The COmet Nucleus TOUR was a NASA Discovery-class space probe that failed shortly after its July 2002 launch. It had as its primary objective close flybys of two comet nuclei with the possibility of a flyby of a third known comet or an as-yet-undiscovered comet.
The two comets scheduled to be visited were Encke and Schwassmann-Wachmann-3, and the third target was d'Arrest. It was hoped that a new comet would have been discovered in the inner Solar System between 2006 and 2008, in which case the spacecraft trajectory would have been changed if possible to rendezvous with the new comet. Scientific objectives included imaging the nuclei at resolutions of up to 4 meters, performing spectral mapping of the nuclei at resolutions of up to 100 meters, and obtaining detailed compositional data on gas and dust in the near-nucleus environment, with the goal of improving knowledge of the characteristics of comet nuclei.
After the solid rocket motor intended to inject the spacecraft into solar orbit was ignited on August 15, 2002, contact with the probe could not be re-established.