Seven NASA astronauts have a challenging mission ahead to pay one last service call to the Hubble Space Telescope in less than two years' time.

On Tuesday, NASA chief Michael Griffin gave shuttle commander Scott Altman and his six crewmates the green light for a May 2008 mission to repair and upgrade the 16-year-old orbital telescope.

Altman, who commanded NASA's last Hubble-bound shuttle flight--STS-109 in 2002--is joined by fellow servicing mission veterans John Grunsfeld and Michael Massimino for the upcoming $900 million spaceflight. First-time flyer Gregory Johnson is serving as pilot alongside mission specialists Megan McArthur and Andrew Feustel, who will also make their orbital debuts during the upcoming spaceflight.

"Obviously, I'm very happy to be here and thrilled to be on this team," McArthur told reporters Tuesday. "It's still sinking in."

NASA initially cancelled the upcoming Hubble servicing mission in 2004 following the 2003 Columbia accident, but later studied the possibility of a robotic servicing mission before returning to an astronaut-led spaceflight.

With NASA's post-Columbia accident shuttle flight improvements and focus on astronaut safety, Altman said he is convinced that the 2008 Hubble flight will be much safer than his STS-109 mission.

"I feel very confident that we've got the whole puzzle put together and the pieces laid out in front of us," the shuttle commander said.

Known as Servicing Mission-4 (SM-4), the 12-day Hubble flight is currently designated as STS-125 and slated to launch aboard NASA's Discovery orbiter sometime during a six-month window that opens in May 2008, space agency officials said Tuesday.

"It's not like we're planning from scratch," Chuck Shaw, NASA's mission director for the Hubble servicing flight, said in a telephone interview. "We're actually picking up where the earlier SM-4 planning left off."

The spaceflight will be NASA's fifth and final mission to maintain Hubble before the agency's three-orbiter fleet is retired in September 2010. About 14 other shuttle flights are dedicated to the completion of the International Space Station (ISS).

"We're scoping out an actual date in May 2008," said NASA's Jennifer Wiseman, Hubble program scientist at the space agency's Washington, D.C. headquarters, in a telephone interview. "The new science instruments have already been built."

Altman and his six shuttle crewmates expect to launch towards Hubble from NASA's Launch Pad 39-A at the agency's Kennedy Space Center spaceport in Cape Canaveral, Florida. A second shuttle must already be atop KSC's other launch site--Pad 39-B--to serve as a rescue shuttle should the Hubble-bound orbiter suffer critical damage during liftoff or in orbit, NASA officials said.

Unlike NASA's ISS-bound shuttle crews, which can take refuge aboard the orbital laboratory for at least two months should their spacecraft suffer serious damage, a rescue mission for the 2008 Hubble flight would have to launch within 25 days of the emergency to aid Altman and his crew. The astronauts will, however, carry extra food and other supplies inside their spacecraft cabin and cargo bay to support that contingency, which NASA officials feel is extremely remote.

"We have a strategy that I think equalizes the risk," Altman said, adding that a rescue shuttle would not dock with his own orbiter.

Instead, the two vehicles would be linked by a robotic arm that would serve as a bridge to evacuate a crew from a stricken craft, he said, adding that the entire process would take the better part of a day.

Hubble upgrades

Altman and his crew will deliver a new Wide Field-3 camera and Cosmic Origins Spectograph (COS) among the 22,000 pounds (9,979 kilograms) of hardware they will haul up to Hubble to amplify its ability to observe some of the oldest objects in the universe.

Five spacewalks in as many days are required to install the new instruments, as well as replace all six of Hubble's vital gyroscopes and perform other necessary upgrades or repairs--including a first-ever orbital fix of the observatory's Space Telescope Imaging Spectrograph (STIS), which made the first detection of an atmosphere around an extrasolar planet before going offline in 2004.

"At the conclusion of this mission, Hubble will be at the apex of its capabilities," David Leckrone, senior scientist for Hubble at NASA's Goddard Space Flight Center, said Tuesday.

During the planned mission, McArthur will serve as chief shuttle robotic arm operator to aid spacewalkers and perform crucial heat shield inspections to determine the orbiter's health. Grunsfeld, Massimo, Feustel and Good will perform the five extravehicular activities (EVAs) in teams of two to be arranged to have one veteran and one rookie working outside at any given time.

"I kind of feel like I found my cause in life servicing the Hubble Space Telescope," said Grunsfeld, who will be making his third trip to the orbital observatory with the upcoming mission. "I think we have a challenging mission ahead of us."

The Hubble story so far:

• NASA Cancels Shuttle Mission to Service Hubble
• Public Bombards Operators to Save Hubble
• NASA Chief: 'Let's Go Save the Hubble'
• Canadian Robot Top Choice for Hubble Servicing Mission
• Study: Hubble Robotic Repair Mission Too Costly
• NASA Prepares for Possible Shuttle Mission to Save Hubble
• NASA Review: Hubble Headed For Deorbit-Option Only
• Health Checkup: Engineers Work to Stall Hubble's Death
• Hubble Puzzle: How Safe is a Shuttle Servicing Mission?
• Hubble Saved: NASA Approves Shuttle Flight to Service Space Telescope

Podcast: Hubble: The First Great Space Observatory