Flying out of the sunlight behind the rings of Saturn and into the dark today, Voyager II suddenly suffered a crippling blow that forced flight controllers to turn off its cameras and telescopes.
As it hurtled blindly away from Saturn, its cameras pointed uselessly into space, scientists and technicians at the flight control center here worked to correct the mysterious malfunction but said it had done little harm to Voyager's historic mission.
Whatever hapened to the silver-and-black spacecraft was still unexplained tonight but it was enough to immobilize the spacecraft's camera platform so that it could not be moved from side to side for almost 17 hours. At 9 p.m. (EDT), flight contollers sent up a command to force the platform into low gear, and were able to force it to move slightly in a direction opposite to the one it had been moving in when it jammed. While nowhere near convinced they had unjammed the camera platform flight contollers were cautiously optomistic that they could get it to resume at least partial operation on Thursday.
"Our hope of quickly zeroing in on this problem was a bit optimistic," Esker W. Davis, Voyager project director, said tonight at the Jet Propulsion Laboratory. "This may take a while."
Whatever the cause, flight controllers turned off the craft's five cameras and telescopes so they would not be blinded by the sun if they opened their shutters in the sun's light and could not be moved out of the glare. They also canceled an experiment that did not involve the cameras but would have turned the craft too close to the sun.
Flight contollers speculated that the 1,800-pound spacecraft may have been hit by dust particles drifting out of the rings when Voyager crossed them late Tuesday. At the time the spacecraft crossed the rings, Davis said, an instrument that detects electrified particles around the craft suddenly "went crazy."
The the same time, the tiny gas thrustres that control the up-and-down pitch of the craft fired 86 short bursts to keep Voyager on an even keel. Said Davis: "This implied a force on the spacecraft as if something was hitting it. The thrusters fired in response to this force in an attempt to damp out whatever force this may have been."
Davis said that the craft's telephoto camera missed three out of five pictures of small regions of two Saturnian moons it tried to photograph, and that wide-angle lens caught only the edge of one moon it attempted to photograph full-face.
"This happened right around the time of the ring-crossing," Davis said. "There was something wrong with the pointing mechanism, which may have been because the spacecraft was hit with something."
Early tonight, flight directors sent a command to the craft to push the camera platform into low gear in an attempt to unjam the cameras. At first, the craft computer refused to accept the command. Flight directosr guessed correctly that the radio receiver on Voyager was not hearing the commands because they were being sent at a frequency a little off-center from the frequency to which the receiver was tuned.
The command was sent again at 9 p.m. (EDT) at a different frequency; the receiver heard it, the computer accepted it, and the camera platform was nudged out of the jammed position it had been in for 17 hours.
Engineers emphasized that moving the camera platform 10 degrees did not mean they had freed the platform completely from whatever had caused it to jam. A second test using the same low gear driving mechanism was to take place later to see if the platform could be rotated freely in both directions.
Flight directors emphasized that the spacecraft will continue on to Uranus, though it might have to navigate the 1.4 billion miles to get there in a way that would minimize its use of fuel. If flight directors manage to unjam the camera platform completely, they will have no problem navigating all the way to Uranus without any concern about fuel.
The disabling blow that came without warning early this morning dealt scientists their first setback on what had otherwise been a flawless flight over the rings of Saturn Tuesday night.
When the camera platform jammed, Voyager's cameras lost scores of pictures they were to take of the exotic moons of Enceladus and Tethyus, of the dark side of the rings and the unlit portion of the planet where scientists had hoped to see lightening strikes in Saturn's seething atmosphere. The longer the camera platform is jammed in the next few days, the more dark side pictures the camera will fail to take on the spacecraft's way out from Saturn.
"We were fortunate that whatever happened did not happen sooner," Voyager Project scientist Edward C. Stone said. "We accomplished most of what we set out to do."
On learning early this morning that Voyager's cameras had been immobilized, engineers first feared they might have to burn more fuel than necessary to complete the mission at Saturn and put the craft on course to Uranus. However, turning the two cameras off cuts fuel consumption until they're turned back on because there is no reason to maneuver the craft to aim the cameras.
If the camera platform stays jammed all the way out to Uranus, the spacecraft can still be navigated and flown by burning fuel to roll and pitch the entire 1,800-pound spacecraft to fix on different stars on its journey to Uranus. The three telescopes in the camera platform are used to navigate by starlight and by turning the whole spacecraft the telescopes can direct it through space.
The spacecraft has 155 pounds of fuel in its tanks right now, 50 of which must be used to cruise from Satrun to Uranus and from Uranus on to Neptune, which Voyager will reach in 1989. A portion of the same 50 pounds is required to maneuver the spacecraft when it flies by Uranus and Neptune.
Ninety pounds of fuel is needed to make mid-course correction maneuvers between Saturn and Uranus anbd betwen Uranus and Neptune to make sure the spacecraft comes as close to the two planets as it safely can. That leaves 15 pounds of fuel as spare, which could be used in small bursts of one-tenth of a pound each to move the spacecraft around if the camera platform stays jammed all the way to Uranus. One-tenth of a pound is about as much as an airline passenger gets in a miniature cocktail or whiskey bottle.
Though preoccupied with whatever jammed the camera platform, scientists were delighted with the pictures the cameras had sent back, of the sunlit side of the planet.