PHM Species Swap au fucks me up so bad to think about (in a good way) because like. Just picture it with me, ok?
Human!Rocky wakes up from his coma to find his crewmates dead. He's an engineer. He is not equipped to solve this by himself. He's all alone and he's not a scientist but if he doesn't do something then his crew died in vain, and so will he, and then so will Earth. So he does the only thing he can do, and tries to teach himself microbiology from scratch, in a last ditch effort to make himself useful. Even with food for three he knows his time is limited, so he's under-eating. He's not sleeping. He's reading pirated textbooks he doesn't understand and all the while he knows this effort is in vain. He can't teach himself to become an expert in Astrophage in a handful of years all by himself. But if he doesn't try, then this has all been a waste. He has to try.
It's been years and he's still trying.
He knows he's going to die trying.
There's only enough food left for maybe another year, if that. There are suicide methods on the ship to use instead of starving, but if he kills himself when he runs out of food then he's admitting defeat. He's going to starve to death reading research papers that still go over his head, trying to brute force his way to a solution he knows is out of reach. He's programmed Mary to look for his life signs and launch the Beatles when he dies, so at least Earth will know. They just contain one message: I'm sorry that I failed you.
Meanwhile Eridian!Grace is a scientist. A scientist who was forced to go to space against his will, because Eridians make decisions by committee, and he got outvoted, his shy voice too quiet to sway the thrum. He doesn't get to make the choice for himself, and there was never any pretense that he would. He still feels betrayed. He tries to run, to hide, at least until launch. He'll take any punishment and social fallout this earns him over the alternative, being sent into the unknown in an untested vessel where any one of a million things can go wrong. They haven't even sent animals to space yet, there's been no testing on living beings, they're moving things far too quickly and it's going to kill them, he knows it will. But eventually he needs to sleep, and Carl, his friend, someone he thought was his friend, someone who knew he was hiding and said nothing, says he'll watch.
Grace settles in for sleep on Erid, and wakes up on a ship already in space. He knows instantly that he's been betrayed, in the worst sort of way, harmed in his sleep by the very person who promised to watch. He no longer wants to be watched in his sleep, and refuses to rest unless he's alone. And he is alone, often, confined to the lab and his quarters, not allowed to leave for fear of sabotage or lashing out, because he made it very clear that he never wanted to be here in the first place. In the end, this confinement saves his life. He's the only one constantly shielded by Astrophage from radiation.
He hates that he was right.
They moved ahead too fast with too little testing, and they're paying for it. The ship is approaching it's destination with a crew of one, and Grace is resigning himself to dying too, maybe slower than the others, but there's no reason he should be spared where they weren't. Even if this doesn't kill him, he can't pilot the ship home, it was designed to be piloted by multiple Eridians at once, he's only able to finish the trip there because it was already plotted out and almost completed when the last pilot succumbed. He can't get home, so even if he managed to figure out how to save his star, he couldn't get the information to anyone that would be able to do anything with it, so what's the point? He's been murdered by his own people and their hubris, and so has the rest of the crew, and it will be a slow, lonely, terrifying death.
Rocky and Grace are both resigned to death and failure, both so sure that these ships will be their coffins, that their deaths will mean nothing.
And then Grace's ship comes into range of the Mary.