What if the Voyagers had remained within the plane of the ecliptic?

Question

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?

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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.

Answer 1

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.

Answer 2

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.

Answer 3

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!