Kerbal Space Program

Kerbal Space Program is a game about a green humanoid species, named Kerbals, starting a space program. Superficially similar to Orbiter, the difference between the two has been likened to the difference between making to-scale miniatures for architectural design and playing with LEGOs with rocket fuel in them.

You're set loose upon a space center with vehicle assembly buildings and a launch pad, a bin full of rocket parts, ground personnel composed entirely of yes-men who build and wheel onto the launch pad anything you design no matter how crazy it is, and some astronauts to crew your creations. There's a whole toy solar system waiting to be explored, and the only limit is your imagination, plus whatever game mode you're playing:

• Sandbox is the original game mode, with no restrictions.
• Science was the first iteration of "career" mode, where parts need to be researched using science points earned by running experiments during missions.
• Career features technology restrictions like science mode, but also has a budget that has to be balanced by accepting and completing contracts.

Its official website and download location is here.

If you want something a bit more demanding and realistic, see Orbiter Spaceflight Simulator.

Tropes used in Kerbal Space Program include:

• Artistic License Engineering: Many of the most tedious and frustrating elements of aerospace engineering (e.g. wiring, piping, life support, random part failure) are simplified, abstracted or eliminated. The remaining challenges of propulsion and control are hard enough on their own.
• Art Evolution: Examining screenshots and videos from even a year apart shows remarkable development of the game.
• Ascended Meme: One of the loading-screen messages is "Adding K to Every Word", an activity that used to be very (some would say overly) popular among fans. It's less of a thing now, but odds are still very good that an addon's name will have hard "c"'s replaced with "k"'s.
• Cloudcuckoolander: Jebediah Kerman (and Valentina, and any other "badass" Kerbal), who always smiles all the time, no matter what is happening. Unless something goes wrong.
• Conveniently-Close Planet: Even though the solar system as a whole is about 10x smaller than ours, it still takes many months to get from one planet to another, and journeys are also subject to the limitations of launch windows, just as they are in the real solar system. However, the distance from Kerbin to the Mun is less than one-thirtieth of the distance from the Earth to the Moon (12,000 km, or about the diameter of the Earth), so it only takes one Kerbin day (6 hours) at the most to get from one to the other. In fact, the Mun starts to edge into being Inconveniently-Close: because it is in such a low orbit, and is in the same plane as the Space Center and Kerbin's orbit around the Sun, a mission to another planet actually has fairly good odds of encountering it on the way out of Kerbin's sphere of influence, which is at best a complication to mission planning, and at worst a collision hazard.
• Fantasy Counterpart Planets: Each planet in the solar system is based on one or more planets in our own.
  • Moho is based on Mercury (small, closest to the Sun).
  • Eve is based on Venus (roughly Kerbin-sized -- actually somewhat bigger than Kerbin, unlike Venus -- with a thick atmosphere).
  • Kerbin is based on Earth (third planet, life-bearing, with very varied biomes, and a big Moon... er, Mun).
  • Duna is based on Mars (red, smaller than Kerbin, with a wispy atmosphere and ice caps).
  • Dres is based on Ceres (quite small).
  • Jool is based on Jupiter (large gas giant, with lots of very varied moons).
    • Laythe, Jool's innermost moon, may be inspired by Titan, one of the Moons of Saturn, with its atmosphere and oceans.
    • The orbits of Laythe, Vall, and Tylo are in a resonance just like Jupiter's innermost three moons Io, Europa, and Ganymede
  • Eeloo is based on Pluto (smallest and most distant).
• Game Mods: Lots of them! Part of what makes KSP so popular is that players can mod almost any aspect of the game to their hearts' content.
• Little Green Men: The Kerbals are these, to make it more acceptable (and funny) to lose them in accidents. Version 1.0 brings Little Green Women to the program as well.
• Obvious Beta: Initially very much so, as the game was released while quite early in its development (2011, with version 1.0 released in 2015).
• Oh Crap:
  • Bill and Bob Kerman always look worried whenever the spacecraft is doing anything other than holding still. All characters do this if something explodes. If Jebediah ever stops grinning... see above.
  • Bob will always look terrified. He'll only calm down if the craft is drifting back to the surface slowly via parachute or has come to a complete, safe stop.
• Purely Aesthetic Gender: Male and female Kerbals perform identically (subject to their statistics and traits), but it used to be the case that some internal model parameters were incorrectly set in the female model, which made them more prone to "Spaghettification"^[1] when they collided with terrain and ragdolled.
• Sci-Fi Writers Have No Sense of Scale: The developers avert this but the use of this trope in media has caused some confusion among players less familiar with actual astronomical scales. Even among those familiar with space fight don't always remember exactly how big space really is. For instance, some have suggested ignoring time acceleration (i.e. being able to increase the speed of the game) and instead allow the program to run essentially in the background to allow for realistic flight during extended missions. Even acknowledging that a single mission would very well be months long, they don't seem to realize that most of that time would be spent doing... nothing.
  • The distances are less than they would actually be, but that's due to the Space Compression. The scales for Kerbin/Mun/Sun are roughly the same as for Earth/Moon/Sun accounting for it.
• Shout Out: The "name pool" for Kerbals includes names of famous astronauts and Kerbal Space Program players.
• Snowy Screen of Death: It used to be that the individual video feeds for each Kerbal would cut to static when they were killed. No longer the case now that Kerbals can exit spacecraft for reasons other than death.
• Space Compression: To cut down orbit times and prevent the game from becoming astronomically boring, the solar system is roughly 1/10th the size of ours. Shrinking the planets also reduces the required sizes of rockets, making rocket design less demanding on players than it is for real-world space agencies. To put it in perspective, a rocket that can get into orbit around Earth could fly all the way to the surface of Duna and back.
• Space Is Noisy: Even in the depths of space, you can still hear rockets and explosions. Partially justified in that most of those explosions happen either in-atmosphere or when you're still attached to the thing blowing up/making noise, giving the sound a medium on which to propagate to your ears.
• Stuff Blowing Up: Rocket fuel is an explosive after all.
• Tertiary Sexual Characteristics: The Kerbals were originally meant to be genderless, but the default set had masculine names (and the list of names for generated Kerbals includes names of famous astronauts and Kerbal Space Program videomakers, most of whom are male), and they all have relatively square (well, cylindrical) jaws and heads and short hair, so they quickly came to be identified as male. Version 1.0 added female Kerbals, which are distinguished by having smaller, rounder heads and ponytails.
• Unfortunate Name: Aside from the first four, Kerbal names are randomly generated. Some come from a pool of names (famous astronauts, aerospace industry members^[2], Kerbal Space Program players), but most are randomly generated by combining prefixes and suffixes. This has resulted in "Anus Kerman" and "Dildo Kerman" (both names are now blocked by the name generator).
• Video Game Caring Potential: Some players equip their crew capsules with parachutes and try to land them as close to the Kerbal Space Center as possible...
• Video Game Cruelty Potential: ...while others take gleeful pleasure in sending the hapless trio to die in the cold blackness of outer space. Or just blowing them up.
• Wide Open Sandbox: The appropriately-named "Sandbox" game mode. Once a Science or Career mode save has enough research, funds and reputation to do just about anything, they become this as well.

1. ↑ A Fan Nickname for a process where a ragdolling Kerbal's body parts jitter further and further away from each other, leading to widely-spaced joints with thin meshes connecting them. It bears a passing resemblance to the hypothesized tidal stretching that would occur to a body falling into a black hole, and like real spaghettification, it invariably ends with the destruction of the Kerbal in question. Unlike real spaghettification, it ends not when the body crosses the event horizon, but when the jitters increase above the maximum impact speed of a Kerbal's body.
2. ↑ e.g. Elon Musk.