An article yesterday incorrectly said space shuttle astronauts are scheduled to blast off as early as Friday on a mission to repair the Hubble Space Telescope. The mission is slated to begin as early as Saturday morning. (Published 12/07/1999)

Since the Hubble Space Telescope was placed in space almost a decade ago, the orbiting observatory has taken some 259,000 exposures of almost 13,000 astronomical targets. Through its soul-stirring images, it has blended cutting-edge science with a broader public appeal.

But for the last month, the telescope has been idle, forced into inactivity by the failure of crucial devices that allow ground controllers to point the powerful instrument. Now, seven astronauts are poised to perform replacement surgery on the Hubble. The astronauts are scheduled to lift off as early as Friday morning aboard the space shuttle Discovery on a mission to rendezvous with and repair the massive orbiting observatory.

Because of the telescope's lack of pointing control, Discovery commander Curt Brown will have to work harder to position the shuttle to grab the 53-foot-long telescope on the crew's third day in orbit. Then, in four straight days of planned spacewalks, U.S. astronauts Michael Foale, Steven Smith and John Grunsfeld, with Claude Nicollier of the European Space Agency, are set to install six new gyroscopes, restoring the telescope's ability to remain stable and pointed at target objects. The flight's secondary goals include putting in a new fine guidance sensor, rolling on a fresh outer skin of protective insulation in a maneuver akin to wallpapering and, in what NASA chief scientist Edward Weiler called a "brain transplant," installing a more advanced computer (based on an Intel 486 unit) to replace the 1960s model now on board.

Engineers designed the telescope for long life, with regular maintenance and equipment upgrades by astronauts expected to keep it robust until 2010. But the gyroscope replacement--already moved ahead of its original summer 2000 schedule--has become more crucial since the Nov. 12 shutdown of science operations. Hubble officials deem the fix second in urgency only to a 1993 flight in which astronauts corrected a small but devastating flaw built into the Hubble's main optics.

The Hubble design, development and operations costs, spread over 30 years, are expected to total about $6 billion, officials say.

While the project has aroused resentment among some scientists, who say it attracts a disproportionate amount of attention and money, NASA officials point out that the telescope's observations have generated more than 2,400 published papers and numerous significant findings on fundamental scientific questions.

As of early this year, Weiler said, those accomplishments include completion of the entire Hubble "triple crown . . . that is, the big three promises of many decades ago" when the instrument was being sold to Congress: to determine the age of the universe within a certain accuracy, to provide proof that massive black holes exist and to detect the faintest and farthest objects in the cosmos.

In a recent Hubble tribute at the Carnegie Institution here, astronomer and theorist Sandra M. Faber of the University of California at Santa Cruz traced the telescope's climb from ignominy as "an icon of failure" to its current status as "one of the best comeback kid stories in history."

The Hubble's most significant accomplishment, she argued, lies not in the sheer beauty of its images, or in the breakthroughs provided by its data, but in its power to convey ideas that speak to all of humanity.

"We say seeing is believing," she said. "We don't say seeing is knowing. Images make us believe--in our guts."

Among the examples of themes made real through Hubble images, she cited an asteroid collision at Jupiter that awakened civilization (and Hollywood moviemakers) to the importance of violent impacts in our past and, almost certainly, our future; the glowing patterns painted in the births and deaths of stars, events that reveal much about our place in the cosmos; and the super-long exposures known as the Hubble Deep Field, images that constitute a kind of core sample of the expanding, changing universe. The images are the equivalent of drilling back to a time as early as a billion years after the cosmos was born in the big bang to reveal bizarre shreds and fragments of galaxies already forming and colliding.

One of the most important collective messages from such astronomical images, Faber said, is about time. "We have the incredible gift of another 4 billion years," for example, before it is the sun's turn to swell toward its gently explosive end, turn Earth to a cinder--and paint a vibrant picture on the sky to be pondered by possible watchers elsewhere.

The Servicing Mission

Astronauts are scheduled to replace equipment aboard the Hubble Space Telescope. Although no scientific instruments will be installed, there are many activities planned for this mission, which will require four spacewalks. Below are some kinds of equipment the astronauts will install:

1. Advanced computer: The new main computer will be more capable, require less maintenance and cost less to operate.

2. Fine guidance sensor: Replacement sensor has enhanced, on-orbit alignment capabilities.

3. New outer blanket layer: Existing panels will be covered to control Hubble's internal temperature.

4. Gyroscopes (rate sensor units): Six sensors of the failing pointing system.

5. S-band single access transmitter: Unit uses radio waves to send data to the ground.

6. Solid state recorder installation: This digital recorder is faster and more reliable.