Because of the earlier discovery of a thick atmosphere on Titan , the Voyager controllers at the Jet Propulsion Laboratory elected for Voyager 1 to make a close approach of Titan and terminate its Grand Tour . Because Pioneer 11 had one year earlier detected a thick , gaseous atmosphere over Titan , the Voyager space probes ' controllers at the Jet Propulsion Laboratory elected for Voyager 1 to make a close approach of Titan , and of necessity end its Grand Tour there .