On their last big-planet flybys, both Voyager 1 and 2 were given substantial kicks out of the plane of the ecliptic.

If this wasn't so, and their flyby's were adjusted to stay within the plane of the ecliptic, in what ways would the missions have been different to manage, and in what ways would the resulting data have differed?

From How well can Voyager 1 separate Earth signals from Solar noise these days?. Data for the Sun, planets, Pluto, Voyager 1 and Voyager 2, from January 1, 1969 (a good year to start things) until now July 2016 (dots), but it will look roughly the same now. Data is from NASA JPL Horizons. enter image description here

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    $\begingroup$ Nice figure! What did you use to make it? $\endgroup$ – WaterMolecule May 3 '19 at 1:51
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    $\begingroup$ @WaterMolecule Thanks! I downloaded positions from Horizons, then just wrote a Python script to make still images. Was too lazy to figure out how to us Pillow to build a GIF, so I think I just used ImageJ. pastebin.com/n2hAmfTW $\endgroup$ – uhoh May 3 '19 at 2:27
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    $\begingroup$ @WaterMolecule updated script (I've deleted a small debugging thingy) pastebin.com/1Y7YmUiU $\endgroup$ – uhoh May 5 '19 at 2:26

The Voyager spacecraft had nearly no propulsion capabilities. They were launched on a "Grand Tour" trajectory that would use gravity assists to provide encounters with Jupiter, Saturn, Uranus, Neptune, and optionally Pluto. In order to follow this trajectory, the route the spacecraft can take past each planet is very strongly constrained.

Major differences from keeping both spacecraft in the plane of the ecliptic:

  1. Less data from a Titan flyby. NASA selected a flyby route for Titan that would take Voyager 1 close over the moon's south pole and through its shadow. Both of these greatly increased the amount of atmospheric data that could be gathered, but the resulting gravity assist of necessity gave Voyager 1 an out-of-plane velocity far beyond what its thrusters could counter. An alternative equatorial flyby would have kept Voyager 1 in the plane of the ecliptic, but an encounter far enough away to avoid disturbing its trajectory would provide limited data, while a close encounter would take it off the Grand Tour trajectory.

  2. Different moon encounters at Saturn. The trajectory change at Titan put Voyager 1 on a different path through the Saturn system than Voyager 2 took, giving different views of Saturn's moons, both in terms of direction of view and in terms of which ones are close.

  3. No polar data from a Neptune flyby, and probably no Triton encounter. Neptune's moon Triton is inclined 60 degrees to the ecliptic. An in-plane encounter with it can only happen at the ascending or descending node of its orbit; an alignment between a node and the Grand Tour trajectory only happens four times per Neptune orbit (164.8 years). Any encounter with Triton would take Voyager 2 off the Grand Tour trajectory, so NASA decided that the greatest science value would come from passing low over Neptune's north pole on the way to a Triton encounter.

  4. A possible Nereid encounter. Unlike Triton, Nereid orbits close to the plane of the ecliptic. A Neptune flyby that's not trying to encounter Triton could be directed to fly by Nereid, or at least pass closer than Voyager 2's 4.7 million kilometers. (Deliberate encounters with Neptune's other moons aren't possible: only Larissa was discovered prior to Voyager 2's flyby, and its orbit wasn't known at the time.)

  5. A possible Pluto flyby. If it had stayed in the ecliptic, Voyager 1 could have been redirected to Pluto by a gravity assist at Saturn. (Voyager 2 could have been redirected at Neptune, but it would have been a much longer flight).

  6. Different observations of the edge of the solar system. The termination shock, heliosheath, and other features of the solar system boundary are believed to vary with solar latitude. Near-equatorial encounters would be different from high-latitude encounters, and under the most popular models, would take place significantly later.

  • $\begingroup$ Excellent answer, thank you! Is this all from memory, or can you recommend a source or two for people read further? $\endgroup$ – uhoh May 4 '19 at 2:09
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    $\begingroup$ A combination of memory, Wikipedia articles, and a general feel for orbital mechanics. $\endgroup$ – Mark May 4 '19 at 2:15

Differences in the resulting data:

  • no close Triton flyby for Voyager 2, differences in what moons could be observed during the Neptune encounter
  • no close Titan flyby for Voyager 1, differences in what moons could be observed during the Saturn encounter
  • possibly a Pluto flyby for Voyager 1
  • probably some differences in the timing of the heliosheath encounter (heliosheath is not spherical AFAIK, so crossing into the healiosheath could be sooner or later)

For more background and detail on the Voyager decisions, see this answer to Why didn't Pioneer 11 visit Uranus/Neptune, and why didn't Voyager 1 visit Pluto?

Differences in mission management: I can't think of any.

  • $\begingroup$ I don't see why the two no close moon flyby's must be true. No matter how inclined an moon's orbit, it passes through the planets orbital plane twice per rotation. The distances and flight times between planets are so vast, certainly there's room to add a short phasing advance/delay to flyby the Moons when they were also in the planets' orbital plane. $\endgroup$ – uhoh May 3 '19 at 23:44
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    $\begingroup$ @uhoh, the Voyager spacecraft had almost no propulsion capability -- their routes were determined based on gravity slingshots around the gas giants. In order to get a close encounter with a moon in an inclined orbit, not only do you need to approach it as it's passing through one of its nodes, you need to do so when that node is close to the Grand Tour trajectory. And even if you could get a close encounter, that encounter would disturb the trajectory, making it difficult to move on to the next planet. $\endgroup$ – Mark May 4 '19 at 0:22
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    $\begingroup$ @uhoh, additionally, Voyager 1's Titan encounter was designed to pass over the moon's south pole. This permits a far greater range of atmospheric observations, but of necessity the resulting gravity slingshot sent the spacecraft out of the plane of the ecliptic. Voyager 2 leaving the plane of the ecliptic was a result of the aforementioned nodal alignment issue: any maneuver to visit Triton would take it off the Grand Tour trajectory, and NASA decided that Triton + Neptune's north pole had greater science value than Neptune's equator + Pluto (or Neptune's equator + Triton). $\endgroup$ – Mark May 4 '19 at 0:29
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    $\begingroup$ @uhoh, ecliptic crossings aren't what's important. It's nodal alignments (the time when that crossing lines up with the spacecraft's trajectory) that matters. Those happen twice per planetary orbit: once every 15 years for the moons of Saturn; once every 82 for the moons of Neptune. $\endgroup$ – Mark May 4 '19 at 0:58
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    $\begingroup$ @Mark Aha! I'm forced to think in 3D before my second cup of morning coffee, ouch. Yes I've got it now. I think it would make an excellent answer, please consider posting it. I think both myself and future readers will really benefit from having it spelled out clearly. Thanks! $\endgroup$ – uhoh May 4 '19 at 1:01

Taking the numbers from July 2016 that I used to make the plot in the question, Voyager 1 was 149 AU away from the sun in-plane ($\sqrt{x^2+y^2}$) and at +108 AU out-of-plane ($z$). For Voyager 2 those are 114 and -100 AU respectively.

That means that seen from the neighborhood of the Sun, including Earth, Voyager 1 and 2 will be 36 degrees above the ecliptic, and 41 degrees below it, respectively.

Why is that important to know? Because the latitudes of the three Deep Space Network locations used to communicate daily with the Voyagers are as follows: Canberra: -35.4, Madrid: +40.4, Goldstone: +35.4

I haven't checked the historical DSN communications records for the Voyagers yet, but almost certainly this means that Voyager 2 talks to Canberra most often and (probably) never to Madrid, while Voyager 1 almost never (if ever) talks to Canberra.

If the two voyagers were much closer to the plane of the ecliptic, scheduling the uploading of commands and the downloading of data would be far more relaxed!

enter image description here


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