http://www.fimfiction.net/story/139225/kerbal-equestria-program
This is a crossover between Kerbal Space Program and MLP:FiM. I think it's pretty bad, but I'm not sure if it's bad enough for a mission.
Here are the problems I've found:

- Portraying Kerbals as completely stupid. That might depend on the interpretation, but they have fairly advanced rocket science, they're not that dumb.
- Even though the author apparently plays KSP, he still managed to get orbital ascent wrong.
- The author definitely didn't proofread their story, leaving things such as "his flight path can now formed a perfect circle" or "He had quite a problem grinning with grinning ear to ear without ears."
- The author desperately tries to be funny, and not only does it fail, it sometimes just doesn't make sense. Example: "The kerbals crashed into Jool [...] at least 67 times due to them trying to open their eyes. Turns out, they had no eyelids." I don't even get what the joke was supposed to be here.
- Making 300 people speak "in perfect unison" is... Creepy.
- By the way, the scale is completely wrong. Weeks to get to Eeloo? More like years. And 3 years (or 4, the author doesn't even know) for an interstellar mission? Seriously?
- Also, no, "hydrogen smoke" isn't "noxious." It's water.
- "MKIII Hitchhiker containers" don't exist. There are either PPD-10 Hitchhiker Storage Containers or Mk3 Passenger Modules. Potential mini? I suggest a mini-Minmus.
- Either the author mixed up periapsis and apoapsis or he just doesn't understand how even basic KSP orbital mechanics work.
- You know, for a story on FIMFiction, there should probably be ponies earlier than the fifth chapter.
- Rainbow Dash seems mostly in character, but I doubt "magical auras" are responsible for supersonic shockwaves.
- Why would an interstellar spaceship have air-breathing engines?
- "The satellites did what they did best; floating around, sending signals, recieving signals, and dropping important phone calls with corporate powerhouses in the middle of important phone calls that could have potentially "their big break."" First of all, paging the Repetitive Department of Repetition. Secondly, paging the Repetitive Department of Repetition. And finally, what phone calls? Again, trying to hard to be funny.
- "They were going to touch down near a beach, 2300 meters above sea level." I... Guess it can work, if it's a lake, or something... Uh, no, wait, it says "until [the craft] reached 500m above sea level" right after that.
- Kerbals don't know what trees are, and are confused by tall grass and bushes? Are you kidding? There definitely are trees on Kerbin.
- Randomly switching units from Celsius to Farenheit.
- Let's say for a second that Rainbow Dash is able to get into space, and can survive by holding her breath even though her lungs would explose. Why would she hear sound? Oh, and, that's not how orbit works. You fall, and you move sideways fast enough to miss the ground. You don't stand still.
- Kerbals don't know about charcoal... But they have gunpowder explosives? Again: what?
- Describing a Kerbal as a "lime paint can atop an asterisk in a blue suit" is... Weird. Also, while "spectral maned pony" is technically a good description of Dashie, it's confusing.
- Making the intellectual and scientific-minded Twilight believe that "There's nothing beyond the sun and the moon." May or may not be true in the FiM universe, but it's definitely false in this crossover, and anyone smart enough with a telescope would have figured it out centuries before.
- Pinkie Pie has spies. Okay. Did I miss something in season 4 about that, or...?

... Wow, I might have gone a little overboard here. I should probably go to bed.