Squad isn't done adding to Kerbal Space Program just because the game's protracted development is over... if anything, the studio is just getting started. The company has unveiled KSP's first expansion, Making History, and this is definitely represents more than your usual downloadable add-on. It earns its namesake from the ability to reenact real-life space missions (insofar as you can in a fictional universe, anyway), but the real highlight is the option to create your own missions. There's a simple "drag and drop" interface, Squad says, and you can throw players for a loop by introducing everything from arbitrary limits (such as fuel or time) to surprise events.
Source: Kerbal Space Program