Space Telescope Dealt Setback by Mirror Flaw
In a stunning setback for the Hubble Space Telescope, the national space agency disclosed Wednesday that a flawed mirror is threatening to prevent half the scheduled experiments from being carried out.
Engineers for the National Aeronautics and Space Administration said that they are confident they can avert such a potential disaster for the $1.5-billion telescope by sending new equipment into space, but that might not be possible for at least three years.
They described the nature of the problem and their anticipated solution as similar to designing eyeglasses to correct myopia.
In any case, space engineers and the world’s leading astronomers now clearly are locked in a high-stakes race against time, since the telescope’s projected life is only 15 years.
One space official described the telescope’s two mirrors as “the heart of the whole thing.”
The imperfection in one of the mirrors has rendered useless now the most important of the five major scientific instruments on board--a “wide-field planetary camera” designed to explore the farthest reaches of the universe. It was designed by and belongs to Caltech and the Jet Propulsion Laboratory, both in Pasadena.
The flawed mirror cannot be replaced, and if there is no way to compensate for the imperfection with new equipment, three of the remaining four scientific instruments will be useless half the time. The fifth will be useless about 25% of the time, according to NASA’s Ed Weiler, program scientist for the telescope.
As matters now stand, the images being sent back by the telescope are no clearer than those obtained by ground telescopes, Weiler and other somber NASA officials conceded at a press conference.
“It would be dishonest to say the mood of scientists is very happy right now. We’re all frustrated--obviously,” Weiler said grimly. “But nobody’s walking away.”
As conceived, the telescope is supposed to be capable of “imaging” objects up to 14 billion light years away. With an orbiting altitude 330 miles above the earth’s surface, the telescope’s images were supposed to be 7 to 10 times clearer than images obtained by ground-based telescopes.
Because of the imperfection--which NASA engineers called a “spherical aberration"--light is not being sufficiently concentrated on the mirror’s surface, they explained. The image provided is therefore blurred.
Weiler insisted repeatedly that the imperfection only means that “we are deferring science, not canceling science.”
It was announced that NASA has decided to appoint a “review board” to investigate all aspects of the latest problem, to be headed by JPL director Lew Allen.
Jean Olivier, the telescope’s deputy project manager, said that it is “possible but not practical” to bring back the telescope, which was launched with the space shuttle on April 25.
He said it is unlikely that the imperfection was caused by stress during the launch. Rather, Olivier said, the flaw might have occurred as long as 10 years ago during the mirror’s manufacture.
Jack Rehnberg, the chief of the space science program at Hughes Danbury Optical of Danbury Conn., manufacturer of the flawed mirrors, said the cause may have been human error during an extensive testing program while the mirrors were being made.
“We don’t know the cause, but its probably something inherently, fundamentally not done right,” he told the Associated Press. “It could have been a human error.”
The two mirrors were tested individually on the ground, Olivier said, and passed all tests. He said they were not tested in combination on the ground because this would have required a test arrangement that would have cost “hundreds of millions of dollars.”
Based on current schedules, the first “service mission” for the telescope, to be carried out by astronauts, is scheduled for 1993.
Douglas Broome, program manager, said that NASA now will see if that timetable can be expedited so that a replacement wide-field planetary camera, already under construction, can be launched by 1993 or sooner.
“I know we are going to succeed at this,” said Lennard Fisk, NASA’s assistant administrator for space science and applications.
Fisk predicted further that the telescope will go on to make “fundamental discoveries” that will leave astronomers and the public “ooing and aahing.”
He flatly rejected a reporter’s question about whether this is “the biggest disaster” at NASA since the 1986 explosion of the space shuttle Challenger.
“The test,” he said, “is how we recover from this.”
Fisk said that a team of scientists and engineers from the European Space Agency is to arrive in the United States next week to help assess the situation and design the corrective equipment.
And he urged against recriminations, advising all to await the findings of the Allen panel, whose members have not yet been named.
The other four major instruments aboard the telescope:
--The faint object camera. Built by the European Space Agency, this camera is designed to obtain images of extremely faint objects at great distances. It now can be useful only half the time, Weiler said.
--The faint object spectrograph. Designed to study light waves emitted by celestial objects, this machine can reveal their temperature, distance from Earth and their velocity of travel through space. It is now only 50% useful.
--The high resolution spectrograph. Even more sensitive than the faint object spectrograph, this machine records and classifies radiation from various celestial sources. It is 75% useful still.
--The high speed photometer. Described as the simplest of the five instruments, it measures the intensity of light and can detect fluctuations in brightness down to microseconds. It will be useful only half the time.
Officials are searching the backlog of scientists’ proposals for experiments that can replace those already approved. Ten times as many proposals were submitted as could be accommodated in the first round of research, officials said, and many of them could use the telescope as is.
The mirror imperfection is the latest, albeit the most serious, problem to plague the 43-foot-long telescope.
Already, one of its two antennae’s movements had to be restricted because it was striking a cable loop, causing an automatic shutdown.
That forced NASA engineers to design a computer software program to deliberately limit the antennae’s movement. In turn, however, that resulted in an inevitable restriction in the flow of communications--including the transmission of data--from the telescope to scientists and engineers.
The experts also discovered that the 25,500-pound satellite developed a vibration that lasted for some minutes each time it went from sunrise to sunset and back again in its orbit. Engineers planned to compensate for the vibration with another computer program.
Another problem with which engineers still are grappling relates to the telescope’s electronics fine-guidance sensors. The sensors are supposed to lock onto guidestars, or objects near the targets to be observed. The sensors are proving to be highly vulnerable to radiation from the South Atlantic, thus causing instruments on board to shut off automatically.
NASA’S HUBBLE SPACE TELESCOPE
One of the two giant mirrors in the orbiting Hubble Space Telescope is so flawed that the telescope is no better than those on Earth. The flaw cannot be corrected from the ground, and the National Aeronautics and Space Administration estimates that it will take three to six years to equip the telescope with reflecting gear that will compensate for the imperfection.Sources: Lockheed Missiles & Space Co., NASA, Sky & Telescope Magazine, Aviation Week & Space Technology magazine