The Hubble Space Telescope's continuing jitter is proving unexpectedly complicated to fix, and the European-built solar panels that are causing it will have to be repaired or replaced by spacewalking shuttle astronauts, according to Hubble engineers.
The flutter is apparently caused when the spacecraft reacts to abrupt temperature changes that occur each time the $1.5 billion orbiting observatory passes between shadow and sunlight. It has been the Hubble's most serious problem other than the major manufacturing flaw discovered in its primary mirror in June.
The motion is so slight that it would not matter in most spacecraft whose work does not require the precision of the Hubble's.
The telescope has already begun to produce significant scientific data and images of the heavens, although it is still in its check-out phase, and scientists have been able to work around the jitter problem. But if it continues indefinitely, officials say, it will reduce the telescope's precious observation time and strain resources on the ground trying to compensate for it.
For six to 10 minutes out of each 96-minute orbit, the booms that support the telescope's 40-foot solar "wings" bend as temperatures rise or fall by 82 degrees Fahrenheit within 57 seconds, according to Joseph Rothenberg, associate director of Hubble flight projects. This sets up a reaction in the telescope's supersensitive controls.
"The thing that causes the spacecraft to jitter is its own control system sensing errors and correcting," he said.
This flutter in turn sets up a second vibration that occurs randomly for a minute or two at a time over periods of up to 20 minutes in sunlight.
The solar arrays were built for the European Space Agency (ESA) by British Aerospace Space Systems Ltd. in Bristol, England, with subsystems from Germany, Switzerland and elsewhere in Europe and were based on a design purchased from Hughes Aircraft Co., according to Robin Laurance, ESA's project manager for the Hubble.
"The main fault is in the analysis," he said in a telephone interview from Noordwijk, the Netherlands. "We didn't predict correctly the rate at which the boom bends."
The bending was expected to occur slowly, over several minutes, rather than within seconds, he said.
Engineers "still haven't got to the bottom of what's really happening. . . . We're studying whether the boom-bending can trigger a motion in the drum," Laurance said.
The "drum" contains a spring-loaded device that can furl and unfurl the solar arrays like giant window shades. Its reaction, or that of spreader bars that support the "shades," or both, may be the cause of the second jitter.
Engineers have said the telescope was designed with a pointing system so precise that if it were a laser mounted on the Capitol and fired at New York City, it could hit a dime on top of the World Trade Center. The vibrations caused by the solar panel wiggle the end of the telescope, at most, only 22/100,000ths of an inch, but that means the bull's-eye at 200 miles expands from a dime to a 10-inch pizza. The problem is even greater over vast astronomical distances.
To neutralize the first jitter, controllers at Goddard Space Flight Center in Greenbelt last month sent up new computer instructions that would tell the control system not to react to the bowing of the booms.
The new instructions "worked too well," however, according to Edward Weiler, chief National Aeronautics and Space Administration scientist for the Hubble. "We fixed the first problem so well, it made the spacecraft more sensitive to the second problem."
The instructions have been shut off again while an additional fix is developed. New instructions are not expected to be ready to send to the telescope's computer until at least mid-February. However, even if the instructions eventually fix the flutters, Rothenberg said, they would use up all of the on-board computer capacity and render it too busy to compensate for any unexpected operational problems. "It mortgages our future," he said.
For this reason, ESA engineers, in coordination with the U.S. team, are working on a redesigned set of solar panels that could be carried aloft by a shuttle in 1993. The new design includes thermal covers to prevent the booms from bending, Laurance said. They are also working on ways to repair the existing set.
"We'll weigh the risks of replacement versus fixing this one, but in any case we'll have a spare aboard the shuttle," Rothenberg said.
The advanced-technology aspects of the solar arrays are working, Weiler said. "They're producing power like crazy . . . more than we expected." The solar power is converted to electric power to run the telescope's five instruments.
Although officials say ESA and NASA are working well together now, the relationship has suffered strains, particularly over the devastating discovery in June of the flaw in the primary mirror. It was caused a decade ago, apparently when a technician at the plant of a Connecticut contractor used a measuring rod improperly, and subsequent tests and analysis failed to catch the mistake.
In 1993, shuttle astronauts are expected to replace the telescope's workhorse camera, the wide field/planetary camera, with an advanced model that will include a built-in correction -- like a pair of eyeglasses -- to neutralize the flaw in the mirror.
Weiler said engineers are also studying ideas that might allow the telescope's other instruments to compensate for the flawed lens, such as replacing one instrument with a robot that could hold the equivalent of eyeglasses in front of the apertures of the remaining instruments.