NASA may fit Hubble with corrective mirrors

WASHINGTON - NASA is weighing a daring plan to fix the nearsighted Hubble Space Telescope in 1993 by fitting four instruments with pairs of mirrors the size of postage stamps.

The plan, worked out by a panel of engineers and astronomers, would require three six-hour spacewalks by a team of astronauts. They would replace the telescope's main camera and install a box the size of a telephone booth to carry and deploy the corrective mirrors for three other instruments.

"This strategy recovers essentially all the science capabilities expected at launch," said the report of a committee organized by the Space Telescope Science Institute.

Engineers at the National Aeronautics and Space Administration's Goddard Space Flight Center have been doing an exhaustive study of the recommendations since they were presented late last year.

"We've got to get serious and pick a design in the next three or four months," said Dennis McCarthy, NASA's deputy program manager for Hubble. A decision will be made by May, he said, leaving a tight schedule to build the mechanisms and to train astronauts to install them.

The telescope, launched with great fanfare last April from the space shuttle Discovery, was designed to detect starlight that has traveled for billions of years and events that occurred in conjunction with the Big Bang believed to have created the universe.

Soon after the Hubble went into orbit, scientists discovered the main mirror was ground to the wrong prescription and that the telescope's view of the universe was blurred.

It was determined that when the telescope was built 10 years ago, a technician improperly assembled a measuring device used to determine the mirror's shape. Tests at the time indicated a problem, but the warning was not heeded.

A 700-pound box called COSTAR will contain the mechanism to put the mirror pairs in place in front of the telescope's faint object camera and two spectrographs. COSTAR would be bolted into the space now occupied by a high-speed photometer.

The 1993 shuttle visit to the orbiting telescope had been planned before the mission to replace the wide-field planetary camera, the main instrument. That camera now will be fitted with the corrective mirrors as well.

The mirrors act, in effect, like high-tech eyeglasses.

In each pair, the first corrective mirror passes an image of the Hubble's 94.5 inch-diameter primary mirror onto a second mirror. The second glass is formed like the Hubble's mirror, only in exact reverse, thus canceling the flaw.

If it works as planned, the $1.5 billion telescope should be restored to near-perfect health, the panel said.

It recommended a number of other fixes, including a replacement of a gyroscope unit that failed in November, replacement of tape recorders and of the solar arrays.

The arrays, which produce electricity, have caused jitters in the telescope every time it passes from day into night - nearly 15 times in each 24-hour period.

"The recommended strategy is to develop COSTAR on an urgent basis," the panel said. "Then, the 1993 (telescope) servicing mission restores the scientific functionality expected at launch."

It conceded there are technical issues that must be resolved, including the risks involved in making the fixes nearly 400 miles above Earth.

Among the problems: "Debris, like the little hooks that break off the Velcro straps each time astronauts remove a tool, have the potential to block the very small apertures that are present in each instrument or jam sensitive mechanisms."

"The definition study is looking at technical obstacles, cost and scheduling," said McCarthy. "It looks feasible technically, cost-wise and schedule-wise."

He estimated the cost of the repairs at $20 million to $30 million, not counting the expense of launching the shuttle.

The telescope institute panel's report said the costs are worth it "to propel astronomical exploration into the 21st century."