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I suppose there's a launch window to Neptune by using Jupiter as often as there are Jupiter/Neptune conjunctions, every 13½ years. But which years are Jupiter and Neptune best aligned for gravity assist? How long does such a launch window last (three months, to take advantage of Earth's orbital speed?)

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  • $\begingroup$ Do you want a fly by or would you like to see a Neptune orbiter? $\endgroup$ – HopDavid Sep 11 '17 at 21:11
  • $\begingroup$ @HopDavid Does it matter for the launch date and flight trajectory? You need a Jupiter gravity assist anyway, and that pretty much sets the date as i understand it. $\endgroup$ – LocalFluff Sep 12 '17 at 5:13
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    $\begingroup$ To minimize arrival delta V you would want to path from Jupiter to Neptune to be nearly Hohmann. Which constrains the windows when the probe would fly by Jupiter. $\endgroup$ – HopDavid Sep 12 '17 at 12:52
  • $\begingroup$ @HopDavid You mean that a flyby could get to a ice giant sooner than a mission to enter its orbit? I thought the trajectory was the same and that entering orbit was just a matter of bringing enough fuel to do it. $\endgroup$ – LocalFluff Sep 12 '17 at 18:50
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    $\begingroup$ No, actually I'd expect flybys to reach Neptune faster than a Hohmann. But if you want to minimize delta V needed for orbital insertion, you'd want a near Hohmann trajectory from Jupiter to Neptune. If it's a fly by, no delta V is needed for Neptune orbital insertion. $\endgroup$ – HopDavid Sep 12 '17 at 23:17
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Using the Trajectory Browser, I was able to figure out the following trajectories that are available.

enter image description here

The dates are Jan-18-2018, Feb-22-2019, Feb-23-2031, and Mar-29-2032.

So basically there are two good launch windows roughly every 13 years To show out the path a bit more, I've included more than just the optimal paths.

enter image description here

Unfortunately the tool doesn't show how long the optimal window lasts, but it can be inferred that it lasts for some time. The total mission time is about 10 years for such a mission, depending on exactly how much one wishes to optimize the fuel expenditure.

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  • $\begingroup$ Ahw, 14 years from now! And Uranus, if I get it right, has its window for a 7.8 year trip in May 2033. It seems too early to design probes for them. $\endgroup$ – LocalFluff Sep 5 '17 at 7:03
  • $\begingroup$ It's not to early to design them, but it is too early to start serious design for them. Putting together a mission proposal probably is reasonable to start. $\endgroup$ – PearsonArtPhoto Sep 5 '17 at 15:19

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