Eventually, Kerbal Space Program becomes a game about triumph. Getting into space. Landing on the moon. Mastering physics to leave the ground behind and blasting into the history books on a rocket of your own creation. When it finally happens, it's one of the most satisfying moments in the history of gaming. What makes Kerbal a classic, though, is what happens before that point. As a great Kerbal once probably said, we do not choose to go to the Mun because it is easy, but because... well, why not? Look at it, hanging up there in the sky like a great big smug thing. Don't you just want to wipe the smile off its poxy moon face?
Achieving this, just one of many goals, is literally rocket science. Take a command module, slap on an engine, some fuel, perhaps a parachute if you're feeling generous, and if you're lucky, the whole thing won't explode on the launchpad or go spinning into an explosive death-dive. The cute characters and playful nature both take the edge off failure and make things more approachable than more serious attempts like Microsoft Space Simulator or the much older Buzz Aldrin's Race Into Space. They're simply a soft felt glove over an iron core, though. Kerbal Space Program both understands, and more importantly, respects the challenge of getting into space, and the tremendous war on physics that it takes. The slightest weight imbalance, the tiniest piloting mistake and: boom. Weather reports everywhere reporting roasted Kerbal raining from the skies. Again.
It takes many, many such failures before you finally get to see the world dropping away to be replaced by infinite space, in a moment as satisfying as any achievement in gaming right now - and even then it comes with a reminder that you've no more mastered the stars than a kid with a dinghy can hope to conquer the sea. It's rare for a game to truly convey that sense of wonder or the promise of something new, but this one delivers - a feeling that only happens because it's made you work your bottom off for every last little scrap of your success.