this post was submitted on 31 Aug 2025
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I'm confused, but this doesn't make sense to me.
It shouldn't matter whether you're moving in the same direction or not for this, because ultimately it's all relative - if you set the planet as the frame of reference, the direction you come in from doesn't matter - just the velocity and angle.
What I can see working is calculating the in and out angles - if the exit velocity is at a sharper angle relative to the planets velocity than the entrance angle, then your exit velocity "gains" more of the planet's velocity than the entrance velocity "loses".
If you were completely stationary, from the planet's point of reference, you're moving with the velocity of the planet. If you then did half an orbit, exiting in the other direction (theoretically), from the planet's point of reference you have the same speed, just in the other direction - but from the sun's point of reference, you're now moving at the planet's speed on top of the planet's own speed, thus gaining double the velocity of the planet.
The issue is, of course, I have no idea if I'm making sense, or missing the point.
This explains it quite nicely: https://youtube.com/shorts/kD8PFhj_a8s
Ayy, I'm not crazy, that sounds like exactly what I described... The only question is, is the explanation of "you spend longer falling" is bs, or if it makes sense if you conceptualize it differently?
It is difficult to conceptualise.
But you also have to choose the most convenient observer to help you get it.
I would say the easiest way to "get it" would be to consider it from the Suns observation point of view. Choosing the planet or spacecraft just means that you have to consider a lot more relative motion.
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This is obviously simplified and the numbers are meaningless. But the concept stands.
Depending of the incoming and outgoing angles; the energy changes are more or less....
Hope this illustrates it a little better.
Well, relative motions are more intuitive to me - they make sense, and I can use calculations for them.
In the first example, you presented 101 speed - this means only 1 speed relative to the planet, and that's all that's getting redirected (in the planet's frame of reference your enter and exit velocity should be the same, since that's how orbits work). The number is just too small, but your velocity would be planet velocity + 1 on a different vector, which will be less than 101 total.
If we estimate the angle on the picture is about 60 degrees from the velocity vector, and the speed to be 100+v1, the speed from the planet's frame of reference is v1 - so, the exit velocity will have components of (100+v1cos(60°)) and (v1sin(60°)), so the final speed relative to the sun should be
Wolfram alpha suggests this simplifies to sqrt(x(x+100)+10000), and comparing the equation by appending
<x+100gives the solution of x>0This means, if my math is correct, with an entry angle of 0° and exit angle of 60°, you always lose speed.
I could try replacing the angle with a variable and setting a constraint of x>0 and see if the free version of wolfram alpha would spit out something, but just replacing the 60 with y is spitting out some convincing solutions, since in those x is never greater than 0.
PS: Thanks for taking the time to explain, the fact that you went out of your way to draw the diagrams is not lost on me. I unfortunately still don't see it, but I really do appreciate the effort!
Taking the planet as the reference point. Complicates the situation a lot, but here we go.
If you contrast "A" and "D". The initial velocity in "A" is 1, whereas "D" is 201. The acceleration due to gravity in "A" SEEMS LOWER (this is why external observer is way easier) on the way in and in "D" it seems higher. In "A" you are literally falling for much longer (gaining much more speed); than in "D".
In "C" and "F" the situations are also different, I over simplified a bit too much. In "C" you would spend more energy than in "F"; since the acceleration due to gravity would seem higher, but not that much more. I should have made the exit angle 90°, to make them exactly equivalent...
The calculations are significantly more complex from the point of view of either the planet or space craft.
Thinking about trying to solve a real set of equations is a bit much; there are other concerns; like the fact that gravity drops off at 1/d^2^; so distance between the objects matters, the integration over distance of the equations is beyond me (I haven't had to do that since uni, 20yrs ago). But the concepts are not too complicated; and for me at least the external observer makes it so much less complicated.
As others have said, you are stealing kinetic energy from the planet to go faster. Or giving kinetic energy back to the planet to go slower.
So, relatively, you slow down and the planet speeds up or the planet slows down and you speed up.
Right, but as I explained, it's the how that doesn't make sense to me - the explanation that you "fall for longer" doesn't make sense, since 1. with how orbits work, it takes the same energy and time to "fall" as it does to ascend, and 2. at these scales you can use the planet as an inertial frame of reference, so the angle of approach doesn't matter for how "long" you "fall", it'll be the same regardless of whether you're moving towards or away from the planet.
You mentioned “from the perspective of the planet” before, and I think perhaps that’s the key, from the planet’s perspective you fall and rise with equal velocities and equal accelerations, but crucially the planet is moving relative to other things and curves your orbit, so whilst you might might have the same falling and rising speeds relative to it, they’re not in the same direction, so your velocity has changed, and from an external perspective you’ve gained velocity from it.
Imagine you start stationary relative to the sun, with Jupiter barrelling towards you (not on a collision course!). From Jupiter’s perspective you fall towards it, and so from the suns perspective you gain velocity opposite jupiters orbit, but you’re not directly head on so it twists your course (let’s say 90 degrees to keep things simple) then as you leave Jupiter it indeed decelerates you relative, but crucially you’re in a different direction now, (from jupiters perspective) you’re pointed right towards the sun, so as you pull away Jupiter is decelerating you in the sun direction (aka accelerates you away from the sun). So you were both accelerated in the anti-Jupiter-orbit direction and then again in the anti-sun direction. Added together those give you a vector which is non-zero, so you’ve gained speed from Jupiter.
If your orbit didn’t curve (eg if you could pass straight through the middle of Jupiter without colliding) I think perhaps it’d cancel out its own effects on your velocity, though I’d need to check to be certain…
I'm sorry, but this comment thread genuinely makes me feel like I'm going insane. You seem to have explained exactly the same thing as me, with the same example, and none of it includes the "fall for longer before you catch up" bit.
As for the orbit not curving, yeah, I think you're right - the obvious case is if you're sitting stationary on the planet's orbit, but the curious case is if you're approaching from the sun, with the planet's velocity plus velocity away from the sun. If I'm not mistaken, in that case you'd end up with the same velocity (minus what you might have lost to the sun's gravity), but on the other side of the planet's gravity well, which means you still gained energy.
youre taking advantage of the planets immense kinetic energy.