Is there a standard or convention for what 'counts' as a low orbit for other solar system bodies?

I'm currently writing code for computing arbitrary Hohmann/ patched conics transfers, and I am looking for a streamlined way of calculating what a Low Orbit ought to be, given an object's size, atmospheric height, etc.. To maximise the Oberth effect, the periapsis of any orbit must be as low as possible, but not too low as to fall foul of local mascons or experience an unplanned high-speed rendezvous with the atmosphere or mountains. That suggests a low orbit 'sweet spot' for such calculations.

Earth's low orbit (LEO) is given as a range from about 200 km to 2000 km. The lower boundary is limited by atmospheric drag and the time the satellite is expected to remain in orbit. The upper boundary seems more or less arbitrary from what I can read.

On this delta-V map, the author gives some standard 'low orbits' for various celestial bodies. These range from 100 km for the Moon and the moons of Jupiter, to 250 km for earth, 400 km for Venus, 1000 km for Uranus, Neptune and Titan and 2000 km for Saturn and Jupiter. I think these numbers are generally reasonable, but it's also clear that they are somewhat arbitrarily chosen.

What is a reasonable heuristic for choosing a low orbit radius, based on the size and atmospheric height of the target planet?


1 Answer 1


Partial answer, too long for a comment, no time to do the math:

Two numbers that come up for Earth are 100/80 and 400 km.

100/80 km absolute lower limit

The math behind the Karman line is a bit arbitrary, but somewhere between 100 and 80 km even a single pass by Earth is in jeopardy of not surviving.

So what you can do is use a scale height estimate for the other planet to estimate an altitude where the pressure is the same as Earth's at say 100 km. Temperature can change a lot at low pressure and data may not be available, so you'll have to guess.

Also remember that if the planet has 10x the diameter of Earth, you'll loose the same amount of momentum at 1/10x the pressure during a single close flyby.

~400 km "typical orbit"

This altitude and above are thought of as "safe for a while". You can put something here and *unless there is a nasty solar storm heating the atmosphere (or a collision with a Starlink), it will be below but within 100 km of the starting altitude a few months later.

Now that scale height was mentioned above, you can use it again. But remember that you need to find out the dominant species at this altitude.

Above 100 km it's mostly atomic oxygen with a mass of 16, but at about 450 km it becomes mostly helium!

upper limit?

Probably impossible to define and not necessary from the point of view of a flyby or short-term parking orbit.

Definitely below synchronous altitude for the planet in question.

The volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on the MSIS-E-90 atmospheric model https://ccmc.gsfc.nasa.gov/modelweb/models/msis_vitmo.php


The volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on the MSIS-E-90 atmospheric model: https://ccmc.gsfc.nasa.gov/modelweb/models/msis_vitmo.php


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