By Irene Klotz
at Kennedy Space Center, Florida
Engineers fully expect to be able to resurrect the Hubble Space Telescope's main camera, which shut down due to an electronics problem last week, but the long-term future of the popular observatory rests on shuttle Discovery's upcoming flight.
Hubble was last serviced by the shuttle in 2002
The US space agency (Nasa) plans to make a second attempt to return the troubled space shuttle fleet to flight with the launch of Discovery on Saturday. Lift-off is scheduled for 1549 EDT (1949 GMT).
The shuttles were grounded following the 2003 Columbia accident for safety upgrades primarily to the ships' faulty fuel tank. The first post-Columbia test flight in July 2005 revealed more problems.
Columbia's tank shed a suitcase-sized chunk of foam insulation during launch, which struck and damaged the orbiter's wing. The shuttle was destroyed and seven astronauts aboard killed as Columbia flew through the atmosphere for landing.
A piece of foam nearly as big fell off during Discovery's 2005 launch, but did not strike the ship. The incident spurred another year of redesigns.
Now, time is running short. The space shuttles, which face a firm 2010 retirement date, are the only vehicles that can finish assembly of the half-built International Space Station (ISS).
Hubble's future will depend in part on July's launch
They are also the only hope for extending the life of the Hubble Space Telescope, which was last serviced in 2002. Nasa wants to keep the observatory operational until 2013, when its successor, the James Webb Space Telescope, is scheduled for launch.
"I'm going to be holding my breath when this shuttle launches," said Ray Villard, an astronomer and spokesman with the Space Telescope Science Institute in Baltimore, Maryland.
After the Columbia accident, Nasa cancelled plans for a fifth and final shuttle mission to Hubble, saying it was too dangerous to fly a ship anywhere but the space station, where astronauts could await rescue if their craft was too damaged to fly home.
Now the agency is reconsidering, but first the shuttle programme must prove that it has resolved the potentially deadly problem of debris falling off and damaging the shuttle during launch.
If Discovery's mission is successful, Nasa expects to decide by October if Hubble gets a final house call, the agency's associate administrator for spaceflight Bill Gerstenmaier has said. The mission is targeted for December 2007.
In addition to new gyroscopes, batteries and other life-extending gear, astronauts would install a new camera on Hubble, as well as a highly sensitive spectrograph, which can split light into component wavelengths that researchers can then analyse.
"There are so many wonderful things we can do with the new instruments, but of course flight safety comes first," Villard said.
In addition to test-flying the shuttle's redesigned fuel tank, Nasa plans to dispatch British astronaut Piers Sellers and rookie flier Michael Fossum, two of Discovery's seven crewmembers, on a unique exercise during the mission's first spacewalk.
The pair will assess if a spindly, 15m- (50 foot-) long extension boom to the shuttle's robot arm can be used as a work platform in case future crews need to repair their ship during flight.
DISCOVERY SHUTTLE FLIGHT
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Lift-off: 1549 EDT, 1 July
Location: Kennedy Space Center, Launch Pad 39B
Objective: To test new safety equipment and procedures
Payload: Cargobay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter
The task would be particularly key to a Hubble servicing mission when sheltering a crew aboard the space station is not possible.
First Sellers and then Fossom will step into a foot restraint attached to the end of the extended boom. They will sway and bounce and lean in all directions so engineers can determine if the pole could support an astronaut for an actual heatshield repair job.
"I'm pretty optimistic it's going to work," Sellers said.
Hubble's flight team has more immediate concerns. The telescope's Advanced Camera for Surveys shut down last Monday due to what engineers believe to be a problem with an electronics circuit. There is a backup system aboard, though it has not been tested since before Hubble's launch in 1990.
"We fully expect that it will be working," said Hubble deputy programme manager Edward Ruitberg.
The camera's recovery work is expected to last several weeks.