Astronomers hope to pull out all the stops for those last five years, which David Leckrone, the Hubble project scientist, calls the “grand finale of the Hubble symphony.”  “This mission is a bit of a hallmark for us, not just as a program but as a country,” LeRoy Cain, NASA’s deputy shuttle manager, said at a recent news conference in Houston. “It’s because we have a shuttle vehicle that we can go back and do this final servicing mission that allows us to grow as an exploration community and a space-faring nation.”  If all this sounds familiar that is because it is. NASA has been here before. Commander Altman and his crew were only two weeks from taking off last fall when a router, officially known as the Science Instrument Command & Data Handling System, broke down, rendering all of Hubble’s instruments useless. The telescope’s engineers realized they could get Hubble back in action by activating a backup circuit, but that would have left the telescope with no backup, one incident away from total failure.  Deciding that it made no sense to spend millions of dollars and send seven astronauts into space just to leave the telescope in such a vulnerable position, NASA postponed the mission until the engineers at Goddard Space Flight Center, could prepare a replacement router.  The astronomers and astronauts had already been waiting a long time. In 2004, in the wake of the Columbia disaster, which killed seven astronauts, Sean O’Keefe, then NASA administrator, canceled the final servicing mission, saying it was too risky. If something happened to the shuttle’s fragile thermal tiles during the launching, as it did with Columbia, there was no haven for the astronauts to await rescue. After Michael Griffin took over as the agency’s administrator in 2006 and the shuttles started flying again, he reinstated the Hubble repair mission.  How has the political roller coaster ride been? “I used to have black hair,” Dr. Leckrone said.  In place of the space station, Commander Altman and his crew will have Endeavour, which is to be launched on a mission to the space station in June, as a safety backup. For a rescue mission, a skeleton crew led by Chris Ferguson would “launch on need.” The astronauts from Atlantis would be transferred along a rope line between the shuttles in a two-day dance of spacewalks and spacesuit swaps. Atlantis carries four suits, none of them big enough for Commander Altman, who takes an extra-large, so Endeavour would have to take him one.  Commander Altman, a former Navy captain, will be making his fourth space trip. His pilot, Gregory C. Johnson, also formerly ofthe Navy, is making his first spaceflight.  Traveling as mission specialists will be veterans — John Grunsfeld, an astronomer on his third Hubble repair trip and fifth shuttle mission, and Mike Massimino, a mechanical engineer making his second Hubble trip — and rookies Michael Good, an Air Force colonel; Megan McArthur, an oceanographer; and Andrew Feustel, a geologist.  Commander Altman, Dr. Grunsfeld and Dr. Massimino were together on the previous Hubble servicing, STS-109, in March 2002. For this mission, Dr. Grunsfeld and Dr. Feustel will be paired on the first, third and fifth spacewalks while Dr. Massimino and Colonel Good will partner on the second and fourth spacewalks.  Dr. Massimino has been Twittering during his training and has some 112,000 followers. At the recent news conference he said he would try to find some time to Twitter a few messages from space.  Dr. McArthur will operate the shuttle’s robot arm. She will be the first to grab hold of the telescope when it finally comes into view on the third day of the mission, and, as she pointed out during a news conference last week, she will be the last one to let go of it when the shuttle departs.  Seven years of loneliness have taken their toll on the telescope.  Only two of Hubble’s scientific instruments are fully functional: the Wide-Field Planetary Camera 2, which was installed 16 years ago, and the fine guidance sensors, which measure star positions.  Hubble’s main camera, the Advanced Camera for Surveys, lost two of its three channels to mechanical failure in 2006; a spectrograph, known as STIS, failed in 2005; an infrared camera known as Nicmos has not worked since last fall when it was shut down as part of the preparations for the coming servicing mission and then failed to restart, probably because it is clogged with ice crystals. Engineers think they will be able to restart it after the astronauts leave.  In addition to replacing Hubble’s batteries and gyroscopes and the data router, the astronauts will install two new instruments on the telescope. They will replace the old wide-field camera with the Wide-Field Camera 3. And they will install the Cosmic Origins Spectrograph in place of a set of corrective lenses called Costar that astronauts installed in 1993 to correct Hubble’s original flawed vision. All the instruments added since that first servicing mission include corrective optics, so Costar is no longer needed.  All that is routine, as far as spacewalk tasks go. A more daunting job will be to repair the STIS spectrograph and the Advanced Camera, neither of which was designed to be worked on in space.  “Hubble needs a hug,” said Dr. Grunsfeld, “and we’re ready to go.”  A version of this article appears in print on , on Page D3 of the National edition with the headline: Shuttle Crew Prepares a Long-Delayed Last Goodbye for Hubble. Order Reprints| Today's Paper|Subscribe
On May 11, if all continues to go well, a crew will blast off on an 11-day maintenance and repair mission that will leave the Hubble Space Telescope at the peak of its powers.