# Why (the heck) did Tianwan-1 insert into a low inclination orbit then execute a plane change from 11.8 to 86.9 degrees?

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).

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

• 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. Commented Mar 16, 2021 at 0:39
• 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?
– user39728
Commented Mar 16, 2021 at 1:27
• (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. Commented Mar 16, 2021 at 2:14
• 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 Commented Mar 16, 2021 at 10:14
• @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?
– uhoh
Commented Mar 16, 2021 at 10:27