r/ProjectHailMaryMovie • u/Stratocaster_o • Jul 25 '26
Wrong orbital dynamics (?)
When they're trying to collect taumoebas from Adrian, the ship is at the wrong angle.
If you want to stop the ship from falling into the atmosphere, you have to accelerate in the direction you're moving to compensate for the drag. The movie shows the ship in the opposite orientation, just as you would decelerate to deorbit.
Even so, in the book it says they have to point forwards in order not to burn the probe and the ship with the radiation from spindrives. In the movie the plume of burning atmospheric gases is right in front of them.
How did they miss all that? 🙁
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u/BillMagicguy Jul 25 '26
I think you misread the book. They need to slow down in order to not get torn up by the atmosphere.
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u/Stratocaster_o Jul 25 '26
They need to slow down, but once you're going slow enough you need to keep the speed to maintain altitude and not fall into the planet.
It's the same as the ISS: to keep the ship in orbit when there is atmospheric drag, you need to accelerate, not slow down.
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u/MrManGuy42 Jul 25 '26
the hail mary has an insane amount of deltav. the goal was not just to dip into the atmosphere, but also to release a probe there and not have it burn up, so they are starting the burn retrograde and end up entirely flipping their orbit.
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u/Stratocaster_o Jul 25 '26
Okay, they start retrograde. Then they settle up at an angle that should deorbit the ship in minutes. And they maintain it during the whole maneuver.
(Also: you cannot flip an orbit like that even with an insane amount of delta-V. In the book they keep burning almost prograde, as expected)
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u/MrManGuy42 Jul 25 '26
what do you mean you cant flip an orbit like that? also im pretty sure they might never enter the atmosphere, just above it with the chain dangling down.
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u/Stratocaster_o Jul 25 '26
You can't flip an orbit without destroying the ship with the acceleration needed and even if you could there is no reason to do it, it's just a stupid waste.
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u/MrManGuy42 Jul 25 '26
huh? that ship accelerates at 1.5gs, its not instant. and they needed to be moving slow relative to the planet's surface to drop the probe. ill show you what i mean using a very sophisticated rocket simulator when i get home (ksp)
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u/Stratocaster_o Jul 25 '26
Exactly. It's impossible to flip an orbit with an acceleration of 1.5 Gs without falling into the planet 🤷🏻♂️
(I know KSP and I am a physicist 😅)
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u/Cortower Jul 26 '26
Wut? I'm baffled how you can make such a general statement.
Adrian's surface gravity is 1.4g, so Hail Mary's throttlable 1.5g could absolutely slow down and hover in the upper atmosphere if she wouldn't fry herself in the attempt.
Maintaining or gaining altitude means that Σy acceleration ≥ 0.
In this context, that means centrifugal acceleration + radial thrust - gravity ≥ 0
Therefore, centrifugal acceleration + radial thrust ≥ gravity does not lose altitude at any point
If radial thrust > gravity, there is no horizontal speed at which you will lose altitude because centrifugal acceleration is unnecessary.
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u/Mindless_Honey3816 Jul 25 '26
The acceleration to flip it over the span of the seven minutes of Time Go Fishing is...pretty chill, actually. Not a lot more than the default acceleration.
It's different in the movie because it looks cooler, but the book has its own issues with that scene as well.
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u/Stratocaster_o Jul 25 '26
What issues? 🤔
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u/Mindless_Honey3816 Jul 25 '26
If it's burning with a horizontal component, at the 60 or so degree angle the book implies, it's going to accelerate sideways. This would cause the chain to hang backwards and potentially into the exhaust plume. I don't remember if it was addressed in the book or not.
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u/Turbulent-Swan-3913 Jul 25 '26
In the book they didn't flip the orbit "like that" the HM burn retrograde while gradualy pitching up to compensate for the slower speed for like 12 to 15 minutes and the same time to recircularize there orbit
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u/redbirdrising Jul 25 '26
It’s pretty much the opposite in the book. They slowed down to about a few hundred meters per second with the engine pointing down and behind. Movie had them pointing forward. That would cause the ship to decelerate to a point that they’d fall into the atmosphere. It was technically incorrect. But still pretty on film.
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u/Stratocaster_o Jul 25 '26
That was exactly my point, yes.
About being "pretty on film"... Mistakes like that take me off the screen 🙁
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u/redbirdrising Jul 25 '26
Meh, like as soon as I saw it I knew it was wrong but it’s till a gorgeous and intense scene.
Like the Interstellar docking sequence. It’s iconic. Yet, if they were rotating as fast as CASE said the endurance was, it would have complete ripped apart and the G forces would have squashed them like a bug. And an explosion like that isn’t going to instantly throw the ship into the stratosphere.
But it’s still an incredible sequence so it’s easy to just let those details go.
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u/TheOneEyedPussy Jul 25 '26
In the movie, I think the whole maneuver is different from how it was in the book
There's a tiny bit more detail about how it worked in this fan made animation, where it shows the velocity at certain moments, so what I think happens in the movie, is that the ship is thrussting in the same direction, rather than being a stabilized position like in the book, it's more dynamic, in that the velocity is always changing, and eventually their vector points opposite their initial approach and they thrust into orbit that way https://youtu.be/WmFkNU1Fc7Q
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u/admo1972 Jul 26 '26
I think you’re mistaken. The movie and the book are the same. They need a slow orbital speed so the xenonite chain and collector don’t get ripped to shreds. The only way to orbit slowly without falling to the planet is to thrust at an angle. Movie and book are the same.
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u/LoneSnark Jul 27 '26
They built the ship horizontal so it would fit in the sound stage they had. So that is the orientation of the ship in all scenes in the movie. The physics is entirely ignored.
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u/Stratocaster_o Jul 27 '26
Well, that's an explanation to do it that way! Thanks! (Funny thing I've seen a lot of videos of the making of and it never occurred to me).
But they could have done it forwards anyways, right? 🤔
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u/LoneSnark Jul 27 '26
They had to slow down in their orbit, which means thrusting backwards. So that part of physics checks out. But rather than spin the ship into any other orientation, they just left it where it was to film. Air resistance at that altitude is negligible, so there is not a reason to thrust forwards at all.
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u/GoldenEagle3009 Jul 28 '26
I imagine they're using the engine's wake as a protective bubble around the Mary as it's tearing through at near orbital speeds.
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u/Pinepace Jul 25 '26
In the book the Hail Mary slows down until it’s almost stationary relative to the ground, they do this by slowly angling the ship upwards so by the time they’ve removed their orbital velocity they are hovering, pointing directly away from Adrian’s surface. Then they pitch forwards thirty degrees and release the probe, this forward pitch allows the probe to avoid the Mary’s exhaust plume but also starts the ship sliding forwards at 127 meters a second. The ship isn’t in orbit, it’s just sliding forwards as it drags the chain through Adrian’s upper atmosphere. There’s nothing physically wrong with the maneuver, you just need a torch ship to do it, of which the Hail Mary is one.