By Irene Klotz
Kennedy Space Center, Florida
For three US astronauts, Nasa's decision to make a fifth servicing flight to the Hubble Space Telescope will mark a homecoming of sorts.
This will be John Grunsfeld's third trip to Hubble
John Grunsfeld, an astronomer by training, will be making a third visit to the orbital observatory, which on Tuesday won a reprieve when Nasa agreed to briefly interrupt International Space Station assembly missions for a final shuttle visit to Hubble.
The Hubble flight is targeted for May 2008.
Without servicing by a shuttle crew, the telescope is expected to last only another two or three years.
"I feel like a mission to Hubble is worth risking my life for," Dr Grunsfeld said at a crew press conference in Houston.
"It's something that's really important for our country and I firmly believe the next mission to Hubble will be much safer than the missions we've flown before."
Dr Grunsfeld will be reunited with his last commander, Scott Altman, and former crewmate Mike Massimino, all veterans of the fourth Hubble servicing flight which took place in March 2002.
Dr Grunsfeld also flew on the shuttle's third house call to Hubble in December 1999.
Joining the newest Hubble repair crew are four rookie fliers: pilot Greg Johnson, robot arm operator Megan McArthur and spacewalkers Andrew Feustel and Mike Good.
Nasa expects the flight to last 11 days and include five spacewalks to install two new science instruments, repair a third instrument and replace the telescope's batteries and gyroscopes, which are needed to position and point Hubble for scientific observations.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass: 11,110kg
Observations have probed about 24,000 celestial objects
Made more than 93,000 trips around our planet
Generates about 10 gigabytes of data each day
The crew also will install a docking system so a future spacecraft can deliver a propulsion module to direct Hubble to a safe atmospheric re-entry point over the ocean once the telescope's useful life has ended.
"We really don't, probably, have to go up there until the 2020 or 2025 timeframe," said Ed Weiler, head of Nasa's Goddard Space Flight Center in Greenbelt, Maryland, which oversees Hubble.
By then, the spacecraft being designed to replace the shuttles, which are being retired in 2010, are scheduled to be ferrying crews and supplies to the Moon.
"If the CEV (Crew Exploration Vehicle) can go to the Moon, it can probably take up a solid rocket motor to Hubble," Dr Weiler said.
The US space agency reinstated the Hubble servicing call following an in-depth review to assess how well safety upgrades implemented after the 2003 Columbia accident were working.
Nasa has flown three missions since the shuttle disaster, which claimed the lives of seven astronauts.
The agency's last administrator had cancelled the Hubble mission, saying it was too dangerous to risk a shuttle crew that could not seek shelter aboard the space station in case of serious damage to their ship.
A 'PIT STOP' IN SPACE
See how the Discovery shuttle will grapple Hubble
Shuttles flying to Hubble's orbit do not carry enough fuel to reach the station.
In agreeing to reinstate the mission, current administrator Mike Griffin ordered a second shuttle to be on the launch pad ready to fly a rescue mission in case the Hubble repair crew found themselves stranded in orbit.
"The safety of our crew conducting this mission will be as much as we possibly can," Griffin said. "We are not going to risk a crew in order to do the Hubble mission."
Devoting two shuttles to the Hubble mission will take some time out of the station's assembly schedule, but so far Nasa managers are confident they will be able to complete all 16 planned flights to the station before the shuttle's retirement.
Having a second shuttle on standby for launch will complicate plans for test flights of Nasa's new ship, the Crew Exploration Vehicle, which recently was renamed Orion.
The agency had planned to turn over one of the shuttle's two launch pads in Florida to the new programme, which will modify the complex.
Dr Griffin said the handover would still take place, but the pad must remain suitable for shuttle use until after the Hubble servicing mission.
"Flying the shuttle carries with it more risk than we would like, more risk than we once thought we had," Dr Griffin said. "It can be flown safely if we are very careful."
SERVICING THE HUBBLE SPACE TELESCOPE
Shuttle Discovery will grab Hubble with a robotic arm and pull it on to a work platform to allow astronauts easy access to its interior
Hubble has six gyroscopes that are critical to its control and pointing systems. These have started to fail and all will have to be replaced
Six new batteries will rejuvenate the electrical system; astronauts will attach new thermal blankets to insulate sensitive components
There are two instrument bays; two new instruments will be slid into racks made vacant by the removal of older instruments
An attempt will also be made to repair the Space Telescope Imaging Spectrograph (STIS) which stopped working in 2004