Parker Solar Probe will end up in it's Sun-kissing orbit though a series of Venus flyby's, which roughly pins Parker's aphelion at Venus's semimajor axis or about 108 million km. Its final perihelion will be about 6.6 million km, making its semimajor axis 114.6/2 or about 57.3 million km.

Mercury's semimajor axis is about 57.9 million km!

Is this just a manefestation of Bode's law (humorous reference to serendipity) or will it in any way be helpful to stabilize Parker's orbit, or be unhelpful?

fyi Horizons now has a projected orbit for Parker that goes out to 2025-Aug-31 so I've plotted the Sun, Mercury, Venus, Earth, and Parker in a frame rotating steadily with Mercury's period. You can see Mercury's actual path is a big loop because it's quite elliptical.

The upper plot starts just after launch on 2018-Aug-13 and the dots are today's positions: 2018-Aug-30.

The lower plot stars 2024-Jan-01 and you can see that it moves into what has the appearance of a resonant orbit with Venus. It may not really be resonant if there's no gravitational effect doing this, coincident parallel motion is not a true physical resonance, which happens when there is a causative exchange of energy.

It can be verified that those three or four very tight, repeating orbits begin in November 2024 after the last Venus flyby.

enter image description here

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    $\begingroup$ Orbital period is proportional to semi-major axis, so the probe's position relative to Mercury will remain constant on subsequent orbits. What are the consequences depends on phase, what that position will be when approaching Mercury orbit - will it get consistent fly-bys, or will it remain far and unaffected. Personally, I believe this is an artifact of Venus being near-resonant with Mercury (4% error) and the probe aiming for Venus assists. $\endgroup$
    – SF.
    Commented Aug 30, 2018 at 10:34
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    $\begingroup$ correction: 4 degrees, or 1.1% error. $\endgroup$
    – SF.
    Commented Aug 30, 2018 at 10:54

1 Answer 1


I'm leaning towards the "just coincident" explanation here.

The semi-major must be >54.5 million km (and even that is too hot), as the aphelion must touch the orbit of Venus to enjoy the repeated gravity assists to reach this orbit, and the perihelion must be outside the sun. So ~57 million km is right were the trajectory constraints would demand the probe be located even before one gives any consideration to Mercury.

The Parker Solar Probe has no obvious stability requirements. It just needs to get close to the Sun, and other than that it has no strong location dependencies other than not smashing into Mercury, which is trivial.

  • $\begingroup$ Yes, this does seem to be the case. $\endgroup$
    – uhoh
    Commented Jul 9, 2021 at 23:16

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