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Mariner also looked for two near-Earth phenomena. It failed to find any evidence of the giant tail of Earth's magnetic field that is supposed to stretch thousands of miles out into space. In another experiment, Mariner measured the shock wave caused by solar pressure against Earth's magnetic field. The wave turned up three times at distances of 138,000 to 154,000 miles from Earth. This indicated, the scientists concluded, that the magnetic field around Earth is constantly expanding and contracting.
Turning Heart. Still, for many tense hours last week, the overriding question at JPL control was not Mariner's confirmed scientific coups but what it had done during the photo sequence. The suspense ended when Mariner broke its silence on schedule and began playing back the bits of digital code representing the pictures it had taken the day before. "When I saw that little printout tape and knew we had a picture," says Caltech's Dr. Robert B. Leighton, chief experimenter of the picture team, "my heart turned around."
His emotion was understandable. That printout tape, with its endless rows of digits, told the men who could read it that Mariner seemed to be obeying the intricate orders built into it so many months before. According to plan, shortly after the scanning mechanisms sighted the planet, automatically activating the photo system, the six-inch vidicon tube focused through a reflecting telescope and took its first picture. It was programmed to take one picture every 48 seconds. Each picture was made up of 200 lines—compared with 525 lines on commercial TV screens. And each line was made up of 200 dots. The pictures were held on the tube for 25 seconds while they were scanned by an electron beam that responded to the light intensity of each dot. This was translated into a numerical code with shadings running from zero for white to 63 for deepest black.
The dot numbers were recorded in the binary code of ones and zeros, the language of computers. Thus white (0) was 000000, black (63) showed up as 111111. Each picture—actually 40,000 tiny dots encoded in 240,000 bits of binary code—was stored on magnetic tape for transmission to Earth after Mariner had passed Mars. More complex in some respects than the direct transmission of video data that brought pictures back from the moon, the computer code was necessary to get information accurately all the way from Mars to Earth.
Because of the great distance and the craft's weak 10½-watt radio transmitter, it took 8 hr. 35 min. to transmit the coded data that made up one picture. And by the time the signals reached a tracking station, they were no stronger than one-billionth of one-billionth of a watt. Those faint whispers were picked up by big-dish antennas and amplified a thousand times as they were piped through a liquid helium maser. So slow was the transmission rate that no complete picture could be received at any one tracking station. As the Earth's rotation carried one station out of range, another moved into position to collect the rest of the message.