The ESA video Juice’s Jovian odyssey linked below is quite interesting to watch. Around January of 2031 it substantially increases its inclination around Jupiter. Big inclination changes can be expensive in delta-v and doing anything in such a large gravity field has an "expense multiplier".

It doesn't look like Juice is doing these at a very distant apoapsis where the cost can be much lower.

It's hard to tell from the video, but I wonder if it is synchronized by some rational number (e.g. 1:1, 3:2, etc.) with one of Jupiter's large moons and using repeated gravitational assists to boost inclination?

If so, how does it maintain synchrony; how does it change inclination but not period? Or does the period change as well?

screen shot from the ESA video "Juice’s Jovian odyssey"


2 Answers 2



ESA JUICE Red book, chapter 5.1.4 page 89

The inclination will be increased by several Callisto flybys.

Interesting that the JUICE team had a tradeoff - higher inclination but less Callisto mapping (becase the flybys would be over the same region), or less inclination but flybys over different Callisto areas.

You can compare the mission evolution readind "ESA JUICE Yellow book" of 2012.



I think you're correct that it's using repeated gravitational fly-bys in a synchronized orbit to do it.

Starting at 4:57 in the video, where the "high inclination orbits" portion of the mission begins, it's clear that each inclination change coincides with the orbiter meeting one of the moons. It then does the same trick in reverse to get back to equatorial orbit.

Per Heopps' answer and the linked ESA document, these are flybys of Callisto.

If so, how does it maintain synchrony; how does it change inclination but not period?

Passing the moon equatorially on the "outside" would be an accelerating gravity assist; passing it on the "inside" would be a decelerating one. So I believe that, as you approach the moon, there's some angle between the north pole and the left/inside edge of the moon that balances the increased north-south velocity component with the decreased counterclockwise component and leaves you with the same orbital period.

(If I'm wrong about that, I also believe that you could fire retrograde as you made the inclination flyby and still come out ahead because Pythagoras.)

  • $\begingroup$ Thanks! Europa (T=3.551181 days) is in orbital resonance with Ganymede and Io, but not with Callisto (T=16.6890184 days) so it would have to do some clever footwork to have multiple close flyblys of both, which it may in fact have. $\endgroup$
    – uhoh
    Apr 15, 2021 at 0:08
  • $\begingroup$ Multiple close flybys of Europa, each increasing the inclination, then no close flybys for a while, then multiple close flybys of Callisto, each decreasing the inclination, is what I'm saying. $\endgroup$ Apr 15, 2021 at 0:35
  • $\begingroup$ Yep got it! I'm just admiring the ability to choreograph that when there's no obvious satellite period that can meet those two different period moons at an ascending/descending node. Joy and surprise! :-) $\endgroup$
    – uhoh
    Apr 15, 2021 at 5:15
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    $\begingroup$ And I’m wrong, it’s Callisto flybys both ways. $\endgroup$ Apr 15, 2021 at 14:23
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    $\begingroup$ Just watch the spacecraft and moon tracks converge and the way the spacecraft's course snaps away when they meet. I've done enough graphical simulations of N-body problems and played enough KSP that an inclination-altering flyby is very distinctive to my eye, I suppose. $\endgroup$ Apr 15, 2021 at 19:24

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