By Irene Klotz
Kennedy Space Center, Florida
The US space agency (Nasa) is debating whether to send astronauts on a mission to repair the Hubble Space Telescope.
Hubble was last serviced by the shuttle in 2002
Without another servicing call by shuttle astronauts, Hubble is expected to last another two to three years.
At the crux of this is whether to risk flying astronauts on the shuttle without the International Space Station available as an emergency shelter.
Nasa administrator Michael Griffin is expected to announce on Tuesday whether the mission will go ahead.
Nasa looked to the station as an orbital safe haven after the Columbia shuttle broke apart on re-entry in 2003.
Columbia's heat shield was damaged unknowingly during launch.
The shuttles cannot fly from Hubble's orbit to reach the station, so if a Hubble repair crew's ship was too damaged to safely fly home, Nasa has little time to mount a rescue before the shuttle's power runs out.
"A lot of things are lining up that says Hubble is going to be a do-able kind of thing," said deputy shuttle program manager John Shannon.
"The real question on Hubble is going to be the launch-on-need. You just don't have the orbital lifetime on a Hubble mission to be able to get another vehicle launched. It's going to be very tough.
"We're going to have to go into the Hubble decision not counting on the launch-on-need vehicle," Mr Shannon added. "That's the difficult question the agency's going to have."
If the mission is approved, astronauts will add two new science instruments and replace the telescope's batteries and gyroscopes, which are used to point and position the observatory.
The improvements and upgrades should extend Hubble's orbital lifetime to at least 2013, Nasa says.
Since the accident, Nasa has made three shuttle flights to test post-Columbia safety and design changes and to resume space station assembly.
The tests bode well for a Hubble flight, with astronauts successfully demonstrating how the shuttle's robot arm and a new extension boom could be used for thorough heat shield inspections during flight.
The boom also proved to be a stable enough work platform if spacewalking astronauts had to leave the shuttle to make any heat shield repairs. Shuttles now fly with an assortment of heat shield patches, fillers and application tools.
The biggest hurdle was to fix the problem that damaged the shuttle Columbia in the first place, namely foam falling off the ship's external fuel tank during liftoff.
Nasa's initial redesign failed, but its next two flights successfully showed far less tank foam shedding during the crucial moments after liftoff. Additional improvements will be made on tanks slated to fly next year.
Confidence in the programme has grown to the point where Nasa plans to resume in December nighttime launchings of the space shuttle, a practice that opens many more flight opportunities for station assembly missions.
Daylight launch requirements were imposed after the accident to assure cameras had clear views of the tank to detect any flyaway foam.
Nasa is under a strict deadline to finish the station by 2010 when the shuttle fleet is scheduled to be retired. Aside from the possibility of one flight to Hubble, targeted for 2008, all remaining missions are devoted to station assembly.
Hubble, which was launched in 1990, has become the world's flagship astronomical observatory, delivering to audiences worldwide unprecedented pictures of the universe and gathering information that has led to new understanding of how the cosmos formed and what it contains.
"I think everyone is quite hopeful that this Hubble mission can happen," said Harry Ferguson, an associate astronomer with the Space Telescope Science Institute in Baltimore, Maryland, "The political desire is there. Quite honestly, it's a technical decision now."