In April 2151, the Enterprise (NX-01) followed a Suliban cell-ship into the atmosphere of a gas giant after following its warp trail. (ENT episode & novelization: Broken Bow) Shortly after entering an unnamed star system near the Klingon Neutral Zone, Lieutenant Gary Mitchell aboard the USS Constitution detected an ion trail in orbit of one of the system's three planets. After following the trail, the Constitution was met by a Klingon D7-class battle cruiser, the IKS Stormwind. (TOS - My Brother's Keeper novel: Constitution)