Posted by beardyw 10/24/2025
You are in a spacecraft orbiting the Earth. You distribute a bushel of apples throughout spacecraft so that they are at rest with respect to the spacecraft. After a long time, where do the apples end up?
Hannes Alfvén (more famous for Alfvén waves) argued that they would bunch up together in the middle of the spacecraft. But Michel Hénon (correctly) proved that half would end up in the bottom front corner, and the other half in the top back corner.
I wrote up a blog post explaining the solution a few years ago: https://joe-antognini.github.io/astronomy/apples-in-a-spacec...
"West takes you In, In takes you East, East takes you Out, Out takes you West, North and South bring you back again."
There's a reason why his nickname among the other astronauts is Doctor Rendezvous.
[0] https://dspace.mit.edu/handle/1721.1/12652
[1] https://history.howstuffworks.com/historical-figures/buzz-al...
Instead of "speed up to slow down", it's "speed up to go higher". I think anyone can get on board with that. You'll come back to the same point you were when you accelerated, but later than before since you went into a higher orbit.
I like the included animations, since it also makes it clear that accelerating or decelerating from a circular orbit makes it elliptical. Your don't just jump to a higher or lower circular orbit.
Imagine writing this 6 years before you were literally standing on the moon.
For every action there is an equal and opposite reaction.
I ask because Earth is an third body scenario between the sun and Jupiter. Jupiter has enough gravity to occasionally pull the Earth slightly (not significantly) out of orbit from the Sun, but Earth's orbit to the Sun is self-correcting due to the difference in mass between the Sun and Jupiter. Quick web searching reveals Jupiter's pull on Earth is only approximately 0.005% of the Sun's after accounting for both mass and distance, but that number rises to 0.011% after accounting for syzygy with the moon.
More commonly we push away propellant, but pushing on the other craft is an option.
> If the rendezvous was in orbit does that mean departing from rendezvous pushes both crafts out of orbit?
Assuming we're pushing on the other craft, it means both crafts will change orbits, but in opposite directions. If we're talking about an instantaneous push in the direction of travel one craft will move to an orbit where it is closer to earth one half rotation later, and the other craft will move to an orbit where it is farther from earth one half rotation later.
Both, either, or neither craft could exit orbit like this, but one would be exiting orbit by crashing into the thing its orbiting around (e.g. earth) and the other would exit by reaching escape velocity and flying off into the distance.
> I ask because Earth is an third body scenario between the sun and Jupiter. Jupiter has enough gravity to occasionally pull the Earth slightly (not significantly) out of orbit from the Sun, but Earth's orbit to the Sun is self-correcting due to the difference in mass between the Sun and Jupiter. Quick web searching reveals Jupiter's pull on Earth is only approximately 0.005% of the Sun's after accounting for both mass and distance, but that number rises to 0.011% after accounting for syzygy with the moon.
Yes, Jupiter constantly (not occasionally) perturbs earths orbit, and technically earth constantly perturbs jupiters orbit (though the influence in that direction is completely negligible), but as you note its not enough for either to reach escape velocity or crash into the sun, and it appears to more or less even out over time.
Still can't leave Eve though...
Also, obligatory XKCD: https://xkcd.com/1356/
Nobody is saying KSP physics is perfect.
Until I played KSP, I had no idea how hard orbit was compared with just going up into space (and generally the greater population thinks the same -- they think that sending New Shephard upto 100km is about the same as sending a Dragon into orbit). I had no idea how you move in orbit, how getting from low earth equitorial orbit to Jupiter takes less energy than getting from the same ship to a polar orbit (and even then that the only real way to change your orbit like that is to go out beyond the moon and back), etc.
East takes you Out
Out takes you West
West takes you In
In takes you East