STS-125 Mission Preview:
A Brief History of the Hubble Space Telescope

Editor's Note...
Portions of this historical review were written for Astronomy Now magazine.

CBS News Space Consultant

The Hubble Space Telescope has cost U.S. taxpayers some $10 billion in the quarter century since the project was approved. But to astronomers around the world, the high-flying satellite is, in a word, priceless.

Since its famously blurred vision was corrected by spacewalking astronauts in December 1993, the Hubble Space Telescope has become an international icon of science, one of the most productive astronomical observatories ever built and the flagship of NASA's exploration of the universe.

The solar-powered spacecraft has helped astronomers confirm the existence of super massive black holes, pin down the true age of the universe and spot the faint building blocks of the first galaxies as they collided, merged and grew just a billion years or so after the birth of the cosmos.

Its mind-bending photographs have charted the life cycles of distant suns in unprecedented detail, providing unmatched views of the vast stellar nurseries where stars are born to the supernova bangs and whimpers marking old age and death.

It has catalogued myriad infant solar systems in the process of forming planets and provided flyby-class views of the outer planets in Earth's own solar system, routinely capturing phenomena as common as dust storms on Mars to once-in-a-lifetime events like the 1994 crash of a comet into the atmosphere of Jupiter.

While ever larger, more sophisticated ground-based telescopes now rival and in some areas exceed the power of Hubble's relatively modest 94.5-inch (2-meter) primary mirror, the space telescope, operating high above Earth's turbulent atmosphere, remains in a class by itself.

"Hubble is the most productive science mission, and has had the highest impact, of all NASA science missions in the history of this agency," said Hubble program scientist David Leckrone. "It's a national icon."

It all began at 3:38 p.m. on April 25, 1990, when astronaut Steven Hawley, operating the shuttle Atlantis's robot arm, released the Hubble Space Telescope into open space as the orbiter and its costly payload sailed 381 miles (613 kilometers) above the Pacific Ocean just west of Ecuador.

It was a moment of high drama as the 13-ton observatory, arguably the most important scientific spacecraft ever built, slowly receded from the space shuttle against the blue-and-white backdrop of planet Earth.

"The telescope really looked great as we flew away from it and we all were remarking about (how) we sure hope it does good work," shuttle commander Loren Shriver radioed flight controllers in Houston.

"Well, it sure is now, Loren," replied astronaut Story Musgrave from the Johnson Space Center. "Thanks for all the great work you've done. Galileo is real proud of you."

Running seven years behind schedule and $1 billion over budget, Hubble's deployment was held up a final hour and a half that day because of problems coaxing one of its two electricity producing solar arrays to unfurl.

Astronauts Kathryn Sullivan and Bruce McCandless were within a half-hour or so of beginning an emergency repair spacewalk when the glitch was resolved by ground controllers, clearing the way for Hawley to finally send Hubble on its way.

"The fun has hardly begun yet," said Richard Truly, NASA's elated administrator. "Because just within a few days and weeks and months she'll begin to return science back to Earth. And it should really be exciting. I think it's a great and historic day for the space program."

Congress approved the space telescope project in 1977, with launch targeted for 1983. But the flight was repeatedly delayed because of problems getting the space shuttle program off the ground and by technical trouble with the telescope itself.

Launch ultimately was retargeted for the fall of 1986, but those plans went up in smoke with Challenger's destruction in January 1986, grounding the shuttle program for nearly three years.

All of those delays pushed Hubble's price tag to some $1.5 billion, making the observatory one the most expensive civilian science payloads ever launched. Adding in the cost of ground equipment, annual operating expenses for the Space Telescope Science Institute in Baltimore, planning and development of shuttle servicing missions and other factors, the total cost of the telescope project was expected to reach $2.35 billion by the end of the first year of operation.

Despite the delays and the high price tag, Hubble's launch marked a major milestone in the history of science.

"We'll be like the little nearsighted child in the classroom who is given a pair of glasses and at last can see what the teacher's been writing on the blackboard," NASA science chief Lennard Fisk said on the eve of launch in 1990. "When Hubble lifts off ... we will witness not just another launch, but rather a turning point in humankind's perception of itself and its place in the universe.

"In the 1500s, Copernicus demonstrated that the Earth was not the center of the universe. In the first half of this century (American astronomer) Edwin Hubble revealed the mind-numbing vastness of the universe with its countless galaxies spread over billions of light years.

"I do not know what (the space telescope) will reveal about the origin of the universe and our place in it," Fisk said. "But I am very confident it will be as profound in its effect on our perception of who we are as such previous revolutions in astronomy."

The Hubble Space Telescope measures 43.5 feet long, 14 feet wide and weighs 25,500 pounds (13.3 meters long, 4.3 meters wide, 11,500 kilograms). Built by Lockheed Missiles and Space Co. of Sunnyvale, California, the telescope was designed to be serviced by spacewalking shuttle astronauts and to operate at least 15 years.

The optical system is built around a 1,827-pound (829-kilogram) mirror built by what was then Perkin-Elmer Corp. of Danbury, Connecticut. It is coated with a highly reflective layer of aluminum-magnesium, ensuring that as much light as possible is captured for study.

Small by the standards of major ground-based telescopes, Hubble's mirror is, perhaps, the most perfect ever made, with no peaks or valleys greater than about half a millionth of an inch (0.0000025 centimeters). If the surface of the Earth was that smooth, Mount Everest would be less than 5 inches (12.7 centimeters) tall.

The observatory uses a Cassegrain optical design. Light enters the instrument, bounces off the primary mirror and back up to a smaller, 12.2-inch (31-centimeter) mirror mounted in the center of the telescope tube. That mirror, in turn, sends the light back down the tube and through a 24-inch-wide (61-centimeter-wide) hole in the center of the primary mirror. It is brought to a focus 4.9 feet (1.5 meters) below the surface of the main mirror.

"The human eye can barely detect a firefly at 200 yards (180 meters)," Hubble chief scientist Edward Weiler told reporters before launch. "If your eye was as good as the space telescope, you could see that same firefly 10,000 miles (16,000 kilometers) away, a distance from Washington to Sydney, Australia.

"Another way of looking at it is the human eye can detect a standard flashlight bulb at about 2 miles (3.2 kilometers). With the space telescope, you could see that flashlight bulb on the moon, a distance of a quarter of a million miles (402,000 kilometers). In general, the space telescope is about 10 billion times as sensitive as the human eye."

Overall, the telescope was designed to have 10 times the resolution, or clarity, of ground-based instruments, the difference between being able to read the big letters on the second row of an eye chart and reading the bottom line.

If the human eye could distinguish between two objects with the clarity of the space telescope, Weiler said, "you could read this morning's Miami Herald headlines from the Kennedy Space Center, a distance of about 200 miles (322 kilometers)."

But a big telephoto lens is useless without some way to stabilize it and as one might expect, Hubble's guidance system was one of the most advanced in the world, so accurate it could keep a laser beam focused on a dime at a distance of 200 miles for 24 hours at a time. That's the equivalent of sinking a 1,500-mile (2,400-kilometer) golf putt.

The telescope can be equipped with four telephone-booth-sized instruments mounted behind the focal plane and one side-mounted instrument. It was initially equipped with a pair of light-splitting spectrographs, a photometer and two cameras, the Wide Field Planetary Camera (WFPC), developed at NASA's Jet Propulsion Laboratory, and the Faint Object Camera, provided by the European Space Agency.

The WFPC (pronounced wiff-pick) was perhaps the single most important instrument in the sense that it was expected to provide the ultra-sharp color pictures of astronomical targets that could be easily appreciated by the public. "First light" - the first images from the WFPC - were eagerly anticipated.

But in the days and weeks that followed Hubble's launch, engineers working through lengthy calibration procedures ran into a steady stream of hiccups and glitches, many of them software related, that delayed the start of routine scientific observations.

At the same time, they were struggling to understand a strange, unexpected vibration that rocked the telescope slightly every time it entered Earth's shadow and later moved back into sunlight. More ominously, scientists also were having problems getting Hubble's optical system in focus.

By mid May, the telescope had become the target of increasingly skeptical stories and the butt of jokes on late night television. Comedian David Letterman poked fun at Hubble in his nightly "Top 10 List."

The "Top 10 Hubble Telescope Excuses" included "The guy at Sears promised it would work fine," "Some kids on Earth must be fooling around with a garage door opener," "There's a little doohickey rubbing against the part that looks kind of like a cowboy hat" and "Ran out of quarters."

With Hubble's calibration and checkout about 10 days behind schedule, NASA managers kept a stiff upper lip, calmly explaining that the telescope was a very complicated piece of hardware and that glitches were not unexpected.

"I don't mean to be a Pollyanna," said Leckrone, then deputy project scientist. "But I do know that this time, right after launch, early bugs and early problem-solving are very characteristic. I think we would have been very naive to think it would have been any different."

Said Weiler: "It's not a doom and gloom atmosphere. Things are running pretty smoothly."

But not for long. After repeated failures to focus Hubble's optical system, engineers made an exhaustive series of measurements, moving the motorized secondary mirror in tiny increments to precisely map out the behavior of the optical system. They were left with a staggering discovery.

"We got a very clear and distinct characteristic, a textbook characteristic, of an optical system that had a significant amount of spherical aberration," deputy project manager Jean Olivier told reporters at a now-legendary June 27, 1990, news conference. "We are in the process of evaluating the ramifications of this."

Because of an error that was never caught during the mirror's fabrication, the supposedly perfect primary had been ground into the wrong shape. As a result, its outer edge was about 2 microns lower than it should have been. In other words, the dish-shaped mirror was too shallow by about two-fiftieths the width of a human hair.

That prevented light from the outer regions of the mirror coming to a focus at the same point as light from the inner regions, Olivier's "textbook" definition of spherical aberration.

Hubble's original design specification called for 70 percent of a star's light to be concentrated in a very tiny circle. Because of spherical aberration, Hubble could only manage 10 percent to 15 percent.

"It would be dishonest for me to say the mood of the scientists is very happy right now," Weiler said. "We're all very frustrated. But we should be able to fix it."

Outsiders were skeptical. Discovering the supposedly perfect Hubble suffered from such a fundamental flaw was a devastating blow. "If you asked us the night before launch to give you the 100 biggest worries or nightmares, none of us would have come up with spherical aberration," Weiler said later.

Sen. Barbara Mikulski, a Maryland Democrat whose district included NASA's Goddard Space Flight Center - home of the Hubble project - called the space telescope a "techno turkey," a morale-sapping sobriquet that summed up the feelings of many in the space science community.

But the key to Hubble's salvation was the very perfection of its flaw. It was utterly uniform, with no variation across the mirror. In short, the mirror really was near perfect, it had simply been ground to the wrong optical prescription. While the mirror itself could not be fixed, camera and other instruments could be developed with built-in corrective optics to precisely cancel out the aberration. Or so NASA managers hoped.

Olivier said the aberration likely was caused when the telescope's mirrors were being fabricated by Perkin-Elmer Corp.

"What we suspect is that in the ... techniques used to measure the figure of the mirror and polish it and hold it steady while you're doing that ... somewhere in this chain, there was a mistake or error made that resulted in the mirror being very precisely made but ultimately to the wrong figure," he said.

A painful investigation would reveal that data from a test rig used to measure the "wave-front error" - the precise shape of the mirror - was thrown off slightly by a speck of paint that had lodged in an opening where reflected laser light passed.

Engineers running the test, unaware of the paint chip, tried to adjust the spacing between the mirror and the test equipment to get the expected results. When that failed, instead of stopping and investigating the matter further, washers were inserted to change the spacing even more. As a result of this test, additional glass was ground off the outer portion of the mirror. In effect, the mirror was ground to the wrong prescription.

At the Jet Propulsion Laboratory, meanwhile, a second-generation Wide Field-Planetary Camera was already under construction. In the wake of spherical aberration, WFPC-2 was equipped with specially ground internal mirrors to exactly counteract the effects of Hubble's flawed main mirror.

"If you're nearsighted, your eye doesn't have the right curve," Weiler said. "When you put your glasses on, it cancels that curve out. So it's almost exactly like that, except we do it with mirrors, not lenses."

Providing properly focused light to Hubble's other instruments was a more difficult challenge.

In the end, Ball Corp. of Boulder, Colo., came up with the Corrective Optics Space Telescope Axial Replacement, or

COSTAR, an instrument that would take the place of the photometer in one of Hubble's four main below-mirror instrument bays.

COSTAR included 10 small coin-sized mirrors mounted on five motorized arms that could be extended into the light path of the primary mirror, directing properly focused light to the other instruments.

"These mirrors are only about the size of a dime," said Weiler. "They had to be ground so smoothly that if you ground Colorado down to the same level, the largest mountain would be reduced to one inch."

While the new optical systems were being designed and fabricated, engineers identified the source of the unexpected vibrations causing Hubble to rock back and forth when entering or exiting Earth's shadow. Tests showed the observatory's two solar arrays, provided by the European Space Agency, were flexing in response to temperature changes. New arrays were built with modifications to make them less susceptible to flexing.

When all was said and done, it took three-and-a-half years to complete testing and to train a crew of astronauts for what would be the most ambitious shuttle mission ever attempted. A record five back-to-back spacewalks were planned to replace Hubble's two solar arrays, to correct the telescope's blurry vision and to replace gyroscopes and other equipment that had suffered problems since launch.

The stakes were enormous. By now, NASA's reputation was clearly on the line and along with it, confidence in the agency's ability to build an international space station.

"This is probably one of the most important missions that NASA will fly for years to come until we start putting the first elements of the space station up," said former Apollo astronaut Thomas Stafford, chairman of a key Hubble review team. "And if we blow this one, it is really going to be bad news. It is going to affect the attitude towards the space station and the attitude towards NASA."

Said Jeffrey Hoffman, one of the four spacewalkers charged with making the repairs: "Is it difficult to walk on a real narrow ridge when there's a 2,000-foot drop on each side? No, it's not. But if you slip, it's a long way down."

Finally, after years of around-the-clock brainstorming, multiple internal and external reviews and seemingly endless training for the astronauts, the shuttle Endeavour blasted off on mission STS-61 at 9:27 GMT on Dec. 2, 1993. Two days later, Swiss astronaut Claude Nicollier, operating the shuttle's robot arm, plucked Hubble out of open space and mounted it on a rotating cargo bay service platform.

Over the next five days, Hoffman and fellow spacewalkers Story Musgrave, Kathryn Thornton and Thomas Akers accomplished all of the mission's primary and secondary goals.

"Ah, look at that baby... We'll take some nice pictures with that!" Hoffman said, preparing to install the $24 million Wide Field Planetary Camera 2. Along with installing the $50 million COSTAR package of corrective mirrors, the astronauts also installed:

Over the next month, eager scientists and engineers worked through yet another round of calibration and checkout procedures, fine-tuning the telescope's myriad systems and taking test photos to precisely focus its optical system.

Finally, at a heavily attended Jan. 13, 1994, news conference at the Goddard Space Flight Center in Greenbelt, Maryland, the results were unveiled by Mikulski and NASA Administrator Daniel Goldin.

"The trouble with Hubble is over!" Mikulski declared. "The pictures are remarkable. The science that will come from the pictures is of historical significance."

Said Goldin: "This is phase two of a fabulous, two-part success story. The world watched in wonder last month as the astronauts performed an unprecedented and incredibly smooth series of space walks. Now, we see the real fruits of their work and that of the entire NASA team."

Using dramatic before-and-after pictures comparing Hubble's pre- and post-repair eyesight, NASA officials demonstrated the telescope's obviously successful overhaul, prompting excited whispers from even the most jaded of space reporters.

In one set of photos, a blurry spiral galaxy became a razor-sharp pinwheel of countless tightly focused stars. In another, a smeared-out blob of light was resolved into a swarm of suns.

"I think Hubble will be viewed as a revolution in astronomy ... going all the way back to Galileo and Tycho Brahe," Leckrone said. "I think Hubble will reveal the clearest possible view of our universe with a telescope of this size and I think we'll have a long list of major breakthroughs in astronomical science."

Weiler agreed, saying "Hubble will accomplish many of its major goals."

Among those delayed objectives: Determining the age of the universe to within 10 percent; confirming the existence of supermassive black holes; and charting galactic evolution by looking further back in space and time than ever before.

"I think it will measure the age of the universe to within 10 percent, I think it will tell us a lot about how matter first evolved into the galaxies stars and planets we see today," Weiler said. "And I think 50 years from now, spherical aberration will be a footnote because of all these things."

He was right. Since the 1993 repair mission, Hubble has revolutionized optical astronomy, becoming the most significant spacecraft ever launched in terms of its overall scientific impact and its impact on Earth's cultural heritage. Pictures from the Hubble Space Telescope abound in grammar school and high school textbooks. Children are exposed to Hubble's legacy almost from the time they're taught to read.

More important, of course, it has become one of the central tools in the ongoing intellectual struggle to understand the world around us and our place in an evolving universe.

As Weiler predicted, Hubble has played a key role in determining the age of the universe, now thought to be about 13.7 billion years. What Weiler didn't know then was that Hubble also would play a key role in confirming that the expansion of the universe is actually accelerating, not slowing down as common sense would dictate.

As Weiler predicted, Hubble imagery and data show beyond any reasonable doubt that massive black holes exist at the hearts of galaxies across the universe. What he didn't know then was that Hubble would detect galaxies already shining within a billion years of the big bang, far sooner than most astronomers expected.

As Weiler predicted, Hubble has provided razor-sharp views of the outer planets that rival imagery from the Voyager flyby missions. What he didn't know then was that Hubble would provide the best views in the solar system when comet Shoemaker-Levy 9 crashed into Jupiter in 1994.

He also could not have predicted Hubble would be the first telescope to directly measure the presence of a chemical element in the atmosphere of a planet orbiting another star. Or that Hubble would detect countless potential solar systems - protoplanetary disks, or "proplyds" - embedded in star-forming nebulas.

"Hubble has really opened our eyes to what the universe is made of, its structure, and it has helped us learn how little we know about the universe," said astronaut John Grunsfeld, a veteran Hubble servicing spacewalker. "It's helped us explore the beauty of the universe in a way that we've never been able to before in terms that people can see.

"When we look back, maybe 30, 40 or 50 years from now, I think we'll see Hubble as the most productive scientific instrument in human history. It's had that big an impact on people's lives."

Hubble has had that impact in large part because it was designed to be serviced in orbit, allowing NASA to upgrade its instruments and to repair broken equipment.

In February 1997, the crew of shuttle mission STS-82 performed five back-to-back spacewalks, equipping the telescope with two new science instruments - the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectrograph (STIS). NICMOS, chilled to just above absolute zero by a block of nitrogen ice, gave Hubble the ability to look through intervening clouds of gas and dust to study phenomena invisible to WFPC-2.

The spacewalkers also installed a new fine guidance sensor, two data recorders, a computer command decoder, new solar array drive electronics and a reaction wheel assembly to help Hubble move from one target to another.

NASA originally planned to launch the third Hubble servicing mission in April 2000. But in early March 1999, NASA managers decided to split that flight into two missions, Servicing Missions 3A and 3B, because of unexpected gyroscope failures. With three of six gyros already out of action, a fourth failed on November 15, 1999, putting Hubble into electronic hibernation.

During Servicing Mission 3A in December 1999, the crew of mission STS-103 conducted three back-to-back spacewalks to install a full set of six new gyroscopes, a half-dozen battery regulators, a new computer, a refurbished fine guidance sensor, a solid-state data recorder and a new S-band radio transmitter. The astronauts also installed equipment to help calibrate the fine guidance sensor and attached three insulation panels to keep sensitive equipment from getting too hot or too cold.

In the most recent servicing mission, STS-109 in March 2002, spacewalking astronauts installed a new instrument - the Advanced Camera for Surveys, or ACS, and two new rigid solar arrays that completely eliminated the temperature-induced jitters that hampered earlier operations. The astronauts also installed an innovative xenon refrigerator to revive NICMOS, which ran out of nitrogen ice coolant in 1999.

Equally important, the spacewalkers replaced Hubble's main electrical distribution system, a "black box" that was not designed to be replaced in space. Again, they were successful, leaving Hubble in better shape than ever before.

"Every year we look at a metric, a way of quantifying how the space science programs of NASA hold up in the whole world of science, all areas of science," said Leckrone. "And by this metric, Hubble is the most productive space mission, science mission, and has had the highest impact of all NASA science missions in the history of this agency.

"Without the servicing that we've done and the refurbishment and the upgrades of the technology on Hubble that we've done, this would not continue to be the case. But it does continue to be the case, year after year. The current demand for the use of Hubble by astronomers all around the world exceeds our ability to satisfy that demand by a factor of eight.

"This factor of eight is a record, it's the highest it's ever been for Hubble and I attribute that to the eager anticipation the community has for using the Advanced Camera for Surveys. We can just hardly wait to get our hands on it."

He did not have long to wait. On April 30, NASA unveiled the first images from the new $75 million camera, including a stunning shot showing some 1,500 discernible galaxies, or fragments of galaxies, sprinkled across space like ruddy gemstones on black velvet.

Some of those galactic fragments date back to earlier than a billion years after the big bang. How soon the first stars "turned on," ending the so-called dark ages immediately following the big bang - and how long it took them to coalesce into galaxies - is a profound mystery and one of the hottest topics in modern astronomy.

While the spectacular image from Hubble's Advanced Camera for Surveys had not yet resolved the mystery at the time it broke down, it leaves little doubt Hubble will remain at the forefront of astronomy for years to come.

Before the camera's installation, Holland Ford, an astronomer at Johns Hopkins University who is the chief scientist on the ACS project, promised a 10-fold increase in Hubble's ability to find faint stars and galaxies.

"I now have the pleasure of reporting to you that we have achieved that 10-fold increase and more," he said when the first pictures were unveiled. "The advanced camera gives Hubble and humanity a new window on the universe. This new window is the widest and clearest that Hubble has ever had. I think it is likely that astronomers will use the advanced camera to make discoveries that will change the way we view the universe."

In the wake of the 2003 servicing mission, Hubble's data output was 20 times higher than it was when the telescope was launched in 1990.

"Hubble is the most frequently cited space mission in the scientific literature, it's also the most frequently cited space science mission in the media," Leckrone said. "It's a national icon."

And that's why NASA Administrator Sean O'Keefe's decision in January 2004 to cancel a fourth and final shuttle servicing mission to upgrade the telescope set off a storm of controversy and criticism.

O'Keefe told engineers and scientists at NASA's Goddard Spaceflight Center that his decision was based on a variety of factors, including a recommendation by the Columbia Accident Investigation Board that would require an autonomous tile repair capability for flights not bound for the international space station.

Hubble Servicing Mission 4, or SM-4, was the final flight on NASA's launch manifest that wasn't bound for the space station, where the crew of a crippled shuttle could attempt repairs or await rescue. The CAIB recommendation would have required NASA to develop stand-alone repair techniques for a single flight.

The decision was announced on Jan. 16, 2004, two days after President Bush unveiled a post-Columbia plan to finish the space station and retire the shuttle by the end of 2010, to develop a new, safer manned spacecraft and to establish a moon base by around 2020.

In a strange twist of fate, John Grunsfeld, the astronomer-astronaut who helped service Hubble in 1999 and 2002, was forced to defend O'Keefe's decision in his new role as NASA's chief scientist.

"This is sort of a sad day that we have to announce this," Grunsfeld said. "But I have to tell you, as somebody very close to the project, I can tell you they made the right decision. It's one that's in the best interests of NASA."

The decision meant that that Wide Field Camera 3 and the Cosmic Origins Spectrograph - both already built - would not be installed. It also left Hubble's continued operation at the mercy of its aging gyroscopes, batteries and other equipment.

"People here are brushing off their resumes," said one official at the Space Telescope Science Institute at Johns Hopkins University in Baltimore. "Hubble has been such a crown jewel for NASA, I would have hoped it would have tilted the balance the other way. ... It's been a sad day. It was like walking around a funeral home."

O'Keefe's decision was sharply criticized by editorial writers, astronomers and key lawmakers in Washington. O'Keefe eventually authorized a study to explore the feasibility of robotically upgrading Hubble, but it quickly became apparent that any such mission would be extremely complicated and, equally important, expensive.

NASA resumed shuttle flights in 2005 and, independent of any Hubble concerns, devoted considerable resources to developing reliable heat-shield repair techniques. O'Keefe's replacement, Mike Griffin, made no secret of his support for a fourth Hubble servicing mission.

Finally, on Oct. 31, 2006, Griffin officially reinstated a final shuttle mission to service and upgrade the Hubble Space Telescope, deciding the scientific value of the observatory justified the additional cost - and risk - of a stand-alone shuttle flight.

"We have conducted a detailed analysis of the performance and procedures necessary to carry out a successful Hubble repair mission over the course of the last three shuttle missions," Griffin said in a statement. "What we have learned has convinced us that we are able to conduct a safe and effective servicing mission to Hubble.

"While there is an inherent risk in all spaceflight activities, the desire to preserve a truly international asset like the Hubble Space Telescope makes doing this mission the right course of action."

Mikulski stood up and led a standing ovation.

"What an exceptional day today is," she said. "I'm so pleased and so excited that Dr. Griffin has just announced that Hubble will be serviced. ... It's a great day for science. It's a great day for Atlantis. It's a great day for inspiration, because that's one of the things Hubble has meant for so many people."

Said Preston Burch, manager of the Hubble program at Goddard: "We're elated. I have a hard time finding the right words to express it, but we are extremely pleased, giddy with enthusiasm about this."

At that time, Hubble Servicing Mission No. 4 was slated for the shuttle Discovery and launch was targeted for May 2008. It later was switched to Atlantis and delayed to early October. The, just three weeks before launch, one channel of a two-channel science data formatter aboard Hubble failed. NASA managers ultimately decided to delay launch to give engineers time to check out and certify a replacement science instrument data system computer that will restore full redundancy to the data system. While engineers initially held out hope for a launch in February, the flight slipped to May 12 when all was said and done.

If SM-4 is successful, engineers believe Hubble will remain scientifically productive at least through 2013, an additional five years beyond what could be expected based on the current health of its aging batteries and gyroscopes. With any luck at all, the telescope will still be operating when its replacement, the huge infrared-sensitive James Webb Space Telescope, is launched around 2013.

In the meantime, Hubble's two new science instruments will help the iconic observatory address some of the most fundamental questions in astrophysics and cosmology, including the nature of the so-called dark energy, believed to be accelerating the expansion of the universe, and the evolution of galaxies in the wake of the big bang.

Huge ground-based telescopes using computer-controlled adaptive optics, which can compensate for turbulence in the atmosphere, rival or exceed Hubble's vision in some areas. But Hubble's resolution, "the sharpness of its vision, is really unparalleled and it will be a long while before that is achieved in the optical by the best adaptive optics," said Mario Livio, a senior astrophysicist at the Space Telescope Science Institute.

"There is some hope it will overlap with the James Webb Space Telescope, which in itself would be incredible. Imagine you would have something (in space observing) from the ultraviolet to the mid-to-far infrared, operating at the same time. This would be incredible."

Grunsfeld, an astronomer by training, said "the biggest discovery that Hubble will make is the next one."

"A lot of folks don't believe that, but from the last mission we were told that and there was this small discussion about something called dark energy, which we now know is about 75 percent of the total energy content of the universe," he said. "Prior to the previous Hubble missions, nobody even knew it existed. So I think that's pretty big. I don't know how we can top that, but I imagine there'll be something."