This sounds complicated, because usually physics relating to space is considered hard. But it's not that hard! If someone has already broken down the algorithm (which I will do), you just have to understand the rough theory. Then it's no harder than writing a bot. If you've ever studied kinematics, you should also be able to follow along with the physics.
This is also the first in a series of posts. I will be solving this problem again using C++ instead of python, and introducing some High Performance Computing techniques.
If you don't care about theory, you can also skip straight to the program. An intro to python can be found here.
ELI15 Gravitational Theory
We'll start with the difference between weight and mass. Mass is a measure of the matter that makes up an object, and weight is how much force it feels from gravity. So your weight on the Moon and on Earth are different, but your mass is the same. That's because the Moon has less gravity dragging you down
Things with mass naturally pull other things with mass toward them in a straight line. As you get closer, the pulling gets stronger. For example, you feel lighter on the top of Everest than at the base (100 lbs feels like 99.7 lbs).
Now let's talk about orbits. Imagine a bowling ball far above the Earth's surface. The ball is high enough it would take 5 minutes to free fall to the surface. To get into orbit, you just need to ball to move "sideways" fast enough in 5 minutes that it "misses" the Earth.
In the sketch below, you can see some paths that crash into the Earth, and the orbit that ensues when you go fast enough to miss it.
For a more in depth explanation, I would recommend wikipedia's page on Orbital Mechanics, or playing Kerbal Space Program 
The formula for the acceleration from multiple bodies of mass is:
The y and z directional accelerations are computed just by changing the final term (x_j_{ }  x_i_{ }) to your desired dimension.
As we already know the velocity and position of the object, knowing the acceleration allows us to calculate the bodies location a small time in the future. The acceleration is always changing though, so the smaller the time step, the more accurate our simulation.
Then we repeat this for all the other bodies, and update the positions simultaneously. This is called the Euler Method.
The Program
If you like to skip ahead, the complete code can be found here.
To run this tutorial, you need Python 3 (or higher), and matplotlib. Matplotlib is how we will visualize the paths of our orbiting bodies. If you have an Anaconda python distribution, it will already be included, or it can be installed by running:
$> python m pip install matplotlib
We start by importing the necessary modules, and creating a point class, and a body class. The point class just holds x,y,z information, and the body class holds mass, location, and velocity data. These will be used throughout the program.
Single Body Calculation
In the theory section, we talked about how this whole thing hinges around calculating the accleration of a single body as a summed acceleration from all other bodies. In python, this can be implemented as follows:
The calculate_single_body_acceleration function takes a list of bodies, and the index of the target_body. Then we iterate through all the external bodies adding up the acceleration of our target body.
Update velocity and position
With the acceleration known, the next step is to calculate the updated velocity. This is done for all bodies before changing the positions, as we want to keep the time steps synchronized.
The new velocity is calculated simply by multiplying the acceleration with the time step and adding it to the current velocity.
With all velocities up to date, we can now update the location for all bodies by calculating the distance traveled in the time step (velocity x time), and adding it to the current position.
These can be combined into a single callable compute_gravity_step function:
Simulating orbits
While we're running the simulation, the real valuable information we're finding is the x,y,z locations of our bodies. So to run the simulation, we simply repeat the calculations outlined above for a desired number of steps, and save a history of locations.
This only requires a list of input bodies(with some optional parameters), and returns a list of location histories.
Visualizing locations
From this history, here is the simplest visualization I could come up with:
This plots a 3d line graph with equal axis scales, sized to the largest dimension. If you provide a filename, it will save to file, otherwise you'll get a graphic popup while running.
Running a simulation

Now all the pieces are in place, and we just have to build up some data and run the functions we wrote in previous sections. You can tweak this however you want, but see below for an example of usage.
Once you get it running, have some fun with the bodies to see what sort of wacky and unstable orbits you can create!
For more fun with orbit simulations, checkout my follow up post introducing the RK4 method.
3 comments:
Orbit calculations are inherently unstable, while multibody interactions are dynamically chaotic. Its a great numerical exercise.
Is it possible we are missing the function compute_gravity_step(). The example above doesn't include it it seems. I'm getting an error saying: global name 'compute_gravity_step' is not defined.
TD, you're absolutely right. Compute_gravity_step was included in the full code link, but missing from the walkthrough. This has now been added. Thank you for the note.
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