Strictly Nonstation Missions Only

''It's great to be back in zero-g again,'' said STS-93 Commander Eileen Collins early on 23 July 1999, as she and her four crewmates set about preparing Columbia for five days of orbital activities, but added darkly: ''A few things to work on ascent kept it interesting.'' Those 'things', within seconds of blasting off, almost forced Collins -the first woman to lead a space mission - and Pilot Jeff Ashby to perform a hair-raising and never-before-tried emergency return to KSC and grounded the Shuttle fleet for almost six months.

Columbia's 26th flight was a long time coming. More than a year had elapsed since her Neurolab mission but the cause of the delay had nothing to do with NASA's venerable old workhorse herself. Originally scheduled for launch at the end of August 1998, it was postponed until just before Christmas, then January 1999, then April and ultimately midsummer by a chain of technical problems with her primary payload for the STS-93 mission - NASA's $1.5-billion Chandra X-ray Observatory (CXO) - and the Inertial Upper Stage (IUS) booster that would propel it into its unusual orbital slot.

Given the delays, it is a pity that Shuttle managers did not opt to refly Neurolab in the summer of 1998 because, with only five missions achieved overall that year - as opposed to the normal seven or eight - the manifest for the reusable spacecraft was remarkably light. That was not to imply that no missions awaited the fleet: more than 40 flights to assemble the International Space Station were waiting in the wings, as was most of the already-built hardware they would truck into orbit, but repeated delays to crucial Russian components had temporarily stalled the construction effort.

By the time the first assembly mission, STS-88, had finally carried a US-built connecting node called Unity into orbit in December 1998 and bolted it onto the Russian control module, the project was already running billions of dollars over budget and years behind schedule. More trouble was in store: the habitat module, again Russian-built, would require another 18 months before it finally reached space, delaying the arrival of the station's first full-time crew until October 2000.

Columbia, sadly, would be excluded from this mammoth construction effort; as the oldest Shuttle, and despite numerous weight-saving measures during her numerous overhauls since 1984, she remained considerably heavier than Discovery, Atlantis and Endeavour and thus could not transport large pieces of station hardware into orbit. It was even more saddening and bitterly ironic that this decision was later partially reversed and, had Columbia not been lost on 1 February 2003, her next flight in November of that year would have been her inaugural trip to the International Space Station.

In the meantime, she was restricted to exclusively non-station flights: such as the deployment of Chandra, repair and servicing calls to the Hubble Space Telescope and a series of ill-defined 'research missions' that would, it seemed, utilise the Spacehab company's pressurised module rather than ESA's now-retired Spacelab. Clearly, although she would not be granted a significant piece of the station pie, Columbia was still a valuable asset to NASA and the Chandra mission demonstrated this: she was the only member of the Shuttle fleet capable of transporting the gigantic observatory into orbit.

The reason for this was Chandra's sheer size. When bolted onto its IUS and mounted on a supporting 'tilt table' in the payload bay, the observatory took up about 17.4 m of Columbia's 18.3-m-long storage capacity and weighed more than 22,600 kg. In anticipation of their back-to-back station assembly missions, the other three Shuttles had already had their airlocks structurally removed from inside the middeck and mounted inside the docking system in the forward quarter of the payload bay. The result: there was simply not enough room on Discovery, Atlantis or Endeavour to house Chandra.

Even Columbia needed to lose 3,200 kg of additional mass before she could take the observatory. To achieve some of these savings, engineers used older, lighter main engines, which lacked the newer, more rugged high-speed fuel pumps and combustion chambers. ''We put Columbia on a strict diet to get to this mission,'' said processing manager Grant Cates. ''That work actually began [in 1996] with the identification of this mission and the weight reduction that would be required.'' Nevertheless, her weight would still creep above NASA's normal safety limit if an emergency landing were needed shortly after liftoff.

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