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Comment by vikingerik | Hacker News Reader

The scenario where a gravity assist doesn't work is if turning sharply enough would require your minimum planetcentric approach distance to be less than the radius of the planet - you'd crash into it instead.

This is why Voyager 2 couldn't also do Pluto - it would have needed to change course by roughly 90º at Neptune, which would have required going closer to the center of Neptune than Neptune's own radius.

The most unusual gravity-assist alignment that we did was for Pioneer 11 going from Jupiter to Saturn. The encounters were separated by roughly 120º of heliocentric longitude. Pioneer 11 used Jupiter to bend its path "up" out of the ecliptic plane and encountered Saturn on the way back "down". Nowadays we wouldn't bother doing that (we'd wait for a more direct launch window instead), but the purpose of this was to get preliminary Jupiter and Saturn encounters done in time before Voyager's launch window for the grand tour alignment.

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Could Voyager have reduced velocity, glanced off Neptune, waited for the return path on a narrow elliptical orbit, and then boosted to effectively make the 90° turn to Pluto at that time? Was that impossible given its Neptune approach trajectory, or would glancing off and waiting have been more fuel?

And, why didn't this vortical model that includes the forward velocity of the sun make a difference for Voyager's orbital trajectory and current position relative to earth? https://news.ycombinator.com/item?id=42159195 :

> "The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU

Breaking that down: Voyager itself couldn't have reduced velocity, it had nowhere near enough reaction mass to do that. Hypothetically a spacecraft could but you might be talking about orders of magnitude more reaction mass. (Which means multiples more of the fuel to launch and accelerate that mass itself, which could quickly escalate beyond any chemical rocket capabilities.)

It also likely wasn't possible to get to Pluto on some future Pluto orbital pass. The limiting factor is likely that Voyager's incoming trajectory to Neptune was already too far beyond solar escape velocity to get into that narrow elliptical orbit you propose. (You'd have to slingshot so close to Neptune's center that you'd hit the planet instead.)

Designing from the beginning to come in slower to Neptune and adjust to encounter Pluto on some future Pluto orbital pass was probably possible, but yeah you might be talking about time scales of Pluto's entire orbit or even multiples of that. (We do similar things for inner solar system missions, like several encounters with Venus separated by multiple Venus-years, but that's on the order of single-digit years and not hundreds.)

The common answer to a lot of these outlandish slingshot questions is usually, yes it's eventually possible by orbital mechanics, but it gets so complicated and lengthy that you may as well just build another separate spacecraft instead. We talk about Voyager's grand tour alignment because it's captivating, but realistically if that hadn't happened we would have just done separate Jupiter-Uranus and Jupiter-Neptune missions instead.

The sun's motion relative to the galaxy doesn't matter for any of this - nothing else in the galaxy is remotely close enough to affect anything, the nearest star is still over 1000x Voyager's distance.

Is it possible to focus on a reflection on the Voyager spacecraft; or how aren't communications ever affected by lack of line of sight?

So there was no way to flip around and counter-thrust due to the velocity by that point in Voyager's trajectory (without a gravitationally-assisted slowdown or waiting for planetary orbits to align the same or in a feasible way)

FWICS; /? spirograph ... "Hypotrochoid" ... Hypotrochoid orbit

Aren't there hypotrochoid orbits to accelerate and decelerate using planetary gravity; gravity assist

Gravity assist: https://en.wikipedia.org/wiki/Gravity_assist

- https://space.stackexchange.com/questions/10021/how-are-grav... :

> How can I intuitively understand gravity assists?:

> Aim closer to the planet for a lower pass for a greater change in direction (and velocity from an external frame of reference), farther from the planet for a smaller change; aim ahead of the planet for a slower resulting external velocity, behind for a higher velocity: gravity assist guide (image from this KSP tutorial)

- Vindication! KSP is what I probably would have used to answer questions like this; though KSP2 doesn't work in Proton-GE on Steam on Linux and they've since disbanded / adjourned the KSP2 team fwiu.

- JPL SPICE toolkit: https://naif.jpl.nasa.gov/naif/toolkit.html

- SpiceyPy; src: https://github.com/AndrewAnnex/SpiceyPy docs: https://spiceypy.readthedocs.io/en/stable/

- SpiceyPy docs > Lessons: https://spiceypy.readthedocs.io/en/stable/lessonindex.html :

- > various SPICE lessons provided by the NAIF translated to use python code examples

TY for the explanation.

Hopefully cost effective solar sails are feasible.

FWIU SQR Superfluid Quantum Relativity doesn't matter for satellites at Earth-Sun Lagrangian points either; but, at Lagrangian points, don't they have to use thrust to rotate to account for the gravitational 'wind' due to additional local masses in the (probably vortical) n-body attractor system?

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