What's the difference between the [astrodynamics] and [orbital-mechanics] tags? Do we need both? Will question authors always know which to use?

Question

• astrodynamics 82 questions:

The study of satellite orbits and space flight dynamics.

• orbital-mechanics 1,527 questions:

Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. For the movements of celestial bodies, see [celestial-mechanics].

Is there a clear distinction between the use of these two tags within this site? Is it always clear to question authors which one to use?

I always use orbital-mechanics, until today didn't know (or at least remember) that the astrodynamics tag even existed.

update:

@BrendanLuke15's comment reminds us that there is yet another somewhat tag. It may be sufficiently distinct that it doesn't interfere here, but I just can't tell:

• celestial-mechanics 67 question:

Questions regarding the movements and interactions of astronomical bodies as it relates to spacecraft, their trajectories, and their maneuvers. For the motion of spacecraft themselves, see [orbital-mechanics].

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Answer 1

I would like to point of that over half, 47, of the 82 astrodynamics questions are already tagged orbital-mechanics, and looking at the rest, most of them can be too.

I'm in favour of making astrodynamics a synonym of orbital-mechanics, since any meaningful distinction does not reflect actual use.

On the other hand, there's a (quite extensive) discussion and consensus on the use of celestial-mechanics, so I propose leaving that tag alone for now.

Since it's plausible that some of the astrodynamics questions are really celestial-mechanics questions, it sounds like a good idea to go through the list of questions re-tagging as appropriate before creating the tag synonym.

Answer 2

First, my own question: if we combine the tags, do searches for the term "astrodynamics" suddenly return fewer results? If so, that would be bad, because so many textbooks have astrodynamics in the title. In that case, I would argue for always tagging with both together. However, I'm only guessing; does anyone know for sure?

If it doesn't screw up searches, I lean toward combining them. There is an argument for keeping them separate, but only if we are going to be clear, thorough, and vigorous in maintaining the separation.

There are many competing definitions of each term, ranging from identical to disjoint, with varying degrees of overlap in between. Where there are differences, the term "astrodynamics" is restricted to artificial bodies, while orbital mechanics includes also natural bodies. When there isn't a difference, it seems generally to be because that author also restricts orbital mechanics to artificial bodies.

If we believe astrodynamics means only artificial satellites, and orbital mechanics includes both those and natural satellites, then we could consider drawing and defending a line in the sand to keep one a proper subset of the other, but it would require vigilance. Alternately, if astrodynamics and orbital mechanics are both restricted to artificial bodies, what other tag should we use for planets, moons, comets, and asteroids?

This question on the main site regards the difference between astrodynamics and "space dynamics", and contains a couple useful links, including Vallado's definition and Wikipedia's (astrodynamics and orbital mechanics are equal).

Bate, Mueller, and White, Fundamentals of Astrodynamics (1971), don't seem to define astrodynamics explicitly, but they do say "our efforts in this text will be devoted to studying the motion of artificial satellites, ballistic missiles, or space probes" (section 1.3, page 13).

Pedro Ramon Escobal, Methods of Astrodynamics (1968) says "astrodynamics is the science of applying the techniques of celestial mechanics to the solution of space engineering problems."

Richard Battin, An Introduction to the Mathematics and Methods of Astrodynamics (1999) says "The term ''Astrodynamics'', attributed to the late Sam Herrick, came into common usage in the 1950s to categorize aspects of celestial mechanics relevant to a new breed --- the aerospace engineer."

Samuel Herrick, Astrodynamics (1971) has the most elaborate definition, and also the most amusing (bold and italics in the original):

Astrodynamics must be defined in terms of celestial mechanics on the one hand and space navigation on the other.

Celestial mechanics is concerned with the motions of objects in astronomical space, with the physical forces that govern these motions, and with the mathematical assumptions, conditions, and processes by which they are determined and predicted for observation and correction. Like Gaul, celestial mechanics is divided into three parts, and although a 'celestial mechanic' is not likely to be concerned exclusively with one of these parts, his heart will nevertheless be in one and he may be slightly less than clairvoyant in the others:

1. Mathematical celestial mechanics... prefers general solutions over special solutions... on the other hand it is inclined to delimit a problem in order to employ general analytical tools that may exclude some real problems, or be awkward in handling them.

2. Physical celestial mechanics is concerned with explanations of observed phenomena, and with the evaluation of physical constants associated with the formulated explanations, especially if these constants are useful in other areas of astronomy, geophysics, or physics in general...

3. Astrodynamics is a term that has come to designate the third field of celestial mechanics, which is concerned with the determination, integration, and improvement of specific 'real-world' orbits. It is fully aware of mathematical and physical celestial mechanics, and frequently must solve its own problems in those fields. The demonstration of the existence of a solution is often the end of mathematical celestial mechanics and the beginning of astrodynamics...

Completing this definition, and then drawing parallels and contrasts with "space navigation", takes the next seven pages (the posted quote is about half a page).