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.