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I plotted some data of the speed of Voyager 2 for various times (around the Jupiter flyby) and noticed that the highest speed that Voyager achieved was not at perijove and was wondering why this might be the case.

I found online that Voyager 2 made its closest approach to Jupiter at 22:29 on July 9th, 1979 (which should also be the point at which its velocity is highest. I then plotted some of the velocity data from JPL and noticed that its highest velocity was actually around 8 hours later. I was wondering why this might be the case? Could it simply be an error in the measurement or is this difference really insignificant for it to matter? I was just curious since I found it strange that the data doesn't match.

Jee
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Alexander Ivanov
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    @uhoh

    Here is the plot but it is not so useful since it is very hard to see the times: (but the velocity is in fact highest at 7 AM Jul 10 1979 whereas the closest approach is 10:29 PM Jul 9 1979) https://drive.google.com/file/d/1nMl09c-ZR0PPG7SmweVvazICROVMR4Zh/view?usp=sharing

    So I didn't actually think about the reference frame which I now found to be the ecliptic of J2000.0. Do you think this might be where the problem is?

    Also, this is where I got the data from: https://ssd.jpl.nasa.gov/horizons.cgi#results

    To get velocity I just took the magnitude of the x,y,z components.

     – Alexander Ivanov Jun 30 '20 at 09:56
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    I think it will be a good exercise to export the same data but choose Jupiter barycenter for the origin instead of the solar system barycenter which is usually the default, then compare the two speed curves and see that they peak at different times. To change it do this: 1: https://i.stack.imgur.com/XeIXL.png 2: https://i.stack.imgur.com/Yjw5o.png but don't do this! https://i.stack.imgur.com/IT9Qa.png see this and especially this answer – uhoh Jun 30 '20 at 11:24
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    @uhoh I just did what you suggested and it matches up with the correct times now. Thank you! – Alexander Ivanov Jun 30 '20 at 19:27
  • That's great news! It's always okay to post an answer to you own question, it will certainly be helpful to future readers. Many never read the comments so won't pick it up here, and comments should be considered temporary; there's always a chance they are "cleaned up" in the future. – uhoh Jun 30 '20 at 21:34

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If we look to the speed of Voyager 2 relative to Jupiter, this maximum of this speed should occur at perijove. If distance to Jupiter increases after perijove the speed of Voyager in the jovian reference frame would decrease.

But the transfer of energy from Jupiter to Voyager by gravity assist is not finished at perijove.

If we use the reference frame of the Sun Voyager and Jupiter are moving in similar directions just after perijove. The gravity of Jupiter is pulling Voyager to follow Jupiter on its orbit.

When the jovian orbit and the trajectory of Voyager are diverging into different directions and the distance between them increases, the gravitational pull decreases and changes direction. Finally the pull does not accelerate Voyager any longer because of the different directions of pull and trajectory.

Uwe
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  • This is OK except for the last paragraph: since the gravitational pull never decreases to zero, this would imply would never be a maximum speed but it would increase indefinitely to some asymptotic value. That's just not the case. –  Jun 30 '20 at 11:00
  • "does not accelerate Voyager any longer" is correct. :-) – LoveForChrist Jul 12 '20 at 05:22