Spontaneous Changes to Solar Orbit of Inactive Vessel

[sjohnson0684]

I was wondering if anyone else has experienced anything similar to what happened to me below with a probe to Moho?

1) I burned retrograde relative to the Sun while around Kerbin, also making as much of the needed inclination change that I possibly can during the burn. This brought my perihelion down to ~4,300,000 km to somewhat line it up with Moho's orbit.

2) I time warped to the Kerbin-Sun SOI transition with no change in my solar orbit once past it.

3) I setup a mid-course correction at the descending node to finish my inclination change, and to get an encounter with Moho (a closest approach of ~4,000 km). This will occur about 80 Kerbin days later.

4) I leave to go back to the KSC.

5) In the tracking station screen, I see that my probe headed towards Moho is on track, and that the maneuver I had setup is correct.

6) I proceed with working on another mission that lasts about 14 Kerbin days within the Kerbin system (this includes several fast time warps).

7) I go back to the tracking station, and now see that the orbit around the Sun for my Moho probe is off, and that my planned maneuver no longer gives me the encounter with Moho...

8) I go to the probe, and see that my perihelion is now over 5,000,000 km (about a 17% increase) and I have also gained ~300,000 km at aphelion. I try messing with the maneuver node to get an encounter again, but my solar orbit has shifted so much that I am now unable to get an encounter with Moho with a reasonable amount of fuel (i.e., if I create a maneuver to intersect Moho's orbit at the same position as the previous planned maneuver, Moho will be nearly a 1/3 ahead of my probe in it's orbit).

Being a programmer, I know how floating point errors can cause issues, especially with SOI transitions and encounter calculations within the game, but the orbit change happened when I was already in solar orbit, after the SOI transition from Kerbin escape but well before the Moho SOI, and it was not the active vessel. From my understanding of the game, the probe was on "rails" at the time and there should have been no calculations that would have suddenly shifted the perihelion and aphelion like it did.

I have experienced this several times over the past few months, across several versions of the game, both modded and un-modded. It usually happens with Moho encounters, but it has happened with Jool a couple of times.

[ThinkOutsideTheHangar]

Maybe the patched conics system isn't so accurate? The map isn't completely reliable, by the way. I learned this the hard way with munar free-return missions

[makinyashikino]

I realise that this is kinda poor advice, especially from an "Answering your question standpoint" but you could always just Hyperedit your way into Moho flyby, it's kinda cheaty I know, but you would fix your problem.

[thereaverofdarkness]

Is it possible that your probe entered the SOI of another planet? Maybe a floating point error caused it to think it would just barely miss and instead it just barely went in.

[sjohnson0684]

I realise that this is kinda poor advice, especially from an "Answering your question standpoint" but you could always just Hyperedit your way into Moho flyby, it's kinda cheaty I know, but you would fix your problem.

I would normally just role-play it off as a "navigation computer error", but this has happened on more than one occasion, so I guess I will start having to go this route or manually change the persistence.cfg file.

Is it possible that your probe entered the SOI of another planet? Maybe a floating point error caused it to think it would just barely miss and instead it just barely went in.

That was a suspicion of mine, but the probe was already well behind Kerbin in its solar orbit but was still too far out from Eve's orbit to have any type of encounter. Maybe it came too close to an asteroid this time, which threw it off? I wasn't tracking any at the time though, and I would assume that the same rules for inactive vessels on "rails" would apply to asteroids too... unless there is some sort of background interaction that inadvertently threw the orbit off.