In orbit you have to slow down to speed up
73 comments
·October 24, 2025lproven
As Larry Niven wrote in The Smoke Ring and The Integral Trees...
"West takes you In, In takes you East, East takes you Out, Out takes you West, North and South bring you back again."
ChicagoBoy11
For anyone who is remotely interested in this, a considerable chunk of the Gemini program was all about solving some of the practicalities involved with Rendezvous, and it is quite interesting even hearing some of the astronauts come to grips with some of the physics while orbiting in space trying the various types of rendezvous and docking maneuvers that were attempted.
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ralfd
I have a hard time imagining physics. For example take a train moving 100 kmh to the north which wants to reverse direction to the south. It has to break and then accelerate again, a very costly operation. Except when the tracks make a turn? But how can a northward momentum change to a southward momentum?
The same confusion I have when trying to imagine satellites going around Earth or slingshot maneuvers. Would an X-Wing turn in space differently than in the atmosphere of Hoth? Would it in space just rotate, but keep its forward (now backwards) momentum instead of turning like a fighter jet?
NitpickLawyer
> The same confusion I have when trying to imagine satellites going around Earth or slingshot maneuvers.
I can't recommend KSP enough. It's a "silly" game with "on rails physics" (so not exactly 100% accurate wrt general relativity stuff) but it's got a very nice interface and it will make you "get" orbital mechanics by dragging stuff around. You'll get an intuition for it after a few hours of gameplay / yt video tutorials. Really cool game.
montagg
This is how I now “get” orbital mechanics better than I ever did trying to study it. Play is the best education.
ErroneousBosh
> For example take a train moving 100 kmh to the north which wants to reverse direction to the south. It has to break and then accelerate again, a very costly operation. Except when the tracks make a turn? But how can a northward momentum change to a southward momentum?
Your train is decelerating, and then accelerating southwards. It really is.
If you were on a train that was travelling in a straight line northwards and the driver applied the brakes, it would decelerate, which really is acceleration with a negative value (and I can hear that in my old high school physics teacher's voice, hope you're doing well, Mr Siwek). You would feel yourself being thrown forwards if the acceleration was strong enough because your momentum wants to keep you moving north.
If you were on a train that was travelling around a U-shaped bit of track looping from northbound to southbound, then you'd be thrown towards the outside of the curve. Guess what? The train is not moving north so fast, and your momentum is trying to keep you moving north.
The difference here is that if you brake the train to a stop and throw it in reverse then you're dissipating energy as heat to stop it, and then applying more energy from the drivetrain to get it moving again, but if you go round a U-shaped track the energy going north is now energy going east. You have not added or removed energy, just pointed it a different direction.
GeneralMayhem
Turning around a track definitely dissipates some heat energy through increased friction with the rails. Imagine taking a semicircle turn and making it tighter and tighter. At the limit, the train is basically hitting a solid wall and rebounding in the other direction, which would certainly transfer some energy.
The energy question is this: going from a 100kmh-due-north momentum to a 100kmh-due-south momentum via slowing, stopping, and accelerating again clearly takes energy. You can also switch the momentum vector by driving in a semicircle. Turning around a semicircle takes some energy, but how much - and where does it come from? Does it depend on how tight the circle is - or does that just spread it out over a wider time/distance? If you had an electric train with zero loss from battery to wheels, and you needed to get it from going north to going south, what would be the most efficient way to do it?
ErroneousBosh
> Turning around a track definitely dissipates some heat energy through increased friction with the rails.
No it doesn't, but we're talking about identical spherical frictionless trains in a vacuum.
floxy
There is no "required" energy to change direction, even for a zero-radius change, think of a bouncing ball:
btilly
What's going on here is that your momentum changes whenever you experience a force. Your energy changes whenever you experience a force towards or from the direction that you are traveling.
The force from the rails at all points is at right angles to the direction of motion. So your energy doesn't change. Your momentum is constantly changing. And you're doing it by shoving the Earth the other way. But the Earth is big enough that nobody notices.
Now to the orbital example. In the Newtonian approximation, an orbit works similarly. In a circular orbit, you're exchanging momentum with the planet, but your energy remains the same. The closer the orbit, the more speed you need to maintain this against a stronger gravity, and the faster you have to move.
In an elliptical orbit, you're constantly exchanging momentum with the planet, but now you're also exchanging between gravitational potential energy, and kinetic energy. You speed up as you fall in, and slow down as you move out. Which means that you are moving below orbital speed at the far end of your orbit, and above when you are close.
Now to this paradox. Slowing down causes you to shift which elliptical orbit you are in, to one which is overall faster. Therefore slowing down puts you ahead in half an orbit, and then you'll never stop being ahead.
ambicapter
In your train example, the rails exert a force on the train as it turns. In orbit, the planets are constantly exerting a force on the satellite.
boothby
What's happening is that you exchange forward momentum for angular momentum. When the track straightens out again, you trade the angular momentum for forward momentum again. The train pays for this in friction losses; the orbital maneuver costs some fuel for steering.
Xerox9213
When you brake you generate a ton of heat.
Doing a U-turn generates less heat, but still quite a bit. The train will have to slow down depending on the radius of the curve, and even then the turn will slow it down some more.
But yeah, less heat generation means kinetic energy is conserved.
Cars have to slow down when they turn because it’s too much to ask of the tires to accelerate (throttle) and turn, since turning is in itself acceleration.
NetMageSCW
Caveat: when the tires are already at the limit of adhesion (e.g. on an F1 car). In a road car, you are not normally turning at 1g and probably can’t accelerate at 1g so you can turn and accelerate when you have enough margin.
It’s just the average driver doesn’t realize how much margin is available.
wongarsu
A train has momentum in the direction of the track. If the track makes a 180° turn the train will lose some momentum to increased friction with the track during the turn, but essentially the momentum still follows the track.
A fighter jet (or X-Wing in orbit) kind of generates its own "track" with the guiding forces of the wings. You can still do a 180° turn and keep a significant part of your momentum. Though the guiding effects are a lot softer, so your losses are a lot worse
A satellite (or an X-Wing in orbit) has no rails that can go in arbitrary directions. Any momentum is in "orbit direction", but orbits work in weirder ways. If you make your orbit highly elliptical then at the highest point you will have traded nearly all your kinetic energy for potential energy and can make a 180° turn pretty cheaply (because it's only a small change in speed)
kbelder
A very related physics issue that boggles my mind is when you roll a disk, like a wheel. You can roll the disk north, and it'll lean, curve, and end up going south. What force changed the direction of the wheel?
I understand it, intellectually. It's pushing sideways against the surface as it leans and spins, but it just doesn't feel right. I have no intuition for it.
stoneforger
They should really teach physics using KSP.
vannucci
I tried to teach a group of HS students about orbital mechanics as a high school physics teacher using KSP. It was... difficult. Not impossible. But I agree it's an excellent learning tool.
hobs
Right, the UI/UX is a lot to just get to the rocket part. KSP is probably the best game that forces that into your head with a classic simulation that's fun, but I gotta say something like Rocket League was better at building my intuition for rocket behaviors.
matheusmoreira
Yeah, it's amazing. With enough docking and maneuvering practice I developed some kind of intuition for moving in space. I could maneuver without meticulously planning the burns.
Still can't leave Eve though...
dabluecaboose
I'm a professional astrodynamicist and I owe my base level understanding of orbital mechanics to KSP. It's a fantastic resource for learning the basics of Keplerian motion.
Also, obligatory XKCD: https://xkcd.com/1356/
taneq
Arguably aerodynamics is confusing on a whole other level to mere orbital dynamics. :D
dabluecaboose
I washed my hands of aerodynamics after I got my first job in satellite navigation. Messy stuff, that Navier-Stokes business
M95D
No, it's simple. Just make sure the airplane falls nose-first if it ever stops (speed<stall).
aaronblohowiak
I wish ksp 2 hadn't been a boondoggle
mikkupikku
I haven't kept up with it, but hopefully Kitten Space Agency will be able to take up the torch.
delichon
I doubt SpaceX could put a satellite in orbit with KSP physics. Just the absence of realistic thermal conduction would prevent it. The outer skin temperature typically peaks around 300–600 °C during the densest part of the atmosphere. If you calculate those forces wrong the rocket has a bad day. Best case it is over engineered and has a reduced payload. They might as well do their calculations with pi equal to 3.
0_____0
What does thermal conduction affect? Is it mostly practical spacecraft construction, or actually related to orbital mechanics?
bogzz
The FAR mod is touted as being realistic; I haven't played it though.
iso1631
https://xkcd.com/2205/ comes to mind with your pi approximation.
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.
yabones
Forwards is up, up is back, back is down, down is forwards.
jayknight
How related to this is the helicopter 90-degree phase lag thing?
geon
Or in the words of Larry Niven (The integral trees)
East takes you Out
Out takes you West
West takes you In
In takes you East
Scubabear68
This brings back fond memories of Heinlein's juvenile sci fi series.
brudgers
A similar thing is true when cornering a race car when measuring time through the corner.
taneq
Slow is smooth, smooth is fast.
dotancohen
How so?
anonymars
"Faster" (higher speed) = wider cornering radius = more distance = slower
wrigby
But you exit going faster, which means you make up time on the straight after the corner.
rascul
If there's banking, it can change things.
everyone
The distance has no effect.. Its all about speed, you want to take the line that lets you get through the corner while maintaining the highest speed. If you are going faster and spend as little time as possible breaking and accelerating you will gain time. Also a higher exit speed means you will be going faster for the entire straight after the corner making a very big time difference.
Your car, depending on how much grip it has + other variables, will have a theoretical minimum diameter circle it can drive around at various speeds. The higher the speed the bigger the circle. Finding your racing line is just a matter of fitting the biggest circular arc inside the space available in the corner.
Ideally you want to break in a straight line before the corner and reach the speed your car can drive the circle at at just the moment you enter it.
Theres more nuance when it comes to compound corners, FR vs FF cars, oversteer understeer, hills bumps etc. But the basic theory is simply fitting circles.
HPsquared
Life and business are often the same.
everyone
I thought I understood all this until I played Kerbal Space Program.
elephanlemon
Obligatory: https://xkcd.com/1356/
QuiCasseRien
could it be possible to flag a thread when you need to pay or register to read an article ??
very annoyoing, the subject looks good, open tab and rohhhhhhhh... paid or register.
ceejayoz
Paste link, good to go. https://archive.is/qrP0p
f4uCL9dNSnQm
There is the small, tiny issue of people commenting just based on title, without even reading the article.
In this case I expected it just links to https://www.youtube.com/watch?v=bcvnfQlz1x4 and didn't even notice in links to Wired.
embedding-shape
Allowing paywalls vs not been discussed for a long time. Latest comment from dang about it seems to be this:
> The answer is that paywalls are allowed when there are workarounds (such as archive links) which allow ordinary readers to read the article without paying or subscribing, while hardwalled domains (i.e., without such workarounds) are banned. - https://news.ycombinator.com/item?id=43876575
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
Buzz Aldrin (who was the second person to walk on the moon) wrote his MIT thesis on methods for astronauts to handle the complexities of orbital dynamics when performing rendezvous maneuvers in orbit:
https://dspace.mit.edu/handle/1721.1/12652