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A while back, I asked this question here, dealing with finding a standard low orbit for a solar system body. The answer I received was mainly in the context of a body with atmosphere. This answer gave me a useful nudge in the right direction, but I wanted to come back to the question for airless bodies.

I am looking for a way to arrive at a general-purpose low orbit value, for the sake of my own back-of-the-envelope calculations.

I want to be able to do this for all airless bodies from Ganymede to irregular 500m wide asteroids. To take advantage of the Oberth effect, these orbits should be as low as possible, while still being stable in the short term (as in tens to hundreds of orbital periods) and suitable for flybys.

For large, spherical bodies, the (arbitrarily made up) rule of thumb I have at the moment is:

(The radius of the object's largest axis + height of the tallest mountain) * 1.01

I think this rule of thumb breaks down for smaller objects like 25143_itokawa.

Are there any other considerations like mascons and irregular shape that go into deciding on a short-term stable low orbit?

Ingolifs
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    There essentially is no Oberth effect from a 500m wide asteroid. There is however a huge "lumpy potato" effect. Even the Earth's Moon (which is considerably wider than 500m across) has mascons that can drastically alter an object's orbit. As objects get smaller their gravitational field get lumpier. – David Hammen Oct 24 '22 at 13:18
  • I fear there is no simple rule of thumb valid for bodies with irregular shape or larger mascons, especially for low orbits. – Uwe Oct 24 '22 at 16:29
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    One of the driving considerations might actually end up being flyby science constraints, since you may as well train instruments on whatever it is as you go by/orbit, but there are going to be some kind of limits on the body speed (as measured in the spacecraft-fixed frame) – Erin Anne Oct 24 '22 at 20:05
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    Would it be a consideration if the object has any emissions (e.g. Io, comets)? – Wayne Conrad Oct 24 '22 at 20:43
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    Alas no time to write an answer, but it would be based on some examples of how deep-space craft approach, enter high orbit, and work their way down as they get a better "feel" for its gravity field. At one extreme would be how cautiously and slowly Rosetta approached 67P, and the other might be the confidence with which Dawn lowered its orbit around Ceres and parked itself into a synchronized orbit for "long term storage" Basically a suitable low orbit would be numerically calculated based on all the gravitational data available at the moment. – uhoh Oct 25 '22 at 06:18
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    Interesting, now that I think about it, just about every moon we haven't sent a probe orbiting around will have an unknown set of gravitational anomalies. The smaller the object, the larger the potential anomalies relative to the overall gravitational strength. I wonder what would be a good rule of thumb to estimate the deviation from point source gravitation. – Ingolifs Oct 25 '22 at 08:32

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