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'The Colonization Application' - Season 8, Episode 17
As Sheldon and Amy choose to buy a pet turtle, Amy is angered to learn that Sheldon has applied for a one-way mission to colonize Mars. Meanwhile, Leonard and Penny take a step forward in the bedroom when Leonard makes an adult purchase, while Raj worries about Emily's reaction when she finds out he's been snooping in her apartment.
Air Date: March 5, 2015.