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In Star Trek IV: The Voyage Home, the Bounty travels back to what looks to be Earth circa 1985 to acquire a breeding pair of whales. Yet at this time, whales were already in danger and people were sophisticated enough that the Bounty might be detected and possibly destroyed.

Why did they not travel back to 900 AD or so before mass whaling or missiles became a thing?

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    Later would've risked going past when they were extinct or tripping over the Eugenics Wars which were in full swing twenty years ago according to TOS history. – IG_42 Mar 24 at 3:23
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    There are flaws with the answers because: 1. It is is may belief that different calendars are used in different productions and thus there is no guarantee that the Eugenics wars happen in the 1990s AD. 2. Transparent material is not necessary for the whale tank; steel will do. 3. Any local library will have books about whales including information about where Humpback whales can be found. – M. A. Golding Mar 24 at 17:21
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    @M.A.Golding Then why don't you post an answer of your own? – Rand al'Thor Mar 25 at 13:23
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    The whole terra incognita thing could have easily failed. It barely worked in 1986. Also, this is an ex post facto justification but had they gone to 900AD, they would have been stranded there with no way to get back and no way to make a whale tank, but we can infer that they chose ~1986 for those reasons. – J Doe Mar 25 at 15:11
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    Their goal was to recover a breeding pair of whales, but also - presumably - to avoid messing with the timeline too much in the process. Going back to 900 AD or earlier would mean removing a pair of whales who presumably would have reproduced (either with each other or with other whales.) The sudden absence of those progeny could influence the timeline in ways that are difficult to predict - whalers might go home empty-handed who previously had caught a whale, etc. By getting as close as (safely) possible to the time that the whales historically went extinct, they minimize these ripples. – Steve-O Mar 26 at 13:42
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This is addressed in the film's official novelisation which contains a slightly extended version of Kirk's conversations aboard the Bounty. Mr Scott was making plans to turn the hold into a holding tank, per the Captain's orders, and evidently worked out that he'd need materials from at least the twentieth century (onwards).

"Scott here, admiral. Wi' the proper materials--the proper twentieth-century materials--I'll be able to build ye a tank."

"Thank you, Mr. Scott."

Coupled with the known extinction date of the humpback whale, this gives them a pretty solid date range to aim for with their time warp maneuver. Although they had quite a broad range in mind ("late twentieth century"), Spock's calculations turned out to be reasonably accurate. Given that Perspex® has been readily available from the 1960s, presumably he'd have been just as happy with 1975 as 1999.

"Ready to engage computer, admiral," Spock said.

"What's our target in time?" Jim asked.

"The late twentieth century."

"Surely you can be more specific."

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    Spock probably wanted to avoid the Eugenics Wars of the late 90s. – J Doe Mar 25 at 15:17
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    @JDoe - It's not especially clear how 'hot' those wars were. – Valorum Mar 25 at 15:18
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    @Valorum: 40 nations were under the control of the superhumans and 30 million died in the war. So I'd say quite hot. – Bobby Mar 25 at 20:57
  • @Bobby -Sure, that was the ultimate outcome. The recent comics (not to mention Voyager: Future's End) suggest that the rise of the genetic superman took until the late 1990s at the very least – Valorum Mar 25 at 21:05
  • @Valorum while this answer is well thought out and corresponds to the ideas in the novel, given that they have transporters and therefore replicators which according to the Star Trek universe are very similar technologies (based on matter/energy manipulation), making a sheet of just about anyhting chemical would be trivial especially a simple compound like Perspex. Especially when you think Mr Scott regenerated the dilithium crystals a much more chalenging task. also Perspex is only a trade name for acrylic that's been used much longer than the 70's. – onepound Mar 26 at 12:07
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They had a narrow timeframe that would be useful to them.

  • They needed to arrive late enough that the whale-tank materials they needed to modify the ship would be readily available -- post-1950s. Not just "possible", but in stock locally.
  • They needed an economy robust and entreprenurial enough that their oddball material purchases needs wouldn't pop up any red flags on any government's radar (for instance doing this in the Soviet Union was right out) - so 1950s and up.
  • If they had any problems with the vessel's drive systems (and they did), they needed a modern enough society where technology, electronics, nuclear things, etc. were well-developed and readily available (so 1970s at least, and the later the better!)
  • Whale science needed to be solidly established, with plenty of experts available - because they needed to catch whales, and they had little chance of getting Klingon sensors to tell them the difference between a gray whale and a sperm whale.
    • Even better if culture had become affluent enough that there were aquatic zoos like Sea World, so we're definitely into the 1970s.
  • Preferably, racism (including war sentiments) have declined considerably, so Uhura, Sulu, Chekov and Spock will have an easier time, so 1970s and the later the better. It would also help if the Cold War was abating, so 1980s.
  • Can't go later than 1991, because in Trek canon, the Eugenics Wars began.
  • There are flaws with the answers because: 1. It is is may belief that different calendars are used in different productions and thus there is no guarantee that the Eugenics wars happen in the 1990s AD. 2. Transparent material is not necessary for the whale tank; steel will do. 3. Any local library will have books about whales including information about where Humpback whales can be found. – M. A. Golding Mar 24 at 17:23
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    @M.A.Golding 1 conceded. 2 the limiting factor is not transparent material but an economy robust enough to not notice or ask questions when somebody wants a bunch of steel, plastic, whatever. The Depression and WWII both made that impossible. 3. Reading books is great for executive briefer summaries, but doesn't even begin to replace a real expert with experience and currency when you're actually doing it. – Harper Mar 24 at 18:00
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    Good point with the availability of materials in bulk. Imagine turning up in Napoleon's time and asking for tons of aluminium. Back then, aluminium was such an expensive novelty that reportedly at a royal banquet only the most distinguished guests got aluminium utensils, the others had to be contented with only getting golden ones. – vsz Mar 25 at 7:12
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To transport the whales the crew used transparent aluminium for their makeshift holding tank which they had to "invent" themselves and produce using 1985 technology which wouldn't have existed in 900 AD.

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    They used perspex for the tank. They exchanged the information to produce transparent aluminium to acquire it though. – Jon Clements Mar 24 at 3:55
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    Knowing the location of the whales on the planet might have been part of the reason, where they were before they were released they were easy to find at a known location and close together which must help when beaming them – SpacePhoenix Mar 24 at 7:42
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    This answer is straight-up incorrect. The crew swapped the plans for transparent aluminium for a big sheet of perspex. – Valorum Mar 24 at 8:57
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    @Bobby - Except that Perspex was invented in the 1930s and would have been available in large sheets from the 1950s onwards. – Valorum Mar 24 at 9:42
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    @M.A.Golding it is necessary to write the same comment under the question and under every single answer? Feel free to write a competing answer that you think is better. – Lightness Races in Orbit Mar 25 at 17:32
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I'm not completely familiar with the "time sling-shot" maneuver, but there are multiple possible reasons for that.

Going further back is dangerous

It might be that ~1980 is roughly the earliest year they can safely travel. Traveling any further back might strain the Bird of Prey (also dubbed "rust bucket") too much and you all might die. Any further into the future and you might not have the chance to encounter whales.

Time travel is not an exact science

From what we can see in the movie they never specify to what year they want to go. Quite the opposite, directly after the jump this dialog happens:

Kirk: Picture, please.

[Picture of Earth appears in the viewscreen.]

Kirk: Earth. But when? Spock?

Spock: Judging by the pollution content of the atmosphere, I believe we have arrived at the latter half of the twentieth century.

So they know how to roughly jump back in time, but they can't target a certain year or period.

Time travel is an exact science

In the TOS episode "Tomorrow is yesterday" the crew of the Enterprise learns the mechanics required to jump through time by accident. They complete a maneuver around a black star which hurls them back into the ~1960s on Earth. At the end, they can execute the same maneuver around Sol to travel back to their time.

Unbeknownst to them, this time travel method might have a fixed time span of roughly ~300 years you can jump backwards and forwards with, as the dates neatly line up. Or maybe they knew about it, but it was never established on screen.

Repairs and Extensions

Last but not least, the Bird of Prey is ill equipped to transport whales. That means that you need the resources to remodel it on the fly in the past. That excludes anything before ~1960 I guess, because since then we have made vast advancements in technology. Additionally, Earth is in a pretty bad shape starting from ~1991 with Eugenic War waging and killing tens of million of people, and undoubtedly binding a lot of resources. So you want to land before that, because afterwards you'd either end up in one of three situations:

  • You can't acquire the resources required because they are not available because of the war.
  • Whales are already extinct.
  • The technology has advanced so far that you have a hard time doing your thing.

With these restrictions in mind, the period around ~1980 sounds like quite a good place to be.

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    There are flaws with the answers because: 1. It is is may belief that different calendars are used in different productions and thus there is no guarantee that the Eugenics wars happen in the 1990s AD. 2. Transparent material is not necessary for the whale tank; steel will do. 3. Any local library will have books about whales including information about where Humpback whales can be found. – M. A. Golding Mar 24 at 17:22
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    @M.A.Golding 1. As far as I know, the calendars for anything before Stardates are the same as ours (though, obviously, a different universe). 2. Yes, but you still need big sheets of steel which are readily available for that (for a private person with no questions asked), means of transport of said sheets and tools to put them into the ship. 3. "Found in oceans and seas around the world, humpback whales typically migrate up to 25,000 km (16,000 mi) each year." Great, so we just have to search all the oceans...also, you are aware the in the ST universe these went extinct somewhen around...now. – Bobby Mar 25 at 18:16
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Spock, master of spin

Spock put a convincing "spin" on his plan when he explained it to Kirk, playing down the margin of error. Just as Scotty learned to pad his estimates when Kirk was likely to make unreasonable demands, Spock knew that Kirk had trouble relying on logic for making dispassionate decisions in scary situations.

Based on the Star Trek IV transcript, Spock determined the earliest date from which they could return. He listed fuel components as a limiting resource:

KIRK: What is our target in time?

SPOCK: Late twentieth century.

KIRK: Surely you can be more specific.

SPOCK: Not with this equipment. I have had to programme some of the variables from memory.

KIRK: What are some of the variables?

SPOCK: Availability of fuel components, mass of the vessel through a time continuum, and the probable location of humpback whales, in this case, the Pacific basin.

A trip deeper into the past would have presumably required a more extreme slingshot maneuver with greater strain on fuel components.

As it turned out, they arrived right on target, so Spock's computations were accurate. Spock's plan broke the ship, but he had a way to fix it:

SCOTT: Admiral, we have a serious problem. Would you please come down? It's these Klingon crystals, Admiral. The time-travel drained them. They're giving out. De-crystallising.

KIRK: Give me a round figure, Mister Scott.

SCOTT: Oh, twenty-four hours, give or take, staying cloaked. After that, Admiral, we're visible, ...and dead in the water. In any case, we won't have enough to break out of Earth's gravity, to say nothing of getting back home.

KIRK: I can't believe we've come this far only to be stopped by this! Is there no way to re-crystallise dilithium?

SCOTT: Sorry, sir. We can't even do that in the twenty-third century.

SPOCK: Admiral, there may be a twentieth century possibility.

KIRK: Explain.

SPOCK: If memory serves, there was a dubious flirtation with nuclear fission reactors resulting in toxic side effects. By the beginning of the fusion era, these reactors had been replaced, but at this time, we may be able to find some.

KIRK: I thought you said they were toxic.

SPOCK: We could construct a device to collect their high-energy photons safely. These photons could then be injected into the dilithium chamber, causing crystalline restructure. ...Theoretically.

KIRK: Where would we find these reactors, ...theoretically?

SPOCK: Nuclear power was widely used in naval vessels.

The way Spock immediately offered a solution to their crystal dilemma (the aforementioned "fuel components", it seems), a suspicious person might think that this was part of his original plan.

(I'm tempted to think that Spock simply remembered reading about the Cetacean Institute's humpback whales, couldn't quite recall all of the details due to post-regeneration trauma, and based the entire plan around a hunch.)

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    The film seems to suggest that the search for the whales was going to be rather haphazard, and that the discovery of the cetacean institute, and of George and Gracie therein, was the kind of blind luck that only happens in the movies. The odds of Spock having read about a place several centuries in the past seems even less likely than luckily noticing the billboard. – VBartilucci Mar 25 at 18:00
  • Until I read this answer I didn't notice that there is a massive plot hole with the whole ST series - if they can use a nuclear reactor to recrystallize dilithium (in theory) then why is this never done in the future? and whats the need to mine this material (that cant be replicated) if it can be recrystallized with a simple nuclear reactor? they have force fields so future reactors would pose little risk. Someone should ask that one – Matt Mar 25 at 21:13
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    @Matt This is only a plot hole if de-crystallized dilithium is abundant and it's only scarce in its proper crystalline form. I'm not aware of any canonical indication that this is the case. – Mason Wheeler Mar 25 at 21:37
  • @Matt I've never come across decrystallization of dilithium even being a problem in Star Trek except in this film. Who knows, perhaps the recrystallization process using a fission reactor is only a temporary solution and then the dilithium crystals are permanently damaged? – RobertF Mar 29 at 15:12
  • The crew also had another option for fixing the dilithium: contact the Vulcans (who had warp technology in the 20th century) and ask for help. – RobertF Mar 29 at 15:14

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