The following image from NASA shows the flight paths of Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2, as well as the positions of the outer planets.

enter image description here

It seems to me that with a little tweaking of the flight paths, it would have been possible for Pioneer 11 to visit Uranus and Neptune, and for Voyager 1 to visit Pluto. So why didn't that happen?

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    $\begingroup$ "..a little tweaking.." It can be surprising what 'a little tweaking' actually translates to in gravity assists, hardware and fuel.. $\endgroup$ Commented Jul 20, 2015 at 13:59
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    $\begingroup$ It is generally not easy to "tweak" an object by several million to billion kilometers when it is moving at 20-30 km/s. Such tweaks require either a gravity assist or a tremendous amount of fuel. The first depends upon timing (need a planet in the right place) and the second depends upon costs and the launch vehicle and the size of its fairing. $\endgroup$ Commented Jan 13, 2016 at 14:59

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The initial plan was to visit all of the outer planets:

The Planetary Grand Tour was to send several pairs of probes to fly by all the outer planets (and Pluto) along various trajectories, including Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, which fulfilled many of the flyby objectives of the Grand Tour except a visit to Pluto.

For Voyager 1, scientists had to make a choice: either visit Saturn's interesting moon Titan, or visit Pluto. A visit to Titan would bend Voyager's path and make a visit to Pluto impossible.

Astronomers decided that in order to optimize their science at Saturn, they’d need an orbit that brought Voyager 1 up close with Titan. But that flyby also would put Pluto out of reach after the spacecraft lifted out of our solar system’s ecliptic plane.

“It was a pretty straightforward decision for them because they thought there was going to be a third Voyager mission that could come along and go to Pluto,” Stern says.

(see also the Voyager FAQ)

Back then, they chose Titan, and then NASA cancelled the third Voyager mission.

There's an alternate point of view in this Slashdot comment:

I had the opportunity to ask Ed Stone, the JPL Director & Voyager scientist, this question. His rather glib answer was, "well, Titan was 3 hours away, and Pluto was 3 years away - and I had to make payroll."

I haven't found a reason why Pioneer 11 didn't visit Uranus or Neptune, or if those visits were even seriously considered. Pioneer 11 was launched out of the ecliptic plane by its Saturn encounter, that made it impossible to visit Uranus or Neptune. But it's possible a different trajectory would have given the opportunity to visit Uranus or Neptune.
Budget pressures may have played a role in not scheduling more flybys. Voyager 3 was cancelled in 1975. At that time, NASA needed money to fund development of the Space Shuttle.

  • $\begingroup$ You were too fast by a minute or so. If you want, incorporate the Voyager FAQ into your answer. That page comes from NASA and it specifically addresses the question "Why didn't the Voyagers fly by Pluto?" $\endgroup$ Commented Jul 20, 2015 at 14:30
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    $\begingroup$ Good answer. It wasn't possible to tell from the image I posted that Pioneer 11 and Voyager 1 left the ecliptic plane on an upward trajectory. The Voyager FAQ also contains an interesting tidbit about Voyager 2 - that it might have been able to visit Pluto if it had been aimed inside Neptune's atmosphere. $\endgroup$ Commented Jul 21, 2015 at 0:50
  • $\begingroup$ @david I tried the link you gave in the comment above and it died. How to solve? Is there an alternative? $\endgroup$ Commented May 5, 2019 at 0:39
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    $\begingroup$ @OscarLanzi - Use the web archive (aka the wayback machine). I wrote that comment almost four years ago. Websites change in four yeats. But if the website in question was anywhere close to being frequently visited, the web archive will have captured it. The web archive happened to have captured the linked page on the very day I posted that comment almost four years ago: web.archive.org/web/20150720064642/http://voyager.jpl.nasa.gov/… . $\endgroup$ Commented May 5, 2019 at 0:45

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