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''Beautiful day for a spacewalk,'' chirped Jim Newman as he entered the payload bay on the morning of 7 March to begin upgrading the observatory in earnest. He and Massimino quickly set to work opening an equipment bay at Hubble's base and pulled out the Faint Object Camera (FOC), the last 'original' instrument still in use since April 1990. It had been out of action since 1997 and was the last instrument to rely upon the corrective optics 'bench' fitted during SM-1. That bench had already been stowed and was due for removal during the SM-4 mission in April 2004.

Sadly, at the time of writing, it seems unlikely that SM-4 will ever take place . . .

Filling the gap left by the FOC - and promising to significantly enhance Hubble's capabilities - was the 395-kg, telephone-box-sized ACS. Riding the cherry picker on the RMS, Newman first removed the old camera and temporarily attached it to Columbia's payload bay wall. Nearby, Massimino partially entered the telescope to install a cooling system cable harness that would later be hooked up the new NICMOS cryocooler. This had forced Grunsfeld to admit some good-natured jealousy of his rookie colleague before the mission.

''His head will basically be at the soul of Hubble because this is where all the light from the telescope comes through into the scientific instruments,'' he told an interviewer. ''The sad thing is, the cover on the telescope will be closed. For me, as an astronomer, that would be a very exciting place to be!''

Massimino, however, had little time to ponder where he was and quickly rejoined Newman to slot the ACS into its new berth.

''Okay, you're right over the lip of the telescope by about four inches,'' he radioed to Newman. ''Keep it coming. Come just a little left, a half an inch to the left. That's good.''

''I think we're going in,'' replied Newman, holding the new camera by a pair of handrails affixed to its outermost edge.

''Looks real good,'' came the guiding words of Massimino. ''Nice and smooth. You've got about one inch to go, maybe two inches. There you go. You got it.'' The two men then bolted the ACS firmly into place with a power socket wrench. Not surprisingly, the camera team was overjoyed. ''What the crew did was put the turbocharger on this telescope,'' ACS Deputy Principal Investigator Garth Illingworth of the University of California at Santa Cruz said of the new camera, which was expected to cover twice as much sky with twice the clarity and four times the speed of WFPC-2.

Its sensitivity was so great, said Hubble scientist Holland Ford of the Johns Hopkins University before the flight, that it would be able to discern two fireflies positioned 2 metres apart in Tokyo - from a vantage point in Washington!

Among the most exotic applications of ACS is the photographic detection of planets outside our Solar System. Although so-called 'extrasolar' planets had been identified through indirect means, research scientist David Golimowski of the Johns Hopkins University expressed optimism that they could be detected around several nearby stars. ''It's a long shot,'' he admitted, ''[but] if all the planets align just right and if we have very good fortune in the telescope performance and also in the way we plan the observations, there is a possibility that we could image a planet.''

Golimowski's team was particularly hunting for reasonably large, Jupiter-sized worlds, which he estimated to be around a billion times fainter than their host stars. In order to pick out such dim objects, the ACS was equipped with a coronagraph to blot out most of the starlight so that it could record much less intense reflected light from nearby objects. Candidate hosts included nearby Alpha Centauri, for which at least 400 minutes of observing time was already booked for planet-hunting before the ACS had even been launched, and a brown dwarf called Gliese 229B.

If the latter, which orbits a 'normal' star, does indeed harbour a planetary world, Golimowski expected it to 'wobble' slightly under the gravitational tug of its smaller companion. In addition to searching for planets outside our Solar System, Garth Illingworth added that the ACS would help astrophysicists to investigate the history of the cosmos by exploring the fringes of the 'dark age' in the Universe.

''In the next decade, that's the place where astronomers are going to be looking to see when the first light of the Universe was actually happening,'' said Wendy Freedman of the Carnegie Observatories in Pasadena. ''When did the Universe light up with stars? When did the galaxies actually form? With the advanced camera, I think we'll get our first glimpse of some early galaxies.'' Added Holland Ford: ''We will be able to enter the 'twilight zone', when galaxies were just beginning to form out of the blackness following the cooling of the Universe from the Big Bang.''

After its 10-week checkout, it was anticipated that the ACS would detect more faint objects during its first 18 months than had been picked up by all previous Hubble instruments. Ford cited the vast amount of data from the 'Deep Field' observations made by the telescope in December 1995: for 10 days that month, it had been aimed at a patch of sky free of nearby galaxies - roughly the size of a grain of rice held at arm's length - and WFPC-2 shot 342 images which allowed astronomers to identify 1,500 galaxies dating to within a billion years of the Big Bang.

With the ACS, it was expected to do similar 'Deep Field' observations in a fifth of the time. Together with the revived NICMOS, it was hoped that the camera could play a part in identifying exploded stars known as 'type 1A supernovae', the light from which strongly suggests that the Universe's rate of expansion is increasing rather than decreasing. The nature of the mysterious force powering that acceleration, known as 'dark energy', has been described by David Leckrone as possibly ''the most important question in the physical sciences today.''

Jim Newman removes the Faint Object Camera (FOC) from Hubble to make room for the soon-to-be-fitted Advanced Camera for Surveys (ACS). In the background, Mike Massimino waits inside the telescope's equipment bay.

Celestial objects of particular fascination for ACS researchers include intrinsically bright, but very distant quasars at the very edge of the detectable Universe, which are thought to have supermassive, 'feeding' black holes at their centres. With the ability of the coronagraph to block out the bright cores of the quasars, it was hoped to gain more insightful views of the physical conditions in their outermost reaches. ''We're looking forward to taking images of quasars,'' said Holland Ford, ''and seeing the structures that surround [them] much better with ACS' higher resolution and higher sensitivity.''

In its first three years of operations, the ACS has captured a stunning collision between a pair of spiral galaxies known as 'the Mice' - which may illustrate what could happen if our Milky Way collides with the Andromeda galaxy several billion years into the future - and revealed vivid colours and glowing 'ridges' of gas in a stellar nursery within the Swan Nebula. Closer to home, it also observed a craggy-looking 'mountaintop' of cold gas and dust called the Cone Nebula.

On 7 March 2002, however, after successfully installing the ACS, Newman and

Massimino had scarcely time to respond to the congratulations from their colleagues on Columbia's flight deck, before plunging straight into their next task: preparing for the fifth spacewalk by installing an electronics module to power the NICMOS cryocooler. This was duly mounted on the 'floor' of Hubble's aft equipment bay. After finally closing the doors to the bay, the two men removed thermal and light shields that had been used earlier to support the PCU replacement.

Eighty percent of STS-109's spacewalks had been completed and yet, said Digger Carey, there was no opportunity for the crew to rest on their laurels. ''These are five long days,'' he had told an interviewer before Columbia lifted off. ''One of the main challenges on the mission is to keep that mental focus: to wake up each morning and say, 'Okay, this is the big day. Got to get this done today. Got to get through the day and do everything perfect.' And then waking up the next morning and having that same attitude.''

There was simply no option, Carey added, to get through the last EVA by coasting; nor was the hard part of the mission 'over' as soon as Hubble was redeployed, or as soon as the crew got through re-entry or even after wheels-stop on the runway. He remembered some advice from Nancy Currie, who had been through three missions before STS-109: ''She said, 'The only time I feel like I can relax is when I'm outside the orbiter and I'm in that astronaut bus to go back to crew quarters. Then you can relax!''

It was, therefore, with renewed vigour that the crew prepared for Grunsfeld and Linnehan's spacewalk on 8 March to install the cryocooler and, hopefully, reactivate the $110-million NICMOS. This instrument had been sorely missed by the astronomical community since January 1999, ''especially by those of us who study protoplanetary and debris disks around young stars'', said Ray Jayawardhana of the University of California at Berkeley. Although ground-based telescopes were catching up to Hubble's resolution, thanks to new technologies like 'adaptive optics', the return of NICMOS was still awaited with great excitement.

Mark McCaughrean of the Astrophysical Institute in Potsdam, Germany, for example, had used the instrument before it shut down and felt it ''definitely has a few tricks up its sleeves'', including the capability to observe celestial objects in certain portions of the infrared band. Cool stars and young giant planets, he said, contain steam in their atmospheres, which has proved difficult to study from the ground because of the intervention of our atmosphere. It was the ability of NICMOS to conduct infrared observations from above the atmosphere that made it so valuable.

The problem with NICMOS was its cooling system - essentially a giant thermos flask containing a block of solid nitrogen - which had sublimated away prematurely when an internal short brought it into contact with the surrounding structure. The instrument required cooling to remove its own excess heat and thus prevent this heat from interfering with the sensitive infrared observations. Columbia's crew carried a new mechanical chiller, developed jointly between NASA and the US Air Force, into orbit to circulate ammonia-cooled neon through NICMOS' imager. This was expected to keep its infrared detectors at an acceptable temperature of 60 Kelvin.

''Kind of like installing an external air conditioner in a house for the first time,'' was how Grunsfeld described fitting the tricky cryocooler. The device itself was experimental in nature, meaning that - although NICMOS' revival was highly desirable from a scientific standpoint - it was at the bottom of the list of priorities for STS-109. It comprised a series of ultra-high-speed microturbines, spinning at nearly half a million revolutions per minute, which had been successfully demonstrated four years earlier as part of the Hubble Optical Systems Test (HOST) experiment on STS-95. This was to be an operational test of the unit.

The installation of the $21-million cryocooler got underway at 8:46 am on 8 March, with Linnehan riding the RMS to firstly open the aft shroud doors on the telescope and then pick up the new device from its storage location in Columbia's payload bay. ''I felt like we were opening the doors to a sacred shrine,'' Grunsfeld later wrote in his journal, ''going inside the area where the scientific instruments on Hubble live. In our training, we were taught to have the utmost respect for the delicacy of the instruments and to treat them with kid gloves. Inside the aft shroud, I tried to move as carefully as I could, even though I was in a bulky and clumsy spacesuit. The interior of the telescope is as clean and pristine as it was when launched. After removing a serpentine vent hose that was originally used for venting gas from the old solid nitrogen cooler on NICMOS, we installed the cryogenic cooler itself. It looked a bit odd to take the pristine, almost spartan interior of Hubble and add a box covered with cables and hoses and valves.''

The men laboured through orbital daylight and darkness to fit it and connect cables from the electronics support module - installed the previous day by Newman and Massimino - to the new 136-kg cryocooler. Their next step was to attach a 4-m-long radiator to the outside of the telescope to expel NICMOS' waste heat into space when fully operational. At first, this proved problematic. ''It wouldn't go on!'' recalled Grunsfeld. ''We pushed and pushed, but try as we could, it didn't seem to line up. Rick and I took it off Hubble and realigned it by moving the latches a bit. This time, the alignment looked good, but still we couldn't push it on. With some significant effort, Rick got his handrail on and I took one latch at a time and pressed it into the telescope as hard as I could. After a couple of tries at my maximum effort, I squeezed the first, then second latch on the handrails. A small success!'' Linnehan, stationed 'underneath' Hubble, then fed tethered wires from the radiator through a hole in the base of the telescope to Grunsfeld, who hooked them up to NICMOS itself.

''I felt as if I was ice-fishing,'' wrote Grunsfeld, ''although I could actually see Rick through the hole. He caught the tether, attached it to a long set of cables and cooling lines and I pulled it through the hole right into where the science instruments live. It was like a giant boa constrictor and much stiffer than we had seen in training. Rick joined me and we began the process of hooking up electronics connectors and the ammonia cooling line to the [cryocooler].'' Their task completed, the men set about cleaning up Columbia's payload bay and stowing their tools.

''Good job, you guys,'' enthused Currie from the aft flight deck.

The final minutes of the spacewalk also gave Grunsfeld an opportunity to ride the RMS, which had been deftly manipulated from within the flight deck by Currie and Altman over the past five days. ''What a view I had,'' the veteran astronaut exulted, ''[with] Columbia below, the bright blue Earth above and the Hubble Space

Telescope on my side. It seemed as if time was standing still: it was so touching a moment for me. My last activity was to remove a protective cover from an antenna on the bottom of the telescope.

''At the end of five spacewalks to improve the telescope,'' he continued sadly, ''I gave Hubble a final small tap goodbye and wished it well on its journey of discovery. It is likely I will never see the Hubble Space Telescope again, but I have been touched by its magic and changed forever.'' The five outings by Grunsfeld, Linnehan, Newman and Massimino had set a new single-flight Shuttle record of 35 hours and 55 minutes overall. Across the board, in all four servicing missions since 1993, astronauts had made 18 spacewalks and spent more than 129 hours repairing the telescope.

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