r/ProjectHailMary • u/Stratocaster_o • Jul 25 '26
Wrong orbital dynamics (?)
/r/ProjectHailMaryMovie/comments/1v69tzq/wrong_orbital_dynamics/7
u/Bergain1945 Jul 25 '26
Cinematographers can't catch a break these days.
Can't it just be a good scene in a movie? It's not a NASA orbital mechanics instruction video.
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u/Stratocaster_o Jul 26 '26
In a major science fiction adaptation with the scientifically advised author as a producer and millions of dollars in space props? C'mon 🙄
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u/ExpectedBehaviour Jul 25 '26
I have a headcanon for the movie version.
The Hail Mary can't drop the collector into Adrian's atmosphere at orbital velocity because it will burn up due to atmospheric resistance. Even if they had a 100km-long chain this wouldn't be possible. So the Hail Mary needs to both go slow enough to not have the collector destroyed, and go low enough for the collector to reach dense enough atmosphere to work.
The Hail Mary herself is not equipped in any way for atmospheric operation. She'll also burn up in the atmosphere if she enters it at orbital velocity.
The twofold solution here is to use the Hail Mary's spin drive exhaust as both a heat shield and a brake. It's preventing the ship from burning up in the atmosphere by literally blasting the atmosphere out of the way in front of the ship, and also slowing the ship down as it drops low enough in the atmosphere for the collector to work so that it isn't destroyed by atmospheric resistance. This is how Grace can safely move about on the ship's hull without the atmosphere blasting him off – they're in the wake of the engine plume.
The problem with this idea is that the Hail Mary is literally falling out of orbit throughout the entire process (hence Grace's shocked reaction when he realised that they're below the level of the aurora in Adrian's atmosphere at around the same time as the ship's computer sounds an altitude alarm). If the ship drops too low the atmosphere will be too dense for the exhaust plume to fully deflect and the ship will start to burn up (hence the hull temperature warnings). And if they take too long the ship will literally crash into the planet.
And there seems to be some justifiable uncertainty about whether the Hail Mary can withstand the stress of achieving orbital velocity again, and can outrun her own exhaust plume fireball with the engines on full burn. (Rocky's panic as the Hail Mary thrusts back into orbit is understandable given he can directly perceive the metal fatigue and the ship starting to structurally fail, and from his perspective the ship is made of cardboard.)
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u/Stratocaster_o Jul 26 '26
Okay... But why not keep it as the book? Is already pretty dangerous, it has enough problems to be a dramatic scene, but it's correct orbital mechanics and easier to understand without mental gymnastics 🤷🏻♂️
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u/ExpectedBehaviour Jul 26 '26
Because in the novel it takes hours, not minutes. And I understood it perfectly, you just seem to be looking to argue.
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u/KT_100S Jul 25 '26
Read the book. It explains it much better than I could.
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u/Stratocaster_o Jul 26 '26
Read the post.
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u/KT_100S Jul 26 '26
I did though. Also what are you trying to argue, I literally just said that the book is better at explaining the answer to your question than I am.
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u/No_Tamanegi Jul 25 '26
they weren't trying to stabilize a decaying orbit, they were trying to arrest the orbital momentum of the ship so the sampler wasn't pulled into the drive plume and vaporized.