Minimal altitude for a stable orbit around airless bodies ?

[Gaarst]

So, a while ago, when I sent my first probes to the Jool moons, I managed to put one on orbit of each moon, quite low for some. After doing a few other missions, and therefore warping time a little, I realised the probe orbiting Bop was gone. Due to the shape of Bop, I assumed that the map view did not correspond to the actual surface (like Gilly) and that the probe crashed.

More recently in my RSS save, I put a probe in very low orbit around the Moon (50x15km), and the same thing happened: after a while, the probe was gone.

I'll add that in both cases, I did orbit the body more than once while focussing the vessel, and I did not crash. Furthermore, while the average surface altitude on the Moon is 8km, I haven't found any feature reaching 15km (it went maybe up to 10-11km at the borders of the greatest craters) so the lunar probe did not crash with the surface.

So, I was wondering if there is a sort of minimal altitude/height below which a probe is considered to crash on the ground ?

If so, is there a way to determine that altitude or height above ground ? Maybe using the surface features or radius of the body.

Also, does this happen for all airless bodies, or just some of them are glitchy ?

[Alshain]

Yes actually. The minimum guaranteed safe orbit is just above the altitude of the highest peak. This can usually be found on the wiki

http://wiki.kerbalspaceprogram.com/wiki/Bop

The highest point on Bop is an astounding 21,758 m, marking the highest point above sea level in the Kerbol System. The point lies at 23.87° W and 64.57° N and is as high as Olympus Mons on Mars.

This is dependent on your orbit, on the Mun for example, the highest peak is near the pole, so equatorial orbits can be much lower. But if you stay above that highest peak there is no possible way to hit the body.

Edited September 27, 2015 by Alshain

[Thatguywholikesionengines]

Well, if you want eeloo for some reason, if your craft is less than 7 meters in any diameter or width and stuff like that, then 2930 is lowest for equatorial, but i usually go for 3000.

[Hupf]

http://www.kerbalmaps.com/charts/elevation_inclination/#

[KerikBalm]

Even if your orbit is in theory above all the peaks, this can still sometimes happen... I forget what the rules are though... I just avoid really low orbits above airless worlds (and its realistic... such low orbits probably wouldn't be stable due to increased perturbations from uneven mass concentration of the body you're orbiting.

[bewing]

Especially with the orbit decay bug that's in 1.1.2. There is no such thing as a "stable orbit" currently.

[KerikBalm]

it doesn't decay on rails

[bewing]

No, it doesn't. And I don't think the landing legs deltaV bug happens on rails, either. But regarding the OP's initial orbit, there is nothing on the Mun that will ever take you out at 15km, so something funny is going on.

Edited June 8, 2016 by bewing

[Stoney3K]

There is some different logic which determines if an orbit is "stable" (not hitting anything on the surface) if the craft is on rails. I believe the minimum altitude (ASL) for checking if a craft didn't smash into the surface when it's on rails is higher than when the craft is focused.

Also, there may be some crazy stuff going on when you're using RSS, since RSS re-purposes the existing bodies in the Kerbol system to act differently.