When Voyager II flew across the rings of Saturn Tuesday night, it may have sailed through the interplanetary equivalent of a sand storm.
That was the assessment of some of the scientists at the Jet Propulsion Laboratory where the flight of Voyager II is being directed. Some of these same scientists have concluded that it was the hypersonic impact of microscopic dust particles that jammed the spacecraft's camera platform, which was freed up today but was still not working well enough to turn Voyager's cameras back on.
"Our principal objective is to recover the use of this platform for our voyage to Uranus," Voyager project scientist Edward C. Stone said. "We do not want to take any risks that imperil that mission."
Now on course to Uranus, the seventh planet out from the sun and one never before visited by a spacecraft from Earth, Voyager II will not arrive there until January, 1986. But scientists want to make sure the spacecraft's two cameras and three telescopes are in perfect working order during the trip out. The cameras were turned off after the platform jammed and Voyager scientists say they will not turn them back on until they are assured that the platform is also in perfect working order.
Keeping the cameras turned off means that as many as 600 pictures the scientists had planned to take of the dark side of Saturn and its rings will be lost. Countless infrared and ultraviolet measurements of the planet's clouds and seething atmosphere will also be lost.
Stone said the lost pictures do not mean the mission is a failure or even a disappointment because the 17,000 spectacular pictures the spacecraft took from the sunlit side of the planet have turned out so flawlessly. Said Stone: "I would say this mission has been a 200 percent success."
The electrically-driven camera platform, which had been jammed for 17 hours on Tuesday, was working once again but far from perfectly. Voyager project director Esker K. Davis said the platform was moved in low gear today but that it moved slower than it should have, responding sluggishly and erraticaly to commands.
"The thing we're after is to restore reliable operations so that we can match up the camera shutters with the appearance of a particular object in the lens," Davis said. "We can't say that we've moved the platform with that kind of reliance."
The plan is to keep driving the platform in low gear through larger and larger swings on its circular access, like pushing a truck up a hill in first gear. This is a laborious and time-consuming process that involves three hours just sending one command and waiting for the return signal that the spacecraft has responded. Even at the speed of light, it takes a signal one house and 26 minutes to travel the almost 1 billion miles from Earth to Saturn.
One theory of why Voyager's camera platform jammed is that there are microscopic dust particles in at least some of its gear boxes, impeding their motions. Evidence is mounting that the spacecraft was hit by hundreds of microscopic dust particles when it passed by Saturn's rings, a crossing that took 48 minutes at a speed of 54,000 miles an hours.
On crossing the rings, an instrument aboard the spacecraft that measures voltages across two antennas on the spacecraft became saturated by electrical discharges. For as long as 48 seconds at the time when the spacecraft was closet to the rings, the instrument measured spurts of electrical discharges up to 30,000 hertz, which means there were voltages traveling back and forth between the two antennas at 30,000 times a second.
"The most likely explanation for this is that the spacecraft was being impacted by ionized [electrified] remnants of dust particles," said Dr. Fred Scarf of TRW, which built the instrument, "causing electrical discharging at the radio antennas."
Despite the fact that Voyager II is no longer taking pictures, the thousands of photographs it had taken and stored on its tape recorders were still pouring back to earth today, showing sights of other worlds that had never been seen before. One picture of a brown and white cloud pattern moving across the surface of Saturn suggested a storm system at about 47 degrees north latitude on the planet which would be the equivalent of Boston in the United States.
"We think it's raining at that point on Saturn, raining ammonia of course," said Dr. Garry Hunt of University College, London. "Don't leave your car out."
One sign that the mission has been a "200 percent success" is that the telecsope that was pointed through the rings at a distant star to catch the star's light blinking on and off as Voyager traveled through the rings was a "superb success."
The Jet Propulsion Laboratory's Dr. Arthur L. Lane said it may take him years to draw up a map of the rings of Saturn, 45,000 miles of which were mapped by his telescope. Lane said the experiment will reveal details of the rings and gaps between the rings no winder than a city block.