On the dynamical stability of Principia's modified Jool system
anyone that doubts the wisdom of retrograde bop needs to get the hell out
―Scott Manley
Principia computes the trajectory of the celestial bodies by integrating the equations of motion1; as a result, if the system is unstable, it may break down in-game. This is in fact the case of the stock system: while the specifics depend on how KSP's Keplerian orbital elements are translated into a Cartesian initial state, with Principia's interpretation as hierarchical Jacobi elements, the Jool system breaks down within 19 days, with a close encounter between Vall and Laythe.
| 18 days | 19 days |
|---|---|
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What would actually happen in that encounter is unclear (impact, breakup by tidal forces, or simply ejection of Vall are all possibilities), and Principia is unable to properly simulate it: besides these physical considerations, the integration error explodes, and the sharp bend in the trajectory causes a failure of the polynomial fitting2. This can be seen in the image on the right above. In any case, the final outcome is likely to have a very wild dependence on the initial conditions.
Principia remedies to this by modifying the Jool system, specifically:
- by increasing the size of the orbits of Vall and Tylo, thereby preventing breakdowns of the inner Jool system due to resonances;
- by making the orbit of Bop retrograde3, as suggested by Scott Manley and @pdn4kd, so that it doesn't get boosted out of the system by Tylo (which now comes closer to it). While the resulting system does not appear likely to break down within a century, we find that it is highly chaotic, mostly because of interactions between Bop and Tylo. In the remainder of this document, we discuss the implications of this chaotic behaviour on the predictability of the system, and look at some features of the motion of the Joolian moons.
When discussing a numerical integration of a physical system, there are several sources of error:
- Round-off error, from using floating-point arithmetic rather than real numbers;
- Truncation error, from using a nonzero step size in a numerical integrator;
- Measurement errors in the initial state;
- Modeling assumptions: considering bodies as point masses, ignoring general relativity, ignoring radiative effects (Ярковский, Poynting–Robertson), etc.
Here we ignore errors from modeling assumptions. While there is no measurement of the initial state per se in our problem, the concept is still sensible, since we are dealing with a physical system.
We estimate numerical (truncation and round-off) errors by comparing the result of an integration at a step of 5 min with the same integration at a step of 2.5 min. The actual error should be of the same order of magnitude as the difference between the computed solutions. The estimated errors are shown in the following table.
| time | error in position in the Joolian barycentre frame (moon) |
|---|---|
| 1 a | +3.74739719791592596e-01 m (Laythe) |
| 2 a | +1.39224971456813140e+00 m (Pol) |
| 3 a | +8.49396516257019307e+00 m (Bop) |
| 4 a | +5.10756172599136136e+01 m (Bop) |
| 5 a | +5.41899643855711815e+02 m (Bop) |
| 6 a | +4.96393026229186398e+03 m (Bop) |
| 7 a | +3.01654772715498148e+04 m (Bop) |
| 8 a | +3.07834670458512919e+04 m (Bop) |
| 9 a | +2.62286978937739448e+05 m (Bop) |
| 10 a | +1.47840118400542252e+06 m (Bop) |
| 11 a | +1.05332515433050469e+07 m (Bop) |
| 12 a | +2.31910395306165308e+08 m (Bop) |
As expected from a chaotic system, the error is amplified exponentially with time, so that after 12 years we cannot tell where Bop lies on its orbit. While we could reduce these errors by decreasing the step size, exponential growth means that eventually the numerical errors will get amplified to the point where we cannot predict anything accurately.
We answer the question of whether these numerical errors are "good enough" by considering whether a hypothetical observer of the system could tell the difference between our integration and the laws of physics.
A property of our chosen integrator is that, being conjugate-symplectic, it does not exhibit energy drift: our observer cannot measure the energy at various times and notice an unphysical systematic drift. There will be some bounded variations, but by lowering the time step, their amplitude can be reduced as needed. The same holds for angular and linear momentum.
The observer would have measurement errors on the initial state: let us say that the errors are millimetric. We take a cluster of 100 systems where the Joolian system is millimetrically perturbed in random directions, and we compare the integration of the unperturbed system with that of all perturbed systems, at a 5 min time step. We record the greatest divergence in the following table.
| time | error in position in the Joolian barycentre frame (moon) |
|---|---|
| 1 a | +2.02320983591245245e+01 m (Laythe) |
| 2 a | +9.58347695454694644e+02 m (Bop) |
| 3 a | +9.56218220833261330e+03 m (Bop) |
| 4 a | +5.76096439285006418e+04 m (Bop) |
| 5 a | +6.17599349619004526e+05 m (Bop) |
| 6 a | +5.87323334232143965e+06 m (Bop) |
| 7 a | +4.96615832782097757e+07 m (Bop) |
| 8 a | +1.97354446171970442e+07 m (Bop) |
| 9 a | +3.14420956432531476e+08 m (Bop) |
These errors dominate the numerical errors, so that our simulation is within the measurement errors of that potential observer. Further, the observer cannot accurately predict the behaviour of the system beyond about five years. We call those first five years "predictable".
Here are the trajectories for the first five years.
| 1 a | 2 a |
|---|---|
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| 5 a |
|---|
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Some properties are better seen as a function of time: here are some plots of the apsides of the various bodies.
| 1 a | 10 a |
|---|---|
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| 100 a |
|---|
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Notice, in the plot over 1 Julian year, that the eccentricity of Vall's orbit oscillates between 0 and ~0.05 over a handful of orbits. Bop's orbit varies wildly; we can look at the individual orbital elements. In the following plots, the blue points are Jool-centric elements as shown by KSP, and the yellow points are elements around the barycentre of the inner Jool system.
| eccentricity | inclination | longitude of ascending node | argument of periapsis |
|---|---|---|---|
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 |
 |
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We can see strong nodal and apsidal precession, as can be expected from orbiting the inner Jool system, which behaves roughly like a very oblate mass4. The behaviour of the eccentricity and inclination is complex. Again the specifics after a decade depend on amplified numerical errors; we look at the first five "predictable" years.
| eccentricity | inclination |
|---|---|
 |
 |
There are many flaws in the error analysis above, and there are additional things to be studied about this system.
We have only looked at the global error above, i.e., the error between the true solution starting from the initial state, and the one we compute. An observer however can look at the state at any time, and see whether the computed solution behaves correctly (within measurement errors) on a short interval. For instance, if a close encounter happens, they might notice the numerical error on that time step, even though their uncertainties on the initial state have been amplified beyond the point of usefulness. We should estimate the local error, and possibly the local backward error too. We should also detect very close encounters specifically: when those happen, the error from modeling assumptions becomes unacceptable, since we do not simulate bodies colliding or breaking up from tidal effects.
While we may take a guess at the Ляпунов time from the errors above, it would be interesting to give a proper estimate, ideally computed by integrating the variational equation.
Once we have local error control and close encounter detection, we could simulate a large number of millimetrically perturbed systems to estimate the probability that the system will last a given time.
There are clearly several periodic effects at work on the first five years, which affect both eccentricity and inclination in similar ways. Do the periods match those of the inner moons? Could we see something from a Fourier transform?
1. ↑ Principia offers a plethora of integrators; in this document we use a 12th order symmetric linear multistep
integrator from a paper by
Gerald Quinlan and Scott Tremaine (1990), Symmetric multistep methods for the numerical integration of planetary orbits.
2. ↑ We call the failure to fit a polynomial to the trajectory an apocalypse; the check was introduced in #612, in response to a bug that reminded us of a tweet by Katie Mack (2014).
3. ↑ We call it retrobop.
4. ↑ As @pdn4kd puts it, an oblate megajool (megajoule?).