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Differentiating Medium Earth Orbits from High Earth Orbits at the geosynchronous altitude makes intuitive sense. Is there some meaningful difference between orbits above vs. below 2,000 km, or is the LEO/MEO distinction purely arbitrary?

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  • $\begingroup$ A good question. These things are usually set as "near to some property", like Karman line, where achieving aerodynamic lift requires exceeding orbital speed. If I were to bet, I'd say MEO would be bound to orbital decay time ("not within lifetime of Earth"), but that's just my hunch. $\endgroup$ – SF. Apr 17 '18 at 4:10
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    $\begingroup$ I was taught a long time ago that MEO was in part defined by the need for rad-harder electronics, as the Van Allen particle fluxes start to rise steeply at the top of the LEO range. I don’t have the data at hand to confirm that, but perhaps somebody does & can write an Answer. $\endgroup$ – Bob Jacobsen Apr 17 '18 at 4:31
  • $\begingroup$ A circular orbit with a period of 2 hours has a height of 1700 km. For 2000 km height, the period is 2 hours, 7 minutes and 2 seconds. $\endgroup$ – Uwe Apr 18 '18 at 20:31
  • $\begingroup$ The inner Van Allen belt is typically from 1000 km to 6000 km. The LEO/MEO boundary at 2000 km is well within the inner belt. $\endgroup$ – Uwe Apr 18 '18 at 20:37
  • $\begingroup$ @uwe Not sure of the sign of your comment, sorry. Take a look at figure 25 of spacewx.com/Docs/AIAA_G-083-1999.pdf, which shows that the dose rate is rising exponentially (log plot) and remains high from 2000km (1.3Re) upward. (I can't find my earlier papers on this, but there are graphs from the late 60's that look a lot like that) It's true that LEO has varying dose characteristics, variation with latitude & solar conditions, etc, but MEO at 2k and up was a nasty place for early solid-state electronics. $\endgroup$ – Bob Jacobsen Apr 20 '18 at 4:19
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Indeed it was originally based on radiation dose rates. Hank Garrett told me years ago that it was considered the altitude above which satellites in relatively low-inclination orbits had to take serious radiation hazard reduction measures. But that justification is obsolete, and was tenuous to begin with. Since the particles (especially electron and protons) are tightly confined to spiraling along magnetic field lines, both the inner and outer Van Allen belts are at lower altitudes at higher magnetic latitudes. We've learned in the past few years, partly as a result of NASA's Van Allen Probes, that the huge variability of solar activity has stronger effects on the Van Allen belts than previously recognized, sometimes filling the entire region between the bottom of the inner belt and the outer edge of the outer belt, and that often drives the bottom edge of the inner belt to lower altitudes. After the filling event, over a period of 1 to 1-1/2 years the region below the outer belt slowly decays, separating the inner from the outer again, and eventually making a more tenuous and smaller (in radial extent) inner belt. So the altitude of the onset of serious radiation effects varies a lot, to some extent unpredictably (on weeks or months time scales) due to the unpredictability of solar activity on those time scales. Some people might now be looking for a different justification, but there's nothing particularly distinctive about 2000 km altitude.

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