Kukulkan's probe was an automated spacecraft that was sent out by Kukulkan in 2270. It approached the Federation home worlds, made a scan of Earth's system, and then sent a signal out into space. Before it could be intercepted, it self-destructed. The USS Enterprise followed the trail of disrupted matter left by the probe's highly advanced propulsion system. (TAS: "How Sharper Than a Serpent's Tooth")