Optical Navigation for the STARDUST Wild 2 Encounter

On January 2, 2004, the STARDUST spacecraft flew by the short period comet P/Wild 2 at a distance of 237 km. The primary goal of the flyby was to collect samples of the coma and return them to Earth on January, 2006. An additional goal was to shutter images of the nucleus during the flyby. In order to meet these goals, the spacecraft had to be guided to a flyby about 250 km from the center of the nucleus with an accuracy of better than 50 km. This was accomplished by the use of standard radio navigation techniques, augmented by optical images of the comet using the onboard camera. This paper describes the optical navigation techniques used on approach to target the desired encounter conditions. The optical navigation phase of the mission began about a month prior to encounter when the first images of the comet were seen. Because of the dimness of the comet, plus unexpected problems with the camera due to contamination of the optics, individual frames could not be properly processed to get good navigation data. Techniques such as multiple frame co-addition were employed to boost the signal. The data was used to design a series of maneuvers to guide the spacecraft to its flyby target. At about 7 days prior to encounter, efforts to clean the camera paid off, and better signal to noise ratios were achieved. The final images for targeting were taken roughly 14 hours prior to encounter. These images indicated the previous targeting maneuver done at 2 days prior to encounter achieved the desired accuracy. The final flyby reconstruction showed the accuracy to have been better than 10 km.