It looks like (if I've parsed right) every one of them stands for "Earth", except that HEO alone can also be overloaded three ways (high-earth, highly-elliptical, and highly-eccentric).

This is unimportant, but: a site:nasa.gov search shows all three "HEO" acronyms in common use, there; and even Wikipedia abbreviates it inconsistently across entries[0-2].

https://en.wikipedia.org/wiki/Medium_Earth_orbit ("A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO)")

https://en.wikipedia.org/wiki/Highly_elliptical_orbit ("A highly elliptical orbit (HEO) is")

https://en.wikipedia.org/wiki/High_Earth_orbit ("In this article, the non-standard abbreviation of HEO is used for high Earth orbit[2]")

[edit]: I overlooked the abbreviation of "geostationary equatorial orbit" for GEO, which brings it up to four different "E's"!

The phrase "highly elliptical" is one where I know exactly what they mean but the more I think about it the more wrong it seems. It should be "Highly eccentric orbit".

All shapes which satisfy {(x,y)| x^2/a^2 + y^2/b^2 = 1} for fixed values of a,b in R are elliptical. Something is either elliptical or not - it's not a matter of degree. A circle is just as elliptical as a more eccentric ellipse in the same way that a square is just as rectangular as a more elongated rectangle.

> geostationary equatorial orbit

I thought GEO stood for Geostationary Earth Orbit, since a geostationary orbit must be equatorial anyway. But actually "Earth" would also be redundant, since "Geo-" already stands for Earth.

I understand it's both, but "equatorial" is more precise to distinguish it from GSO, a non-equatorial [g]eo[s]ynchronous [o]rbit. Otherwise, they would both be "GEO".

LEO is Low Earth Orbit

MEO is Medium Earth Orbit

The E is short for the same thing in this case.

GEO for Geostationary and HEO for High-Eccentricity are interesting, though.

MEO is Middle Earth Orbit. We have to keep an eye on what Gandalf is up to.

That orbit has only really been possible since the sinking of Numenor. Better make use of it now that we can.

SubEarthOrbit for the dwarves

…but they orbit too deep, and too greedily.

It is the most common letter, but I agree that is funny.