Likely failsafe

As the commander of such a vessel, he would, perforce, be the most powerful man in the galaxy. The thought had certainly occurred to him that not even the Emperor could stand before him, did he choose to challenge Palpatine's rule. Then again, Tarkin knew the Emperor. If their positions were reversed, he knew that there was no possible way he would sanction anyone having such power-not without some kind of fail-safe. Was a destruct device already built into the station somewhere, with the red button safely installed in Palpatine's chambers? Was some equivalent of Order 66 known only to certain onboard officers and troops? Or was it something even more devious? Tarkin was certain the Emperor had some kind of insurance against any theoretical rebellion. Not that the Grand Moff intended such a course; he wasn't a foolish or suicidal man.

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Star Wars: Death Star Part 1 Chapter 2