Is this plot of deep space trajectories correct? Did most launch retrograde from Earth? Why do some change direction between planets?

Question

This answer contains some nice plots of deep space spacecraft trajectories. Noticing that Voyage 2's heliocentric velocity dropped substantially just before 1990 I wanted to see why. Wikipedia's Voyager 2; Neptune describes its August 25, 1989 flyby.

But later in the mission the article shows the plot below. Since the planetary orbits are fixed I assume this is in an inertial frame.

1. I see that Voyage 2 (red) and Pioneer 2 (green) have substantial clockwise (prograde) "knees" or bends or deviations in their trajectories in the space between Unanus and Neptune's orbit. Are these accurate?

2. It shows all orbits traveling retrograde around the Sun until probably encountering an inner planet. That can't be right, can it?

3. I also noticed that when trajectories change course, the radius of curvature of the deflection is several AU. For scale note that the first planet plotted is Saturn's a = 9.6 AU orbit.

Is it possible that the plot mixes frames somehow? I can't make heads or tails of these plots.

Image cropped from source Voyager 1 extends to the right (purple) and Pioneer 10 extends to the left (dark blue).

Answer 1

The diagram you show is the digital version of a drawing by someone with an Etch-a-Sketch: completely inaccurate. The diagram below is accurate, showing Pioneer 10 & 11 and Voyager 1 & 2 trajectories in a heliocentric, inertial reference frame, of course with the ecliptic N-S dimension collapsed. No retrograde, no dog-legs between planets.

Every now and then a spacecraft will do a broken plane maneuver (illustrative figure here) between planets but the trajectory-change angles involved are very small; by far the largest component of those maneuvers is in the ecliptic N-S direction, so they wouldn't show up on these diagrams anyway.

This is not to say that no mission headed to the outer solar system can launch retrograde with respect to Earth, i.e., slower in an inertial heliocentric frame than Earth. Multiple missions have used Venus gravity assists, along with Earth gravity assists, to get to Jupiter. To get to Venus you have to slow down from Earth's orbital speed. But this is a far cry from launching to retrograde motion in a heliocentric, inertial frame; the idea of doing that with current launch vehicles would get trajectory designers laughing.

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