Are there subdomains that use it a lot? I am not sure what the diffeq landscape is exactly although it sounds related to dynamical simulations?
https://github.com/JuliaSpace/
Out of curiosity, what Python, Fortran, and C/C++ packages do you use / can you recommend?
I personally end up doing a lot of work that uses the HEALPix sky tesselation, so I use healpy [2] as well.
Openorb is perhaps a good example of a pure-Fortran package that I use quite frequently for orbit propagation [3].
In C, there's Rebound [4] (for N-body simulations) and ASSIST [5] (which extends Rebound to use JPL's pre-calculated positions of major perturbers, and expands the force model to account for general relativity).
There are many more, these are just ones that come to mind from frequent usage in the last few months.
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[1] https://healpix.jpl.nasa.gov/
[2] https://healpy.readthedocs.io/en/latest/
[3] https://github.com/oorb/oorb
[4] https://rebound.readthedocs.io/en/latest/
[5] https://github.com/matthewholman/assist
Will look into those. I recently wrote a little n-body simulator to become familiar with Julia's DifferentialEquations.jl and that motivated me to learn more about astrodynamics.