# Why did ESA's Solar Orbiter mission's target inclination drop from 34 to 25°?

During the planned 7-year mission the orbital inclination will be raised to about 25°

But the information below shows that originally the target final inclination was much larger at 34°!

Question: The original planned maximum inclination was 34°, why is it now only 25°?

According to this answer to What is ESA's Solar Orbiter's target inclination? ESA's Solar Orbiter:

From Solar Orbiter: Exploring the Sun-heliosphere connection (ArXiv, Researchgate, and published in Solar Physics) Figure 3 shows the solar latitude of the spacecraft versus time, and the extrema reflect the orbit's inclination for a January 2017 launch which didn't happen.

after flyby        approx. date   approx. inc
-----------        ------------   -----------
Venus 1            06-2017           7
Earth 1            04-2018          14
Earth 2 + Venus 2  06-2020          17
Venus 3            04-2022          25
Venus 4            01-2024          31
Venus 5            04-2025          34


• – uhoh
Feb 8, 2020 at 6:54
• The sequence of flybys for actual feb 2020 launch is different than for planned 2017 launch. The ratio of orbital periods of Earth and Venus can't be written as simple proportion (e.g 3/2) - actually it's about Te/Tv ~ 1.612. This means the configurations of Earth-Venus relative positions don't repeat (at least in years-decades timeframe). So 2020 lauch have different set of flyby configurations. Each flyby has the constraints, and the main of them - it should give trajectory to the next flyby. Feb 13, 2020 at 7:21
• This ESA video shows planned flybys for feb 2020 launch. You can see it differs from 2017 - esa.int/ESA_Multimedia/Images/2020/01/… Feb 13, 2020 at 7:24
• As result the inclination gain of Solar Orbiter differs too for diffetent launch years. Wikipedia article is rather outdated. Yes, it should be answer, not comment. I'll try to write answer later, l hope ) Feb 13, 2020 at 7:32

## 2 Answers

Nothing changed. Note that your plot covers 10 years and not 7. You cited a line from Wikipedia, but didn't check the source given for it:

The nominal mission of seven years will see a maximum orbital inclination relative to the solar equator of 25°. During the extended mission, additional Venus GAMs [gravity-assist manoeuvres] could allow the orbital inclination to increase to about 34°.

• Okay this makes sense, thanks!
– uhoh
Feb 8, 2020 at 16:28

But the article in Wikipedia is rather outdated.

The sequence of flybys for actual Feb 2020 launch is different than for planned 2017 launch. The ratio of orbital periods of Earth and Venus can't be written as simple proportion (e.g 3/2) - actually it's about Te/Tv ~ 1.612. This means the configurations of Earth-Venus relative positions don't repeat (at least in years-decades timeframe). So 2020 launch have different set of flyby configurations.

This ESA video shows planned flybys for feb 2020 launch. You can see it differs from 2017 https://sci.esa.int/documents/34903/36129/Solar-Orbiter_journey_around_the_Sun_768x432.mp4

Each flyby has the constraints, and the main of them - it should give trajectory to the next flyby. But there are more - for example gravity assists should not happen during Solar conjunction, when Sun is between Earth and the spacecraft.

As result the inclination gain of Solar Orbiter differs too for different launch years. I couldn't find the detailed info about current trajectory, but in this pdf from 2011 there are examples of 3 different trajectories in for 2017-2018 launch. Page 76 and further.

Screenshot of summary table:

• Thanks for your answer! Why "gravity assists should not happen during Solar conjunction, when Sun is between Earth and the spacecraft" is probably obvious; it will be a homework assignment to me once I have my morning coffee and read the linked material.
– uhoh
Feb 15, 2020 at 0:51