Imagine a world where everyone is just a tad bit inept. A world where rocket parts are found lying conspicuously on the side of the road and space capsules are built by a paper products corporation. A world populated by little green men with dreams of going to the stars. This is the world of Kerbal Space Program (KSP), a spaceflight simulator in development by the independent studio Squad. The premise is simple: design and build your own rockets, launch them into space, and (optionally) return your Kerbals safely back to their home planet of Kerbin. Oh, and one more thing—at every step of this process it's absolutely necessary to pray that nothing blows up.
Because explosions are the true essence of Kerbal Space Program. Even the most skilled rocket designer and pilot will sacrifice countless Kerbals in brutal and often hilarious ways on the path to space. It could be a harmless miscalculation: maybe launching that rocket with 30 solid rocket boosters in the first stage wasn't such a good idea... It could be a slight design flaw: you really thought the stitching on that parachute would've held together better than that. Apparently aerobraking is far safer (though, admittedly, less effective) than lithobraking. Maybe your Kerbals suffered the classic launchpad failure, sitting on top of a rocket that doesn't fly up so much it lurches sideways towards the nearest and most expensive building. Or maybe you just neglected to give your heroic Kerbonauts enough fuel to return home to kerba firma and now they'll orbit for eternity in their Mk. 3 Space Coffin.
Like many recent indie hits (most notably Minecraft), the developer, Squad, has a somewhat unconventional buisness model. Instead of front-loading the development costs and selling a finished product, Squad's strategy is to fund development of the game by selling the alpha version with the promise that early buyers will recieve all subsequent updates for free. On the one hand this means that many of the features you might expect from a spaceflight sim—such as atmospheric heating on reentry, planetary base construction, resource mining, etc.—are labeled as Coming Soon™ though there are many mods available which fill some of these gaps. The flipside of this is that regular updates maintains interest as the release of new content continually changes the gameplay, adding new features to play around with and places to explore. Personally, I'm actually a fan of this business model because updates give me something to look forward to and because it allows the community surrounding a game to shape the development of new features, though I can understand why others find buying a less-than-finished product grating.
The first step in taking Kerbalkind to space is constructing a rocket. This is, in itself, half the fun, whether you choose to refine your design to achieve a given mission with the most efficient rocket possible or choose to construct enormous monstrosities that contain more power than a small nation. The stock version of the game has fairly full selection of available parts including various sizes of solid and liquid engines as well as more experimental technologies like aerospike engines, NERVAs, and ion engines. Also provided are necessary accesories to the rocket itself such as decouplers for staging your rockets, landing legs, docking ports, parachutes, RCS thrusters, and steering fins. There is also another set of parts that can be used for spaceplanes which features various wings, cockpits, fuselages, and air-breathing jet engines allowing for atmospheric flight or, for the ambitious, SSTOs. Design is an extremely intuitive click and drag process that anyone can pick up with only a couple minutes experience though there are some poorly-documented features that allow you to tweak your designs even more.
In addition to the stock parts, Squad has intentionally made KSP easy to mod and as a result many players have created their own rocket parts which you can download and add to your game. These user-created parts range from recreations of real rocket components like the Soyuz capsule to more generalized technologies like engines running on different fuels, life support, and resource mining to completely fictional antigravity devices and hyperdrives.
After you've completed your rocket you take it to the launchpad for the 'spaceflight sim' part of the game. This is mostly straightforward: various instruments tell you your speed, heading, and altitude. In one corner of your screen there are live video feeds of the faces of the Kerbonauts inside the cockpit. Most of the time they have expressions of pants-shitting terror (except for certified badass Jebediah Kerman). Piloting is generally done in 3rd person but there are first person options available for most capsules. Unlike many flight sims which often require a joystick to play effectively, using the keyboard is a more than acceptable way to control rockets. That said, a joystick may help add to the immersion.
While flying your craft, you can change to the map screen which shows the entire solar system with all the planets, moons, capsules, and space junk currently in it. This is extremely useful for showing the characteristics of your orbit like periapsis, apoapsis, and any upcoming sphere of influence changes. Unfortunately the developers have chosen to solve the three body problem by ignoring it altogether and simulating gravity with only two objects at once. This was done for performance issues but had the unfortunate result of there being no Lagrangian points to play around with.
Currently (as of the 0.24 update in July 2014) the game features 5 planets (including your home planet of Kerbin), 2 dwarf planets (a Ceres and a Pluto analogue) and 9 moons with more planets promised for future updates. The Kerbals' solar system is roughly analogous to our own with some changes, generally in the number of moons. Kerbin, for example, has two moons: the Mun and a much smaller and more distant satellite named Minmus. Still, the similarities greatly outnumber the differences. There are four rocky inner planets resembling Mercury, Venus, Earth and Mars, as well as a gas giant with its own system of five moons. There are also randomly generated asteroids which you can visit and even capture and push around.
Currently Squad is working on adding a campaign or 'career mode' to KSP. There is a tech tree which you can unlock by collecting scientific data during your missions. There is also a budget and contract system which pays you money for completing certain objectives. However these features are still under development and, in my opinion, lack depth. But there is still a lot of fun to be had in setting and achieving your own challenges. Achieving orbit is an obvious place to start, then maybe landing on the Mun, and finally setting off for another planet. Getting back from those places adds another challenge. Or maybe one of your landers ran out of fuel, stranding a poor Kerbal millions of kilometers from home and requiring a rescue mission with new rocket designs and precision landing skills. A challenge I find myself returning to frequently is designing an efficient and reliable SSTO that can fly from runway to orbit and back.
There's a certain kind of person that this game appeals to (generally the slighty-OCD, space geek type). To wit: because of this game I found myself learning honest-to-god rocket science for fun. Calculating the delta-v of my rocket designs using the rocket equation, the ideal thrust for a given atmospheric density, and figuring out the most efficient paths for interplanetary travel using gravity assists, the Oberth effect, and Hohmann transfer orbits. Even just flying your ship requires learning some orbital mechanics and concepts like the gravity turn, prograde and retrograde, and apoapsis and periapsis. That's not to say there's an incredibly steep learning curve on KSP—for most intents and purposes throwing together a really big rocket can get you almost anywhere no matter how you fly it. But for those looking for more, there's a ton of depth available.
If it sounds like something you might be interested in, give the demo a try. It features a limited number of parts and features compared to the full game but it should give you a good idea of what the game is about. As a warning, KSP is surprisingly resource intensive, especially on your computer's memory. However, the game continues to be optimized as it continues develpment.