Remember when they found 00's Cartographer? He says that they implemented security protocols or something and that it initiated some kind of barrier, then cuts off. The planet was the core of the Ark, and the barrier a shield around the core - hence the shield world reference.

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When cities burn and armies turn and flee in disarray,
Cowards will cry "tis best to fly and fight another day."
But warriors know it, in their marrow when they die and fall:
It is better to have fought and lost then ne'er have fought at all.