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I often struggle with the term "stable orbit" for objects in earths sphere of influence, since all real orbits are affected by disturbing forces.

Sentences like:

It is not possible to achieve a stable low lunar orbit

or the question

Is a stable orbit around a Lagrange point possible?

make sense to me, but it seams to me like the term is used very inflationary in LEO without really questioning its meaning. It is not uncommon to read sentences like:

Satellite XY reached a stable orbit!

In most cases the autor wanted to say "The Satellite XY will not reenter in matters of hours" which for my understanding is wrong terminology since a circular orbit above 300 km altitude is as "stable" or "unstable" as a elliptical orbit with a perigee below sea level. And all object in LEO will reenter, only the time of survival varies.

From the engineers perspective "stable" would mean "no change" (mostly the term is used about a systems energy). For real orbits this is not achievable since orbit parameters will change due to perturbations (again it doesn't matter how long the object will survive in this orbit).

So I assume the term was used in early rocketry for public relations reasons and somehow have been adopted into everyday language and than came back into professional wording. Which leads me to the conclusion: As long as there is no adopted convention or definition for this term, its usage is wrong for professional purposes.

So my question is: Is there any (official) definition of "stable orbit"? (not including theoretical keplerian orbits)

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  • $\begingroup$ Small nitpick: In Engineering (and Physics), stable does not mean "no change". It means that the system state equation does not diverge, and also does not diverge under small perturbations. $\endgroup$
    – throx
    Oct 11, 2023 at 23:48
  • $\begingroup$ Can you track back and explain what 'stable orbit' means to you? $\endgroup$ Oct 12, 2023 at 19:16
  • $\begingroup$ @RobbieGoodwin "an orbit, for which the orbital elements won't change (significantly) over (a longer) time" that would be "a stable orbit" for me. Which in reality cannot be achieved in LEO or even GEO. We can talk about "more stable than". Like: "LAGEOS-orbit is more stable than GOES-Orbit" but I do question sentences like "Satellite XY was released from the upper stage in an stable orbit". $\endgroup$
    – CallMeTom
    Oct 13, 2023 at 6:37
  • $\begingroup$ Thanks and d'you not think that's very vague; perhaps too vague for any useful, let alone official definition? $\endgroup$ Oct 14, 2023 at 22:55
  • $\begingroup$ @RobbieGoodwin, since my point is to abandon the term "stable" orbit and don't use it anymore, I do not think that I am the one who should suggest stand-fest definitions. $\endgroup$
    – CallMeTom
    Oct 16, 2023 at 5:11

1 Answer 1

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This response doesn't answer the question as written

"Is there any (official) definition of "stable orbit"?"

and instead aims at the last sentence of the penultimate paragraph in the OP and looks at:

"Is it wrong for professional purposes"

I agree it is messy language. What distinguishes some terminology as right or wrong has to take account of the expected use of the information given. In the orbital case it is going to include the timescale, e.g. hours, months, decades, i.e. you could say its "wrong" if one doesn't describe the context, though that applies to most engineering anyway.

Also, as I'm sure you are aware, stability has a special meaning in engineering and physics. A typical use is to determine whether some entity will return to its initial state after being perturbed. This obviously depends upon what one considers to be the perturbation and varies according to the context, e.g. drag or Solar radiation pressure.

See also this related topic on Physics SE, though this is firmly in the physics rather than being about the casual use: https://physics.stackexchange.com/questions/183726/what-makes-an-orbit-stable-or-unstable

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    $\begingroup$ "Stability" seems to have a bunch of meaning in engineering. What I was taught is en.wikipedia.org/wiki/BIBO_stability It means "do nothing crazy and nothing crazy will result". One system that does not confer to that is Earth's weather, another is military jet fighters. $\endgroup$
    – Vorac
    Oct 15, 2023 at 21:05

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