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In researching the answer to this recent question, I came across some information that was new to me. A third Voyager mission was planned, and then canceled. Apparently, Voyager 3 was cannibalized during construction:

I am currently reading the book Voyager: Seeking Newer Worlds In The Third Great Age Of Discovery by Stephen J. Pyne. On the second chapter, it is listed that there were three Voyager spacecraft. The second Voyager, VGR 77-2 had flaws and it was used for spare parts for Voyager 1 (VGR 77-1) and Voyager 2 (VGR 77-3).

At one point, NASA had a Planetary Grand Tour plan that consisted of 4 missions (Mariner 11-14). Was Voyager 3 one of these spacecraft?

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Voyager 3 would have been Mariner 13, before the name of the mission was changed. It's actually fairly common when multiple spacecraft are designed to have one with flaws that is used primarily for spare parts. It is also common practice to build a spare spacecraft.

In any case, when there were 4 spacecraft there would have been two Jupiter-Saturn-Pluto missions, and 2 Jupiter-Uranus-Neptune missions. As these two were redundant, they canceled one of each. My guess is that at that time, they had the parts to build 3, and decided to build all 3 of the spacecraft. One of these had issues, and thus was never launched.

As for what the original purpose was, it is necessary to realize the primary purpose of the Voyager probes was to study Saturn and Jupiter. Voyager 2 received an extension after Voyager 1 made a close pass of Titan, and not before. Thus, there are 4 possibilities:

  1. Jupiter/ Saturn/ Pluto
  2. Jupiter/ Uranus/ Neptune
  3. Jupiter/Saturn (Only)
  4. A spare/ EDU system (Engineering Design Unit, used on the ground to verify commands and troubleshoot problems

If Voyager 3 had launched, I predict it would have followed the Jupiter/ Saturn/ Pluto approach.

As to why it was canceled, that is certainly part of the Planetary Grand Tour budget crisis that happened post-moon launches.

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