Masterful Performances

An astonishing sight greeted Grunsfeld and Linnehan as they floated out of Columbia's airlock at 6:37 am on 4 March to begin their first spacewalk: an unrestricted view of 'seas' of Saharan sand, together with stony plains, rock-strewn plateaux and interspersed with dark oases of life. The Shuttle was flying 580 km over a slumbering North Africa. ''Oh, wow!'' exulted Grunsfeld, now well into his element on his fourth Shuttle flight and - counting his SM-3A experience - his third spacewalk in total. ''Beautiful view!''

Next, turning to the glistening silver cylinder standing on end at the rear of the payload bay, he introduced himself to 'Mr Hubble', announcing that Columbia's crew had come to ''give you more power to see the planets, stars and the Universe''. Without further ado, Grunsfeld set to work removing the old starboard solar array from the telescope and Linnehan stowed it in a carrier on the FSS pallet. Next, Grunsfeld installed the diode box to ensure that power from the soon-to-be-fitted new arrays flowed into the telescope's batteries, and not vice-versa.

Meanwhile, after some difficulty removing the new, $9.5-million array from its storage canister, Linnehan - his feet anchored into a 'cherry picker' on the end of the RMS - carefully brought it into position. Grunsfeld, keeping himself steady by hanging onto one of Hubble's handrails, then connected it, cranked its two halves open and began wiring it to the diode box. Next, in readiness for the PCU replacement during the third spacewalk on 6 March, they placed thermal covers over vulnerable components to ensure that the telescope's sensitive scientific instruments would not freeze or overheat.

''You guys did a superb job today! We enjoyed watching you work,'' Capcom Mario Runco told the entire crew after Grunsfeld and Linnehan returned to the safety of Columbia's middeck. ''You made it look easy once again.'' The first spacewalk had lasted a minute over seven hours and its only real problem had been the failure of a telemetry sensor in Grunsfeld's suit, although the flight surgeon was still able to monitor his biomedical data. The problem was quickly corrected by resetting power to the suit's built-in communications system.

A similar procedure, albeit for Hubble's port-side array, was the order of business for Newman and Massimino early on 5 March. They were also charged with carrying out the replacement of the troublesome reaction wheel - which had actually, in the last few months, eclipsed the PCU changeout as the most critical task on STS-109 - during a spacewalk which lasted 7 hours and 16 minutes. As with so many other first-time spacewalkers, Massimino, a former professor of mechanical engineering before joining NASA's astronaut corps in 1996, was gripped with childlike wonder as he floated out of Columbia's airlock.

''This is incredible!'' were his first words.

''Welcome to the wonderful world of spacewalking,'' replied Grunsfeld from his choreography station on the aft flight deck.

''Thank you, John,'' said Massimino. ''Let's start with my first task.'' Most of his time was spent anchored to the end of the RMS, while veteran colleague Newman -making his fourth Shuttle flight and participating in his fourth spacewalk overall -scaled hand-over-hand up and down the side of the telescope. Using a pistol-grip power socket wrench, Newman made short work of unbolting the old solar array and quickly handed them to Massimino, who stored them on the FSS pallet for return to Earth. Next, he hauled the new array and its relay box up to Hubble for Newman to install.

At times, not surprisingly, Massimino seemed to need reassurance from his veteran colleagues, particularly when moving the new array up to the telescope. ''You're gonna be fine,'' Newman told him. ''Keep it coming. Looking good. Very nice. Take a big breath and relax. You're doing a great job.''

After the pair had wired the port-side array into its diode box, Massimino removed the troublesome reaction wheel and carried it down to the payload bay, where Newman handed him a replacement which he promptly fitted. Initial validation tests performed by the Space Telescope Operations Control Center in Greenbelt, Maryland, indicated that both the new array and new reaction wheel were functioning perfectly. To wrap up their marathon excursion, the two men set up foot restraints and removed a thermal cover in anticipation of Grunsfeld and Linnehan's challenging PCU operation the next day.

The most challenging spacewalk of the mission began with a water leak from John Grunsfeld's $12-million suit; after first pondering whether to postpone the excursion for 24 hours, mission managers directed him to use one of the other, drier ensembles. It was subsequently decided, said EVA coordinator Dana Weigel, that the leaky suit would not be used again during the mission. The suit swap meant that he and Linnehan actually departed Columbia's airlock two hours later than intended at 8:28 am on 6 March and quickly set to work setting up their tools and tethers.

''A kind of late, but hopefully powerful start,'' Grunsfeld said as he ventured outside. His first task to fit thermal covers on the diode boxes of the new solar arrays and deploy the thermal shields set up by Newman and Massimino the previous day. Linnehan, his feet again fixed to the end of the RMS, then began the process of disconnecting Hubble's six batteries, effectively cutting off power to the telescope at 9:37 am. This gave the two men, at most, eight hours to replace the refrigerator-sized PCU.

Complicating matters was that it was encircled by bulky bundles of cables, which made the 36 electrical connectors incredibly difficult to reach. In fact, they were spaced so tightly that Grunsfeld and Linnehan had to use a wrench, rather than their gloved hands, to remove them. ''What makes it difficult is, as you're facing the PCU, those connectors are on the left-hand side: they're not staring right at you,'' said Bryan Austin. ''That's on the side that [the equipment] bay door is hinged. For the suited crewman to reach his hand in there, he's pretty much reaching in there blind. I kind of equate it to changing sparkplugs on your car. There's always those sparkplugs down there where you can't see real well. You've just gotta go down and feel and make sure you're oriented such that you're unscrewing it without a lot of offset force.'' The astronauts quickly set to work opening and latching the doors to the Number 4 equipment bay and Linnehan started at the top and worked down through the double rows of cables.

''The cables are pretty thick, John,'' he said as he peered into the open equipment bay, located a third of the way up the four-storey telescope.

''Yeah, I can see that,'' replied Grunsfeld. Undeterred, they methodically disengaged the connectors one at a time and fastened them temporarily to a cable caddy. Several rubber loops on the caddy broke, forcing them to tuck a few connectors behind the nearest wire harnesses, while still others were stiff and difficult to unfasten. Fortunately, they managed to keep close to the timeline and it took them barely four hours to unplug each of the connectors, remove the old unit and substitute it for a new version.

''These are not our typical, fully EVA-friendly connectors with the big 'wing tabs' on them that make it convenient for a suited crewman to manipulate,'' said Austin. ''We've worked really hard to have a special tool that we're going to use to get a good grip on these and get through this.'' Had Grunsfeld and Linnehan been unable to complete the full procedure, Hubble could have been left safely overnight, as long as at least 11 connectors to the PCU were in place. Fortunately, the operation proceeded without a hitch and by 2:02 pm the telescope came back to life.

''You did it, buddy! You did it!'' radioed a jubilant Linnehan with a laugh. ''Good job.'' Partial cutting of power had been done during SM-1 to install

As Nancy Currie operates the RMS, two of her colleagues labour outside in the payload bay on the towering Hubble Space Telescope.

corrective optics and again on SM-3A to fit the new brain, but neither went so far as the SM-3B requirement to have the entire telescope shut down for a period of several hours. Mario Runco radioed the welcome news that all seven astronauts were waiting for: ''Columbia, Houston, with a post-operative report. We have a heartbeat!'' At a press conference later that day, Anne Kinney called their work a ''dramatic and masterful performance''.

It was a triumphant end to a series of problems with the old PCU that spanned nine years. In fact, a simple loose screw had impeded its ability to satisfactorily route electricity from the solar arrays to the instruments, control systems and batteries. Without the surgery, it would have been impossible for Hubble to run more than one of its instruments simultaneously. ''As with any beloved relative, you're worried about sending them in for bypass surgery or even a heart transplant,'' said the telescope's chief scientist David Leckrone, ''but you realise the risk of not doing it is severe.''

For John Grunsfeld, who half-joked before the mission that failure would mean he could never show his face at meetings of the American Astronomical Society again, it was perhaps the pinnacle of his astronaut career. The following year, within months of the STS-107 disaster, he was transferred to NASA's Washington headquarters to become the agency's Chief Scientist. His journal notes of the critical PCU replacement highlight the intense levels of "concentration, patience and a little bit of skill'' that he and Linnehan demonstrated during their 6-hour 48-minute spacewalk.

"On board Columbia,'' Grunsfeld wrote of that evening's activities within the safe confines of the Shuttle's middeck, "I went to sleep satisfied I did an honest day's work, and very tired.''

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