How realistic is KSP now? I remember trying it out when getting to low-Kerbal-orbit was the only thing you could do and you had about a dozen parts.
You have a solar system with 5 planets+2 dwarf ones (roughly analogues of our solar system planets but with some differences)+a bunch of moons around half of them including the Mun (like Luna) and Minmus (captured comet or something) around Kerbin. There's docking and spacecraft persistence (so you can meet up landers on the ground) along with orbital fuel transfer, I have like 40+ spacecraft on my main save file orbiting or landed at various places around the system. There's a ton of stock parts now, from batteries through lander legs, probe parts, scientific sensor and solar panels to NTR engines, RTGs, Ion Engines. You can make space planes (there are wings, jets, ailerons, intakes, landing gear and so on).
Next update is supposed to add resource extraction from the soil/ice, oceans and atmosphere of various bodies, stock rover parts, re-entry heat and probably a ton of new surface base/space station parts.
For realism though it's fairly decent though it's no orbiter. Orbital mechanics are based on patched conic approximation so there are no Lagrange points and stable "capture" trajectories are impossible. Planet sizes and orbits are to a 1/11th scale, gravity is unchanged though (but the R^2 of the formula for gravity force at a distance means it's easier to escape a SoI). The advantage is that it allows greater time warp with no risk of orbital changes due to floating point approximation which happens in a pure simulation. No re-entry heat yet (next update probably a month or two away) and the drag model/aerodynamics are very simplistic.
There's a new demo out, but it only has the Mun and Sun. You can try that to see how it looks/runs now as it did get a facelift (they hired one of their modders and he remade a lot of the parts and is making new ones as well), although the demo still has limited parts. You can land on the Mun with it though.
It runs pretty good for me now, you can time warp more in low orbit in 0.18.4 from what I noticed. I launched a huge ass 1000+ ton 400+ part rocket yesterday and got like 4 fps! That's at least double what I would have had on the previous version.