. This game is not only fun - it is inspiring.
Pro Teaches orbital dynamics & rocket design fundamentals in an accessible manner
The physics engine does a reasonably accurate job of replicating the fundamentals behind both orbital dynamics and rocket design, so by going through the tutorials and experimenting, you're actually learning some real rocket science.
Pro Construction sandbox for building and launching rockets
Kerbal at its core is a rocket building and launching sandbox.
Pro NASA working on a mission pack for the game
While not released yet NASA is working on a mission pack for the game.
Pro Huge modding scene
Pro Fun sandbox mode to experiment with rocket designs
The rocket designer lets you experiment with as many different rocket designs as you can think up. The designer is based on components such as engines, fuel cells, structural elements and various other items such as parachutes.
Pro In constant development
KSP is a shinning example of early release done right. The currently available game is fun to play, and the developers are constantly adding new major additions.
Con Graphics are not the most detailed
The graphics of KSP are not the most detailed but they work non the less.
Con Steep learning curve
Kerbal Space Program is not really a pick up and play title. There is much to learn before grasping the core concepts of the game,