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


  • 5
    $\begingroup$ 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 at 0:39
  • $\begingroup$ @RussellBorogove yikes, yes. Duly noted and now reaching for more coffee... (as usual) $\endgroup$ – uhoh Mar 16 at 0:55
  • 1
    $\begingroup$ 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 at 1:27
  • 2
    $\begingroup$ (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 at 2:14
  • 1
    $\begingroup$ Speculation: maybe they wanted some observations of Mars’ moons as well? $\endgroup$ – CourageousPotato Mar 21 at 10:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.