Kerbal Space Program

Sometimes is really really difficult, especially with realism mods (the best part of this game, besides the game itself is the modder community, which is huge). it offers potentially infinite hours of missions and things to do.

BUY this. This is a full-blown *realistic* aerospace simulator AND rocket/aircraft constructor. You will learn to assemble rockets, satellites and shuttles of your own design from numerous parts, fly them off planet and get back, orbit planet, do EVA, travel between planets, do inter-planetary save-and-rescue. The stupid green men are just to not confuse you, should they be killed in action. This game is to Buzz Aldrin Race into Space, like what Quake is to Wolfenstein 3d. Worth the money!!

Well, contrary to many opinions, this game is NOT for everyone. The complexity is huge, and while I like complex games, here it's worsened by the trial and error nature of the game. It feels impossible to predict all the factors that contribute to success or failure of your space mission, which includes a minigame of manually steering the ship. The experience felt tedious and repetitive. If you don't like optimization games - I recommend not to buy it.

I bought the pre-release from the developers' store and for easier updates later bought it on Steam as well. 6 years later I'm still playing it and have learnt the basics of orbital mechanics and rocket science, written a book of 35 KSP vehicle designs (very outdated now), built a network of satellites, stations, ferries and landers across the system and bases on the most important bodies. There is no feeling in the whole of gaming, and it's rare in real life, like your first moon-landing in a ship you designed, built, launched and flew to get there. Unlike in other games where that means collecting resources and filling an arbitrary, game-created, blueprint in KSP you'll know that the only reason it works is because physics says so, and you'll have learnt why and understand it (sort of; this isn't a degree-course or anything). Be prepared for a steep learning-curve though, this is a game for people interested in real space operations, physics and learning. There's also very little to do or see on the planets and moons, it's all about learning to get there and back. Youtube and other external sources will probably be necessary, depending on what you already know. Scott Manley taught most players ;-0

I love this game; there's no better one in this area. The closest is Juno, but I hope they improve the new one. 10/10 with mods