The reason to save Hubble is simple, according to NASA administrator Michael Griffin.
"Hubble is one of the great observatories," he said. "It has revealed fundamental things about the universe of which we had no idea and would have had no idea without that mission. It is one of the great scientific instruments of all time."
The mission to repair Hubble will launch no earlier than May 2008. The shuttle Discovery will carry an experienced crew of seven to fix the ailing space telescope.
This risky mission means NASA will process two space shuttles at the same time and have a second shuttle ready to launch if the Hubble mission encounters trouble and the crew needs to be rescued.
The price tag for the fifth and final mission to service and save Hubble is close to $900 million.
Griffin contends it is money well spent, because Hubble has rewritten much of what we thought we knew about the universe.
The legacy of Hubble is stunning, including such accomplishments as helping scientists:
Date the age of the universe at 14 billion years.
Confirm quasars are actually galactic nuclei in distant galaxies.
Find proof of a black hole several billion times the mass of the sun.
Is Hubble outmoded? Griffin said no.
"It needs some refurbishment and repairs, but its contributions and capability to contribute remain quite robust," he said.
What Hubble needs are six new nickel hydrogen batteries to keep it powered up and working, plus new units containing two gyroscopes to help the telescope lock onto targets.
While the astronauts are up there on what will be the last mission to Hubble, they will also install a new wide field camera, and a Cosmic Origins Spectrograph, plus a new fine guidance sensor, and attach a new outer protective layer.
These fixes will increase Hubble's capability and keep Hubble running until its replacement, the Webb telescope, can be launched early next decade.
This mission under current NASA guidelines will certainly be risky. Five intensive back-to-back spacewalks with two spacewalking teams will refurbish Hubble to extend the wildly successful telescope's life span through 2013.
This will be the fifth time the space shuttle has come to Hubble's rescue.
When Hubble was launched in the spring of 1990, its life expectancy was just 15 years, but thanks to four prior servicing missions by the space shuttle, Hubble's life and usefulness have been extended.
A Hubble mission had originally been scheduled for 2005, but was canceled following the February 2003 accident that killed the crew of the space shuttle Columbia.
Former NASA administrator Sean O'Keefe said in early 2004 that such a mission was too dangerous.
He canceled SM4 -- the shuttle mission to save Hubble -- right after President Bush announced the Moon Mars Initiative.
"Could we do this and take the risk? Sure, but somebody else would have to make that decision not me, because I'm not doing it," O'Keefe said.
Hubble became the first casualty of Bush's new exploration vision.
So Hubble's batteries would wear out, and the space agency would guide it through Earth's atmosphere to plunge to its death in the Pacific Ocean.
The cancellation ignited a firestorm of protest.
Sixty-four astronauts signed a letter volunteering to fly a Hubble mission and sent it to key members of the Senate.
When Griffin took over as NASA administrator, he revived hope that Hubble could be saved.
Twenty years ago, Griffin worked as a project engineer on Hubble on a team designing a Fine Guidance Sensor for the space telescope.
Retired Shuttle Cmdr. Jim Wetherbee says the desire to save Hubble isn't just sentimental.
"Technically speaking, it is more risk. Is it worth taking that risk? I would say yes. If I were the administrator, I would go save Hubble. I can't even begin to tell you all the technological advances Hubble has given us. More importantly, think what we can gain in its future," Wetherbee said.
Brent Jett commanded the latest shuttle mission to the space station. He believes Hubble is one of the space shuttles' most important legacies because of the observatory's ability to inspire.
"When you see the excitement the images from Hubble generate, you don't know how many children are going to look at those images and be inspired to become scientists or astronomers," he said.
Hubble's pictures are so stunning because the space telescope observes wavelengths, which is what the human eye sees.
Therefore Hubble occupies a unique position among the great space observatories launched by NASA.
Chandra observes X-rays, SIRT observes the infrared band. Hubble's pictures are so spectacular they are some of most frequently downloaded photos on the Internet.
Astronauts have been training for spacewalks to service Hubble for months now in the neutral buoyancy lab at the Johnson Space Center, and also studying the guidelines for Hubble spacewalks written by veteran astronaut Bruce McCandless.
McCandless, who is now retired, helped deploy the space telescope in 1990.
"Hubble's contribution to our understanding of the universe," he said, "has earned it the right to be saved."
"The science is so outstanding. Hubble looks at objects in time -- in such a great amount of time -- and what we have learned has redefined so much of what we thought we knew about the universe."
Space Shuttle Program manager Wayne Hale was a flight director for earlier Hubble missions.
He has weighed the risk versus benefit of a shuttle mission to save Hubble for years. He recognizes Hubble's scientific importance.
"I think Hubble is the shuttle's crowning achievement, because it is the most important science instrument of the last century. I think the capability to keep Hubble going is a huge capability," he said.
But Hale also understands the drawbacks.
"We will take risks when we go fly that mission," he said.
Since the Columbia mission, NASA has only flown to the International Space Station.
If something goes wrong with the space shuttle, a crew could seek safe haven for about six weeks on the ISS until another shuttle could reach them.
But the space station and Hubble circle Earth in entirely different orbits, so a Hubble crew would have no refuge if it could not return home in its orbiter.
Astronaut Joe Tanner is one of NASA's most experienced spacewalkers.
He flew on the second Hubble servicing mission. "I'm a Hubble fan, even if I hadn't touched it with my gloved hands on an EVA, I would still be a Hubble fan. It's been rewriting the astronomy books and doing good science."
Tanner flew on the last shuttle mission, and he understands the risks a Hubble crew will take when it launches next year.
"You can't get to the station from a Hubble orbit. It is physically impossible. Commit to Hubble, you are going to Hubble. Or you are coming home. Without a safe haven how long can you stay airborne, while you are trying to fix whatever problem you have? If it's not fixable, obviously you want a rescue capability. How quickly can you get that there?"
The crew would depend on very basic repair methods if it found damage to the space shuttle while orbiting Earth.
Griffin, NASA's administrator, says his agency has learned much about inspecting for damage in the last three flights.
He is also confident NASA officials have solved the problem of foam shedding from the external tank.
A 1.67-pound piece of foam was determined to be the cause of the Columbia accident. The foam punctured a hole in Columbia's wing, allowing hot gases to penetrate the orbiter when it re-entered Earth's atmosphere, and triggering its destruction.
NASA has not been able to develop a repair method to fix a hole of that size on the orbiter should one be found on a future flight.
Milt Heflin was the lead flight director at the Johnson Space Center for the first Hubble servicing mission in December 1993. He says there is one picture from Hubble he cannot forget.
"It is the picture after the very first servicing mission. And it is the picture in the area of the Big Dipper -- that is the area where the handle and the Big Dipper comes together. It is an area if you stick your finger out, is the tip of your finger, is the size of the area Hubble is looking at where the handle and the Dipper comes together."
"What did Hubble see? Hubble saw hundreds, maybe even thousands of galaxies. Galaxies! We live in a galaxy. We are the star and the sun and planets around it. How arrogant of us to believe that we are it in this universe?"
Every day, the Hubble archives 3 gigabytes to 5 gigabytes of data and delivers between 10 gigabytes and 15 gigabytes to astronomers all over the world.
Hubble's gigabytes divulge precious information, and Hubble's images capture our imagination.