Because the switching of the radio receivers was still controlled by the special protection sequence discussed earlier, flight engineers would have to wait for seven days—until April 13—before they could attempt communication with the spacecraft again. During that week special procedures were established and rehearsed so that commands could be sent to Voyager in the short time that the backup receiver would be on. On Thursday, April 13, 1978, the seven days were up and the spacecraft should have shifted from the dead main receiver to the sick backup system. There was just a twelve-hour “window” in which to restore communication. At about 3:30 a.m. PST the Madrid tracking station of the Deep Space Network sent its first order to the spacecraft, approximately 474 million kilometers away. Almost an hour later, word arrived from Voyager that the command had been accepted. (One-way light time for a signal to travel the distance from Earth to Voyager at that time was almost 27 minutes.) Elated flight controllers went ahead and transmitted nine hours of commands to the spacecraft. Voyager 2 was successfully commanded again on April 18 and April 26. The April 26 commands included a course change maneuver that was executed properly on May 3. On June 23, Voyager 2 was programmed for a backup automatic mission at Saturn in the event that the secondary radio receiver should also fail. These backup mission instructions would operate all the science experiments, but only a minimum amount of data would be returned, since the scan platform would only be programmed to move through three positions rather than thousands as it would in normal operation. Instructions for a backup minimum automatic encounter at Jupiter were transmitted to Voyager 2 in two segments, the second of these on October 12, 1978. With the backup instructions recorded on board the spacecraft, Voyager personnel felt their fears partially allayed. If Voyager 2’s secondary radio receiver failed, the spacecraft would still obtain some science data at Jupiter and Saturn. But that would mean that there would be no mission beyond Saturn; our first opportunity to explore Uranus, its satellites, its newly discovered ring system, and possibly even to get a look at Neptune, would not come in this century. Another major concern affecting both Voyager spacecraft was the proper management of hydrazine fuel reserves. Hydrazine is used by the thrusters on the Voyagers for stabilization of the spacecraft and for trajectory correction maneuvers (TCM). Each Voyager was loaded with 105 kilograms of hydrazine budgeted for use on the long flight to Jupiter, Saturn, and beyond. Because of the excellent performance of the launch rockets, both Voyagers required less hydrazine than anticipated for their final boost into proper trajectory toward Jupiter, and at first it looked as though both spacecraft would have plenty of propellant to spare.