Bonanza from a Ringed Planet

Pioneer reports an eleventh moon, more rings and a frigid Titan

Battered and pitted from its encounter with the rings of Saturn, the Pioneer 11 spacecraft headed into deep space last week, its mission accomplished. In its sweep past Saturn, it had provided the best look yet at the solar system's second largest planet, discovered what is probably an eleventh Saturnian moon and two more rings. It also confirmed the existence of another ring and a magnetic field, and dimmed hopes that Titan, Saturn's largest moon, might harbor some form of life.

The computer-enhanced Saturn pho tos taken by Pioneer were far better than any pictures of the planet shot through earthbound telescopes. More details of the famous rings were evident, and for the first time the bands formed by the yellow-and orange-hued clouds enveloping Sat urn could be clearly seen. Still, compared with the spectacular shots of Jupiter and its moons transmitted earlier this year by the twin Voyager spacecraft, the Pioneer pictures were disappointing. The difference is that Pioneer is equipped with a relatively crude camera-like instrument called an imaging photopolarimeter; the Voyagers have far more sophisticated TV-camera systems. But NASA will have an opportunity to try again late next year and in 1981 when Voyagers 1 and 2, respectively, reach Saturn with their superior cameras.

If Pioneer lacked a sharp eye, it made up for that deficiency with its other sensors. Last week, as scientists at NASA'S Ames Research Center near San Francisco skimmed data transmitted during the spacecraft's flyby of Saturn, they made an exciting discovery. While Pioneer was close to Saturn's rings, a detector recording a bombardment by charged particles fell practically silent for twelve seconds, then began registering particles again. Analysis indicated that Pioneer had been briefly shielded from the rain of particles as it flew under a massive object.

Based on these readings and others that showed changes in the surrounding magnetic field, scientists concluded that the spacecraft had passed within about 2,500 km (1,560 miles) of what appears to be a previously undiscovered moon* with a diameter as large as 600 km (370 miles). "The object was very close," says Physicist John Simpson of the University of Chicago. "It could be rocky or composed largely of ice. Either material will effectively block high energy particles." The moonlet, in orbit about 90,000 km (56,000 miles) above Saturn's cloud tops, was nicknamed "Pioneer rock" by the scientists, and it is being officially designated as 1979 S-l (for the first new moon of Saturn discovered this year).

Further study of radiation data revealed that besides the fifth and sixth ("E" and "F") Saturnian rings observed a few days before in Pioneer photos, there was an outermost and tenuous seventh ring as much as 960,000 km (600,000 miles) from the planet. Other facts disclosed by Pioneer's telemetry: Saturn, as expected, has a magnetic field. But it is only 700 times stronger than the earth's, a fifth as intense as scientists had expected. Because this field traps particles radiated from the sun, Saturn has radiation belts that Pioneer detected as it neared the planet. But when the spacecraft passed through the rings on its approach to Saturn, the radiation abruptly ceased-"as though cut off by a guillotine," says Physicist James Van Allen of the University of Iowa. The radiation had apparently been blocked by the icy particles in the rings. Says Van Allen, discoverer of the earth's own radiation belts: "As far as we know, this is the best shielded place in the solar system."

Perhaps the biggest disappointment of the mission was the failure to record atmospheric and surface temperatures on Titan, Saturn's largest moon. Scientists have been particularly fascinated by Titan because it is slightly larger than the planet Mercury and has a thick atmosphere that may be similar to that of the primitive earth. Spectroscopic readings by earthbound astronomers showed, for example, that the atmosphere is rich in methane gas, which is believed to be one of the primary ingredients in the earth's early atmosphere. Given the right temperatures, scientists speculated, some form of life, or at least its precursors, might have evolved on Titan.

But early last week Pioneer scientists announced that interference from solar storms, occurring just as Pioneer was transmitting Titan temperature readings, had obliterated the data. Two days later, NASA explained that signals from a Soviet earth satellite, not solar storms, had. caused the interference (NASA took the blame, explaining that it had failed to notify the Soviets, who would have cooperated by silencing their satellite's radio at the crucial time).

Finally, the Pioneer scientists shamefacedly confessed that they had found in their recordings some Titan temperature data that were partially garbled -- not because of satellite signals but because of interference from solar storms and communications problems between a tracking antenna in Spam and the Ames control center. Still, enough information was retrieved to confirm that the temperature at Titan's cloud tops was a frigid -200DEG C (-328DEG F). That seemed to rule out surface temperatures warm enough to allow the formation of amino acids, the building blocks of life. But scientists were withholding final judgment until the Voyagers get their closeup look at Titan.

At week's end Pioneer was already millions of miles beyond Saturn. Its systems, sustained by a tiny nuclear power source, were still operating; but other than to record an occasional micrometeorite hit, there was little for Pioneer to do. Yet the little spaceship is destined for even greater adventures. Some time in 1993, Pioneer will pass beyond Pluto, leave the solar system and head for the stars.

* Because the orbits of Saturn's moons are not precisely defined, some scientists think that the object could be one of the planet's ten known moons.

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