Delta-V free capture?

[A35K]

So I was just trying to get an encounter with Vall for my probe headed there, and then this happened:

It seems that my probe will encounter Vall, and will directly be in orbit? Is this a glitch or is this actually possible?

This is what the probe's orbit around Jool currently is:

If this is actually real, is it rare? Do I win KSP:cool:?

[Red Iron Crown]

It's a glitch, and only happens rarely. Any trajectory entering an SoI is by definition an escape trajectory for that SoI (if it doesn't impact the body, skim atmosphere or snag a gravity assist on a body orbiting inside the SoI).

But...

If your entry trajectory is very similar to the body's orbit (and it looks like yours is), it is possible for a combination of floating point imprecision and frame stepping to create a new trajectory with an Ap barely inside the SoI boundary.

So congrats on hitting a KSP rarity!

[A35K]

Well, if it works it will be helpful, since my probe is running low on fuel!

[magico13]

At the very least, it will be a very easy capture that won't require much fuel!

[Geschosskopf]

BEWARE, BEWARE when the map shows you capturing for free. What invariably happens is that you want to warp to the SOI change shown on the map. However, because your whole trajectory plot is glitched up, the SOI change shown isn't in the right position, either. So you'll be warping along at high rate thinking you're still far from the SOI change, and suddenly BAM, you're in the target SOI with no warning. And almost invariably, you're on a collision course. And because this only seems to happen when you're going to a moon, the SOI isn't very big. And because you're a high warp, you can't reduce warp fast enough to save yourself before impact.

So, on those rare occasions when I see an "auto-capture" on the map, I do a little burn to force the map to redraw and get rid of this glitch.