An Astronomical Icon

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Beginning in the autumn of 2000, two Shuttle crews began training for Columbia's first missions after her modification period: the 16-day STS-107 research flight and the Hubble Space Telescope servicing. For a time, it was unclear as to precisely which order the missions would be flown, but eventually Altman's STS-109 flight took precedence. As NASA's scientific showpiece, it was essential that Hubble was kept operational until a next-generation telescope could replace it in 2010, and a number of important upgrades and new instruments were waiting in the wings to be installed in the observatory.

Sent aloft in April 1990, Hubble has become perhaps the most widely known and best-loved icon of astronomical technology. ''I think the most important thing Hubble has done is answer questions we didn't even know how to ask in 1990,'' said Ed Weiler, NASA's Associate Administrator for Space Science, before Columbia lifted off on STS-109. ''Hubble keeps telling us that our textbooks aren't being followed by the actual Universe.'' For at least one of the STS-109 astronauts, however, visiting Hubble to conduct repairs was more of a pilgrimage or a Holy Grail than 'just' a Shuttle mission.

''Servicing [Hubble] is by far and away the most meaningful thing I've ever done in my life,'' said John Grunsfeld, a professional astrophysicist before becoming an astronaut. In addition to STS-109, he flew a previous Hubble servicing in December 1999. ''People have to decide for themselves what kind of things they want to do [in life] and what they're worth risking and, for me, the Hubble is worth risking my life. It's that important. It's just an incredible worldwide resource. It's teaching us so much about our world, the Universe, who we are and our place in the cosmos.''

The telescope that received such praise dates from the 1970s and has been billed as nothing short of a cosmic time machine, capable of unravelling fundamental mysteries about the origin, evolution and fate of the Universe. ''That was the culmination of decades of work by hundreds of astronomers [and] thousands of engineers,'' said Weiler. ''It was probably the best day of our lives [when Hubble launched]. We all felt on top of Mount Everest that day." Already, Hubble had waited patiently for four years as NASA recovered from the loss of Challenger. Sadly, more trouble was afoot.

In June 1990, six weeks after it was placed into orbit, scientists realised with horror that the telescope's primary mirror had been polished to the wrong specification and was suffering from a complaint called 'spherical aberration'. In effect, its 2.4-m-diameter mirror had been ground too flat by only the tiniest of measures - a fiftieth the thickness of a single human hair - but more than enough to have a detrimental impact on Hubble's observations, blurring all of its images of stars, galaxies and other celestial objects and rendering it the butt of jokes rather than an icon to be revered.

Weiler has referred to that dismal time as like a journey from the lofty summit of Mount Everest down not to the surface of Death Valley, ''but perhaps six feet underl We had to explain to the American public that Hubble had a major flaw in its main mirror that would affect all the science we had to do.'' Nevertheless, a servicing crew was launched in December 1993 and resolved the aberration by fitting new corrective optics. Starting the following spring, the rejuvenated Hubble began a decade of work that has significantly changed humanity's perception of the Universe.

Among its notable discoveries is the possibility that the cosmos is younger than previously thought - around 13 billion years old, as opposed to earlier estimates of 15-20 billion years - and its observations have lent weight to theories that, rather than collapsing in a cataclysmic 'Big Crunch', it will continue expanding forever. Other research has yielded persuasive evidence for the existence of supermassive black holes at the centres of many galaxies, planets encircling distant stars and views of stellar nurseries and graveyards. Quite simply, no other telescope has done so much to rewrite textbooks in such a short space of time.

''It's Americana,'' said Rick Linnehan, who would accompany Grunsfeld on three spacewalks to repair the orbiting observatory during STS-109, becoming one of only a handful of astronauts to have flown Columbia three times. ''It's like cheeseburgers and Clint Eastwood. I mean, there was a Pearl Jam CD cover a year or two back that had one of the Hubble images on it. It just pervades everything now. That's really impressive because this is raw science - photographs coming down that not only are important to science, but they're beautiful. They're considered art!''

The photographic artwork flowing from Hubble has also included views of Martian dust storms and the once-in-a-lifetime collision of Comet Shoemaker-Levy 9 into Jupiter in July 1994. In terms of expense, the joint US/European project is estimated to have cost around six billion dollars in the quarter of a century from conception to the present day, including a decade and a half of orbital operations and four servicing missions by the Shuttle. From the outset, it was always planned that in order to have a fully operational, world-class, long-term orbiting observatory, there had to be an option to upgrade it in space.

By the late 1970s, the Shuttle was nearing completion and Hubble was accordingly equipped with grapple fixtures to allow the RMS arm to retrieve and anchor it into its cavernous payload bay for servicing. Seventy-six yellow handrails were attached to its hull to enable future spacewalking astronauts to crawl easily along its 13 m length to remove and replace scientific instruments. To ensure that it operated high enough to be outside the 'sensible' atmosphere, but low enough to be reached by the Shuttle, it was inserted into a 530-km orbit above the Earth.

Original plans called for the return to Earth, refurbishment and relaunching of the telescope every five years, with a servicing mission every couple of years to remove and replace worn-out scientific instruments. By the early 1980s, the possibility of contamination and extreme structural loads on the Shuttle during re-entry would severely hamper such a plan. Ultimately, NASA settled on repair missions at three-year intervals. Four were completed: an inaugural one to replace Hubble's solar arrays and fit corrective optics; a second to add new instruments; a third to replace the telescope's 'brain'; and a fourth - STS-109 - to conduct further upgrades.

To understand more fully the observatory that Scott Altman's crew finally saw as a dot in space on the morning of 3 March 2002, it is important to summarise the first three servicing missions, since Hubble had changed a great deal both internally and externally since April 1990. The first repair mission, in December 1993, fitted corrective optics, new solar panels and replaced the first Wide Field Planetary Camera (WFPC) with an improved model capable of operating with the new optics.

Three years later, in February 1997, another Shuttle crew removed two instruments and installed two new ones: a Space Telescope Imaging Spectrograph (STIS) to investigate the chemical composition of various celestial sources and a Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) to provide Hubble with the capability to conduct infrared imaging and spectroscopy. Unfortunately, the latter relied on cryogenic nitrogen to keep its detectors cool enough to function, and this became exhausted earlier than planned in the spring of 1999. That was a pity, because NICMOS had already produced impressive infrared results, exploring in particular the atmospheres of Uranus and Neptune.

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