When Columbia blasted off on Jan. 16, 2003, Servicing Mission 4 was scheduled for takeoff in early 2005. The major objectives of the flight were to install two new scientific instruments, to replace the observatory's batteries and to install a fresh set of gyroscopes.

O'Keefe's cancellation of SM-4 in January 2004 was based on his belief that a non-station flight was too dangerous. The decision touched off a storm on protest and as a result, NASA looked into the feasibility of an robotic servicing mission.

The goals of the unmanned mission included the attachment of a propulsion module that could drive Hubble to a safe, targeted re-entry at the end of its useful life. But such a robotic flight ultimately was deemed too technically risky and too expensive. Griffin has supported a shuttle servicing mission since coming aboard as NASA administrator last April.

With relatively minor exceptions, the goals of the current mission are virtually unchanged from the flight O'Keefe cancelled:

• Installation of the Wide Field Camera 3 (in place of the current Wide Field Planetary Camera 2), providing high-resolution optical coverage from the near-infrared region of the spectrum to the ultraviolet
  
• Installation of the Cosmic Origins Spectrograph, sensitive to ultraviolet wavelengths. COS will take the place of a no-longer-used instrument known as COSTAR that once was used to correct for the spherical aberration of Hubble's primary mirror. All current Hubble instruments are equipped with their own corrective optics
  
• Installation of six new nickel-hydrogen batteries to replace the power packs launched with Hubble in 1990
  
• Installation of three new rate sensing units, or RSUs, containing two gyroscopes each to restore full redundancy in the telescope's pointing control system
  
• Installation of a refurbished fine guidance sensor, one of three used to lock onto and track astronomical targets (two of Hubble's three sensors suffer degraded performance). The refurbished FGS, removed from Hubble during a 1999 servicing mission, will replace FGS-2R, which has a problem with an LED sensor in a star selector subsystem
  
• Attachment of new outer blanket layer - NOBL - insulation to replace degrading panels
  

"We believe right now it's very feasible and affordable to do that and we've looked at all aspects of the repair," said Preston Burch, Hubble program manager at Goddard. But he added, "it really needs to be regarded as a bit of an experiment."

NASA has no plans to launch a propulsion module. Engineers believe the observatory will remain in orbit through at least 2020 and possibly longer. As such, a propulsion module will not be needed for more than a decade.

"It was pretty straight forward," Griffin said. "If it isn't going to re-enter until the 2020s in the worst case, then I'm not going to spend money on it now."

But the Goddard team is considering the addition of a sophisticated grapple fixture on Hubble's aft bulkhead to make it easier for future astronauts, or even a robotic spacecraft, to attach a propulsion module at some point down the road.

"While we're up there, for a few extra bucks we could put a piece of structure on the back and some navigation aids that would make the next (manned or unmanned) visit a lot easier and a lot more friendly," Burch said. "We might put something like that on, but we have not made a final decision on that and we'd need headquarters concurrence."

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