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Intercept 2 is the callsign of the Viper initially assigned to Serina.
However, due to Lieutenant Starbuck's stunt in taking Intercept 1, which was assigned to Captain Apollo, Apollo takes Intercept 2 and pursues.
Intercept 2 later aids Starbuck in keeping a fix back to the Fleet by keeping a fix on Intercept 3, piloted by Serina, who in turn keeps her fix on Galactica. However, Intercept 2 believes that Starbuck becomes forever lost when all transmissions from Starbuck break up (TOS: "Lost Planet of the Gods, Part II").