On February 23, 1978, during a series of movements or slews, Voyager 1’s scan platform slowed and stopped before completing the maneuver. This failure caused a great deal of concern, since the scan platform houses the optical instruments that are crucial to the observation of the Jovian system—the ultraviolet spectrometer, the IRIS, the photopolarimeter, and the two TV cameras. At JPL, tests were run on a proof-test model—an exact copy of the Voyager spacecraft—to try to find out why Voyager 1’s scan platform had become stuck. On March 17, Voyager 1’s scan platform was tested—JPL engineers instructed the platform to move slowly for a short distance, and Voyager responded as ordered. Further tests were conducted on March 23. This time the scan platform was ordered to execute a sequence of four slews, moving away from the part of the sky where the original failure had occurred and ending with the position that it would be most useful to leave the platform in—just in case the platform should become stuck again. On April 4 the scan platform was commanded to perform a sequence of 38 slews, and fifty more slews were performed on April 5. All were successful. Yet engineers were still hesitant to force the platform to move through the region where it had originally stuck, and extensive discussions were held to determine if the Jupiter observations could be carried out without risking a return to the danger area. It was argued, however, that full mobility of the scan platform really was required, and on May 31 commands were sent to maneuver the scan platform through the danger region. It moved normally: The scan platform was operating properly again. After additional slewing tests were run in mid-June, the scan platform was pronounced fit for operation. Engineers suspected that the material caught in the platform gears must have been crushed or moved out of the way by the continued slewing, allowing the platform to move once more. [Illustration: Voyager 1 was launched on September 5, 1977. The launch was delayed 5 days to make last-minute adjustments to avoid the postlaunch difficulties experienced by Voyager 2. [P-19480AC]]