The Comet Nucleus Tour (CONTOUR), a NASA Discovery mission will visit and study at least two comets.

CONTOUR’s two baseline targets, the comets Encke and Schwassmann–Wachmann 3. Encke, CONTOUR’s first target, Encke, has the shortest orbital period of any known comet. CONTOUR will visit Encke in November 2003. CONTOUR plans to fly by Schwassmann–Wachmann 3 in June 2006. The CONTOUR mission uses Earth-gravity assist maneuvers to accomplish the multiple encounters. Three Earth gravity-assist maneuvers are used to accomplish the two comet encounters over a 3-year period

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The sequence begins with a 1-year Earth-return loop that positions the spacecraft for an encounter with Encke in November 2003. Three Earth-swingby maneuvers are then used to retarget the spacecraft for an encounter with Schwassmann Wachmann-3 in June 2006.

CONTOUR will fly by each comet at the peak of its activity, close to the Sun. During each encounter, the target comet will also be well situated in the night sky for astronomers worldwide to make concurrent observations from the ground. Protected by its dust shield, the CONTOUR spacecraft will fly by each comet at a distance of about 100 km (54 nmi). The most intensive data taking will occur within a day or so of each encounter.

The spacecraft design is simple. CONTOUR has few articulated mechanisms; the solar array is body mounted and does not require drive motors. The mission geometry allows CONTOUR to use fixed, passive, existing antenna designs. With the exception of a long-track pointing of CRISP, all instrument pointing and antenna pointing is controlled by moving the spacecraft. A dust shield made of Nextel and Kelvar protects against impacts for the dust sizes and densities expected at the three encounters.

The mass spectrometer and dust analyzer will be mounted to look in the direction of flight at closest approach. NGIMS will be provided by NASA Goddard Space Flight Center. It will be placed on the spacecraft within the dust shield as far forward as possible

CONTOUR's mission design is flexible so that the spacecraft can be retargeted to intercept an unexpected comet. The appearance of such a comet cannot be predicted in time to plan a space mission, but CONTOUR can take advantage of the opportunity if a "new" comet passes close enough to Earth's orbit. To identify suitable candidates for a CONTOUR intercept as early as possible, the CONTOUR project supports a worldwide early warning search program for comets approaching from the edge of the solar system.