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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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3 Answers 3

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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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    $\begingroup$ this comment illustrates why it's important to set a good example for new users and include sources in answers. Answers need to do more than demonstrate personal knowledge, they need to demonstrate where the information is coming from when possible. $\endgroup$
    – uhoh
    Commented Mar 13, 2021 at 0:14
  • $\begingroup$ That's right... I remember seeing something about a spare Soujourner rover. I can't remember if it was displayed someplace, but I remember seeing something about it online. $\endgroup$ Commented Mar 8 at 21:01
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I read somewhere that Voyager 3 was to have been launched in November, 1979 and flyby Jupiter in April 1981 /May, 1981 approximately, then fly onto Uranus in 1986/87, and finally Neptune in 1990/91. One article had mentioned something like that, and i believe it was the August 1970 issue of National Geographic, and Planetary Encounters book by Robert M. Powers.

from the National Geographic, page 185:

One voyage, beginning in 1977, is planned to visit Jupiter, Saturn, and Pluto, making the trip in 8 1/2 years instead of the 40 it would take to go to Pluto alone if this game of celestial billiards were not played. The other Grand Tour, starting in 1979, aims for Jupiter, Uranus, and Neptune (diagram) diagram showing two grand tour trajectories; one in green leaves Earth Sept. 1977, at Jupiter Apr. 1981, at Uranus Jul. 1985, and passing Neptune Nov. 1988. The other in orange leaves Earth Nov. 1979, passes Jupiter Feb. 1979, at Saturn Sept. 1980, and at Pluto Mar. 1986

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  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Commented Mar 8 at 6:38
  • $\begingroup$ I can't find any mention of Voyager in the National Geographic archive for July-December 1970. I can't tell if the text search is working because it doesn't find any mention at all in the entire National Geographic archive there, and I assume they write about the spacecraft at some point. $\endgroup$
    – Erin Anne
    Commented Mar 8 at 7:37
  • $\begingroup$ The cover story in the August 1970 NG is Voyage to the Planets. amazon.com/National-Geographic-Magazine-August-Vol-138/dp/… $\endgroup$ Commented Mar 8 at 13:37
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    $\begingroup$ @OrganicMarble interesting. Okay, Voyage to the Planets is in the archive I linked, and page 181 shows the Jupiter-Saturn-Pluto "grand tour" without the Voyager name. Well-remembered! I'll edit the link into this answer, along with a screenshot. $\endgroup$
    – Erin Anne
    Commented Mar 8 at 19:04
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    $\begingroup$ @OrganicMarble I actually found a full index from 1888-1988 of National Geographic but that also didn't link this story to Voyager, presumably because it doesn't use that name. The "Voyage to the Planets" is more conceptual, and broader than just the Grand Tour missions, including blurbs on all the recent and upcoming planetary missions (prominently Mariners and Pioneers) $\endgroup$
    – Erin Anne
    Commented Mar 8 at 19:14
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NASA decided that the Jupiter/Saturn/Pluto route was not important enough. They were low on budget of the moon missions (which costed 2 billion per launch). However New Horizons, launched in 2006, did do that route.

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    $\begingroup$ Do you have a reference that supports your statement "NASA decided that the Jupiter/Saturn/Pluto route was not important enough."? $\endgroup$ Commented Mar 11, 2021 at 17:03
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    $\begingroup$ There's no disagreement. You simply need to back up your statements here with references, otherwise it's Some Internet Person Saying Something, which is worthless. $\endgroup$ Commented Mar 11, 2021 at 19:44
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    $\begingroup$ The other answer has no reference either. $\endgroup$
    – Saturn 5
    Commented Mar 12, 2021 at 16:46
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    $\begingroup$ @OrganicMarble see comment. $\endgroup$
    – uhoh
    Commented Mar 13, 2021 at 0:16
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    $\begingroup$ @Saturn5 Welcome to Stack Exchange! I do see that the other answer also cites no sources and I've left a comment there. Nonetheless we should all strive as much as possible to cite sources to back up assertions we make in answers. Do you think that you can try to do so here? Thanks! $\endgroup$
    – uhoh
    Commented Mar 13, 2021 at 0:18

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