Why did ONLY Janeway need to give her codes but on the Enterprise (D and E), and Defiant the Captain and a Bridge Officer needed to put theres in as well.
Picard also had that authority in the TNG films. Starship captains carry an enormous amount of authority and power, so such a function might by at the captain's discretion. Or it might relate to the class of ship, or the mission of the ship. Or it could simply be that the first officer died, Tuvok was gone, and she never gave Tuvok or Chakotay that responsibility when they joined Voyager.
Maybe because the original crewmembers with authorization died. And Janeway didn't trust the Maquis enough
If memory serves me, Worf and Dr. Crusher had to put their voiceprint in to get the self-destruct sequence activated.
You're right. I forgot that they do that in First Contact. I was thinking of Nemesis, but it may have been the same case if it wasn't "Offline."
One would expect it to fall to the two or most three senior crew alive. After all if the Captain is killed in an attack you might still need a way to activate the Self-destruct. But I gues it's a good thing the Janeway doesn't talk in her sleep. And accidently give the self-destruct sequence.
Wouldn't be too surprised if Janeway did a little reprogramming so that her sole authorization would be sufficient
That would be a terrible idea. If I were Janeway, I'd deactivate the ECH & the EMH, reset the app, and only use it when necessary.
Idea of original requirement is in case the captain goes insane or something, but sometimes having two people is inconvenient for storytelling.
I could see the self-destruct system being "hard-coded" in some manner, and with Cavit dead, the system would only recognize Janeway as an authorized user. Plus since Voyager is a much smaller vessel than the Enterprise and less militarily-capable vessel than Defiant (I know, I know...) perhaps it wasn't felt that it needed as many safeguards.
Janeway: Computer, start autodestruct sequence, authorization Janeway Pi-one-one-zero. Set it at twenty minutes. Computer: First officer Chakotay, do you concur? Janeway: Computer, do you question the Power Bun? <Computer is silent for about 5 seconds> Computer: Warning, self-destruct sequence has been initiated. Warp core overload in twenty minutes.
That reminds me of: Warning. Warp core breach in 30 seconds ... COMPUTER: Warning. Warp core breach a lot sooner than you think. TORRES: We've got to get someone inside the warp core. Try to eject it manually. SEVEN: No one could survive the current level of plasma radiation. BOTH: The Doctor. EMH: This isn't real, is it? KIM: Doc, you're the only one who can help us! COMPUTER: Warning. Last chance to be a hero, Doctor. Get going. Was there any self destructs in the later seasons?
I suspected that the reason we never see auto-destruct used in the same way on two different ships is because Starfleet intentionally made it that way. It would be harder for an enemy to trigger or disable self-destruct if they don't know how it works from one ship to the next...
Recall her first officer was killed in the caretaker incident. Chakotay was promoted on the basis of unifying the two crews and keeping the Maquis content. It makes sense she wouldn't want Chakotay to have access to the self destruct sequence-he if he proved to be disloyal she wouldn't want him using the self destruct protocol as leverage. To be sure he proved very loyal and extremely unlikely to mutiny but Janeway wasn't going to share control of perhaps the most important and solemn protocol of a Starfleet captain. And I doubt that bothered Chakotay.
In that case, could she have entrusted it to Tuvok, the third man on the ship, whom she (usually) trusted implicitly? Or would that just have been impossible?
Or she did trust him, but Tuvok would see it as a security risk to allow Chakotay access to the auto-destruct.