RENDEZVOUS WITH THE RED PLANET

FOR five months the little spacecraft sped through the interplanetary void.

Last week, on the 157th day of an arcing, 242-million-mile journey across the solar system Mariner 6 reached its destination. In the closest approach to Mars ever achieved by a man-made object, the U.S. spacecraft flew within 2,130 miles of earth's planetary neighbor.

On command from its computer, Mariner 6's electronic gear poured out an endless stream of data from the red planet--information about the density and composition of its atmosphere and its varying surface temperatures. On board the ungainly, 850-lb. ship, whose four solar panels gave it the look of a stubby windmill, tiny transmitters also sent back to earth, some 60 million miles away, the best close-up portrait man has ever had of Mars. At week's end, an identical twin named Mariner 7 moved into position for similar electronic observations. Mariner 6 aimed its close-up cameras on Martian equatorial regions, Mariner 7 at the planet's south polar area. Together they were programmed to photograph about 20% of the Martian surface.

Canals and Craters

Despite ingenious techniques of optical and radio telescopy, astronomers have piled up more questions about Mars than answers. What is the true nature of those strange seasonal dark-enings? Are the white polar caps composed of frozen water or, as many astronomers believe, dry ice (frozen carbon dioxide)? Do those long controversial "canals" really exist, or are they optical illusions? The 1965 flight of Mariner 4 showed that Mars is pocked by moonlike craters, apparently as a result of meteor bombardment. But the unmanned probe did not determine whether Mars can support anything remotely like earthly life.

Seeking to answer that age-old puzzle, scientists from Caltech's Jet Propulsion Laboratory at Pasadena crammed Mariners 6 and 7 with vastly improved electronic gear, ranging from tape recorders to miniature computers to extraordinarily sensitive infra-red and ultraviolet measuring equipment. The total cost of the Mariners and their launch vehicles: $148 million.

Though Mariner 6 needed only one mid-course correction throughout its long journey, and Mariner 7 was almost on target, the flights were not completely trouble-free. Last week one of Mariner 6's infra-red spectrometers balked just as it was supposed to search out the gases and vapors in the Martian atmosphere. JPL technicians explained that the spectrometer, which should be cooled to below -- 400DEG F. to operate efficiently, refused to chill at all. Mariner 7 caused even greater concern at Mission Control when it went off the air entirely for seven hours. Apparently struck by a tiny meteoroid, the spacecraft lost its fix on the star Canopus and its directional antenna spun away from earth. A new roll-and-search command went up from Pasadena. Mariner 7 obeyed, and though performing at less than capacity, its radio functioned again.

Both probes worked at incredibly high speeds. Once the Martian image was captured by their video equipment, it was rapidly translated into electronic signals of varying intensity representing 64 shades of light and dark. Those impulses, in turn, were beamed to earth in the binary language of the computer. To make one complete picture, the spacecrafts' equipment had to scan 665,208 points of light and dark, each of which was converted into six bits of computer information. That five-minute job involved more than 4,000,000 bits for each picture. The poky equipment on Mariner 4 needed 81 hours to make a less detailed picture. After being picked up by NASA's huge, 210-ft. Goldstone radio "ear" in the Mojave Desert, the signals were translated by a Univac 1219 computer back into light and shadow, line by vertical line.

The first pictures arrived from Mariner 6 when it was still 771,500 miles from the Martian surface. But by the time the spacecraft's cameras finished their day's work, they had recorded 33 pictures and brought earthly viewers within 453,350 miles of the red planet. None of the initial photographs were particularly startling. But Caltech's Robert B. Leighton, director of the photographic work, noted that at least one picture showed a ragged edge at the south polar cap--"possibly caused by the presence of mountains or craters."

Bleak and Forbidding

The following day, Mariner 6 produced 17 more Martian photographs from as close as 111,400 miles. Far more stunning than the earlier series, these pictures brought out what seemed like cloud formations near the south pole, verified the presence of numerous craters, and revealed shadings in what had previously been thought to be an all-white polar cap. Leighton spotted at least two features that he thought might be "canals," although he speculated that they might have been caused by electronic noise.

As expected, the final 24 photographs that were shot just before Mariner 6 ducked behind Mars were the most dramatic. Taken from as close as 2,170 miles, they showed a bleak, forbidding landscape, unmistakably similar to the harsh terrain of the moon. "I've seen so many craters I'd hesitate to count them," said Leighton. "We expected craters, but not in such fantastic number."

Mariner 6's final photographs did not show any signs of life--but JPL scientists had already warned that even at the spacecraft's relatively close distances, vegetation would be all but unobservable. The two Mariners, moreover, were designed only to determine whether Mars could support life. At week's end, investigators were already mulling over two important observations. Mariner 6 had failed to detect any nitrogen --an ingredient of all earthly life --but it found signs of water in the form of ice in the Martian atmosphere or on the surface.

More definitive findings are expected in 1971, when the U.S. plans to send two more Mariners into 90-day orbits around the planet, and in 1973, when another two American spacecraft are scheduled to land on Mars itself.

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