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eoPortal's Tianwen-1 (China's first Mars Exploration Mission) shows the following graphic, caption and link.

It's often discussed that orbital plane changes are like certain table wines in that they are for lying down and avoiding. They are generally costly in delta-v. The image shows that the maneuver is likely done at the apoapsis of a 400 x 180,000 km orbit and so the cost has been minimized. At that distance the orbital velocity around Mars is only about 485 m/s 97 m/s and so at least several hundred tens of m/s of delta-v would be needed for such a large plane change.

Question: Why not enter directly into a high inclination orbit? From Earth wouldn't it just mean aiming a few thousand km higher at a distance of hundreds of millions of km? This plane change seems unnecessary and a bit costly to me at first glance (a bit of delta-v and an extra burn and maneuver).

Tianwen-1 orbital activity at Mars

Figure 10: Planned orbital trajectory at Mars. A scheme of the different orbits that the Chinese probe Tianwen-1 will use around Mars, with informations on the orbital parameters of each of them13

13https://en.wikipedia.org/wiki/File:Orbital_trajectory_of_Tianwen-1_around_Mars.png

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    $\begingroup$ Apoapsis velocity is only 97 m/s (485 m/s would be for a circular orbit at 140,000 km), so the plane change cost works out to only around 118 m/s. That doesn't answer your question as to why they don't go direct to polar, of course. $\endgroup$
    – Russell Borogove
    Mar 16, 2021 at 0:39
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    $\begingroup$ Interesting! I didn't imagine plane-change maneuvers could be so cheap in delta-v, but it makes sense at those altitudes. Any numbers on the delta-v cost at 400 km? It would have to be super expensive then, yes? $\endgroup$
    – user39728
    Mar 16, 2021 at 1:27
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    $\begingroup$ (Meant 180,000km, not 140,000, also clearly coffee deprived.) At 400km periapsis, the orbital speed is ~4700 m/s, so the plane change cost would be proportionally increased (I think?) to about 5700 m/s -- not a recommended strategy. In general, for large plane changes, you usually want to raise apoapsis, make the change, and then get to your final periapsis/apoapsis, and for plane changes above 60º in particular, you want to raise apoapsis as high as possible. $\endgroup$
    – Russell Borogove
    Mar 16, 2021 at 2:14
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    $\begingroup$ In order to go nearly polar, you need to aim above or below mars (depending in which way you'd want to orbit). If your insertion maneuver fails while you are in-plane, you might be able to salvage a mission because you are in an heliocentric orbit in the plane of mars. if you go above/below, and the insertion fails, you get a gravity assist boosting you completely out of the plane of the solar system, making any kind of salvaged mission nearly impossible. I don't know if that was a consideration, but to me capturing near-plane seems much safer than capturing directly to a highly inclined orbit $\endgroup$
    – Polygnome
    Mar 16, 2021 at 10:14
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    $\begingroup$ @Polygnome Interesting, but I'm not sure it's actually and demonstrably any easier to return to Mars one way or the other. Can you think of something specific that can be done more easily if it misses Mars in-plane vs out? $\endgroup$
    – uhoh
    Mar 16, 2021 at 10:27

1 Answer 1

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A direct insertion into a polar orbit gives you only one option for an orbital plane: one that's initially pointed straight at the Sun. This is sub-optimal for an observation mission, since you're more or less directly over the sunrise/sunset line, and can only see deeply shadowed ground. It will be several months before the planet moves far enough in its orbit to give you good lighting.

If you instead capture into a highly elliptical orbit, you've got a far greater selection of orbital planes to choose from, and they're generally more useful, too. For example, a capture burn over planetary midnight (the natural choice for something coming in from a Hohmann transfer orbit) lets you enter a polar orbit over noon, giving you a nearly-unshadowed view of the ground.

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    $\begingroup$ I don't really understand this, can you include a diagram or some better explanations of or better yet sources for these assertions? Thanks! $\endgroup$
    – uhoh
    Jul 21, 2021 at 11:22
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    $\begingroup$ I disagree, many Mars missions have utilized a "more or less directly over the sunrise/sunset line" orbit to stay sun-synchronous and always in direct view of the Sun, an example given in this answer. Scientific instruments are specifically designed with a certain orbital geometry in mind and I highly doubt (put can't prove) that they would build the spacecraft, let alone launch it, without determining exactly what the final orbit would be (re: options of orbital planes). $\endgroup$
    – BrendanLuke15
    Jul 21, 2021 at 11:26
  • $\begingroup$ @BrendanLuke15, two and a half of the three main instruments on Mars Odyssey don't care about the lighting. $\endgroup$
    – Mark
    Jul 21, 2021 at 22:10

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