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Alarm in Analysis. On encounter day—July 14—JPL technicians arrived at their control center at dawn. They were filled with nagging doubts. The TV and tape-recording equipment had not been tested since February, and if something did go wrong, there would be no time to correct it because it would take 24 minutes for radio signals traveling at the speed of light to make the round trip between Mariner and the control station. Just to be on the safe side, JPL control sent a series of four last-minute direct commands to back up the programmed instruction. It was the first time the lab had talked to its ship in five months, and Mariner answered like a good boy.
The first command was sent when Mariner was still 107,000 miles away from Mars. This turned on the camera's shutter mechanism, started the scan platform searching with a wide-angle sensor for light from Mars, and turned on the tape recorder's power. Everything was going unbelievably well. Newsmen and families of the scientists gathered in JPL's Von Kármán Auditorium to await the cryptic reports from the primary tracking stations at Johannesburg in South Africa, Woomera in Australia and Goldstone in California.
At 4:55 p.m., P.D.T., the wide-angle sensor detected the edge of Mars. Twenty-three minutes later, the narrow-angle sensor also picked up Mars. Presumably, the picture-taking sequence had begun. At 5:30 p.m., Jack James, Assistant Deputy Director of JPL in charge of lunar and planetary projects, grinned broadly as he received a report by telephone. Goldstone, he told newsmen, had just verified that the tape recorder was running. The chances of getting pictures were excellent. Mariner's cheering section broke out in applause.
The elation soon turned to despair. JPL control began receiving conflicting signals about the performance of the two-track, continuous-loop tape recorder. "There is alarm in the analysis team," James announced. The signals hinted that something was wrong with the recorder's stop mechanism. Quite possibly it was not cutting off for a 24-second interval between each picture. If so, the tape would have run through its two tracks twice as fast as it should have; it would have recorded only half of the 21 pictures. The confusion was compounded when a disembodied voice over the intercom announced: "All indications are that all was normal during the recording sequence."
Thin & Dusty. For hours no one would know for sure. While the JPL crew waited anxiously, Mariner swooped around the back side of Mars. It was out of touch with Earth for 54 minutes. During this maneuver, it performed one additional and highly important experiment. Mariner beamed radio signals back to Earth through the atmosphere of Mars. By examining the changes in amplitude and frequency of the radio waves as they arrived on Earth, scientists hoped to get a better idea of what Martian atmosphere was like.