Jool-System Gravity

[GarrisonChisholm]

Good morning all!  I encountered something which I cannot go back and observe for clarification, so I thought I would ask my experienced compatriots their opinions on the matter.

 

I had a probe in an approximate 2-day orbit of Jool (a polar orbit again; I'm getting better, honest...), and I had contrived to have the orbit pass through Laythe's circuit so-as to get a fly-by or two.  After the second flyby the orbit had its Ap reduced by Laythe, but it was still quite stable and just about 2 days in length.  Then I shifted my focus to another mission, and time-warped it to its course correction.  Then I looked back at Jool, and saw that the probe was no longer in orbit- checking the tracking station, I saw that it had been kicked out in a Huge, 37 year orbit to solar north.

 

I had of course considered that the probe's path could be changed when I left it in an orbit where it could encounter Laythe, but I did not think it would be ejected from the system, let alone nearly sent on an interstellar path (450+m km Ap).  Unfortunately, I cannot find a Save far enough back to witness how this event transpired.  So I am curious and seek opinions;, was this ejection possibly caused by an encounter with Laythe itself, or is it more likely that Laythe changed its orbit and then a close encounter with Jool was the culprit?  I am afraid I have no frame of reference to judge the matter, and couldn't even fathom the required math to deduce it.

[fireblade274]

Yes Laythe and i'm assuming Vall and Tylo have the power to kick you far out of the Jool system. I have had similar issues with satellites and derelict ore movers that were too close in orbit and caused the object to be flung out of Jool into usually a highly elliptical orbit that goes far out past Eloo. Even when I saw no intersections a few orbits ahead. You will hit that SOI if a part of your orbit is in the wrong place at the wrong time.

And on multiple occasions upon trying to leave Jool from Laythe's orbit, just after gaining an elliptical orbit, my projected path after breaching Jools SOI, will immediately show it as leaving the Kerbin system.

Edited December 7, 2015 by fireblade274

[Snark]

Planets and moons don't change their orbits in KSP; they run on rails and are absolutely fixed in their ellipses, nothing will budge them.

Almost certainly what happened was that you got a kick from Laythe.  If you care about maintaining a stable orbit, never leave a ship parked anywhere that its orbit passes nearby a planet's or moon's orbit.  Both Laythe and Tylo have powerful gravity wells, they can really send you shooting out.

If you have your ship on that polar Jool orbit and want to make sure it will stay there, you need to make sure that it doesn't pass within the SoI of any of Jool's moons when it crosses their orbital planes.  This is not hard to do; do the necessary burns at Ap and/or Pe so that at the points where you cross the moons' orbital plane, your orbit is midway between two moons and not close to one.  You'll do fine.

Suppose you want to "peek ahead" several orbits, i.e. "will I encounter any moons in my next dozen orbits?"  There's a little trick you can use to do this.  It relies on the fact that orbit encounters are always shown after the latest maneuver node.

So what you can do is"

1. Drop a maneuver node somewhere on your orbit.  Note that you don't show any encounters with any moons. That means you're clear for the next orbit.
2. Click on the little "+" button on the maneuver node to move it ahead one orbit.  Does it show any moon encounters? If not, you're clear for one more orbit.
3. Repeat step 2 for as many times as you like, until you encounter something or get tired of clicking.

If you encounter something, then you can hover over the encounter marker and that will tell you how far in the future it is.  If you don't encounter anything, then you can look at when your maneuver marker is, and you know "I'm safe at least until such-and-such time."

Of course, all those shenanigans are unnecessary if you make sure to set up your orbital path so that it will never hit any moon's SoI.  But if you're unsure whether you've met that condition, this can give you a way to check.