In such a dire eventuality, Columbia would tip the scales at 113,000 kg, some 590 kg heavier than safety rules determined to be the maximum-allowable landing weight. In the case of STS-93, a one-time-only waiver was granted to this rule, based on a detailed analysis of the payload, the Shuttle's centre-of-gravity constraints and a host of other interrelated factors. For Eileen Collins, the challenge of possibly having to perform a heavier-than-normal emergency landing did not faze her: ''We would land at 205 knots, which is very close to the maximum certification [of] around 214. There are some challenges there, [but] I feel very confident we've looked at the abort landings and they're well within the safe limits
of landing the Shuttle.'' Collins' confidence in her abilities and those of her crew would come very close to being tested. Joining her and Ashby on board Columbia were Mission Specialists Cady Coleman, the notoriously launch-delay-prone Steve Hawley and Frenchman Michel Tognini. All but the Pilot had flown into space before: interestingly, Ashby had originally been assigned to another mission in the summer of 1997 but had resigned that post in order to care for his cancer-stricken wife.
In some ways, STS-93 harked back to the early days of the Shuttle programme. Not only would it last a mere five days - making it NASA's shortest-planned mission for almost eight years - but it would also feature the deployment of a payload on top of the Boeing-built IUS. This two-stage, solid-fuel booster had been used on several occasions to deliver a series of TDRS satellites and a number of military payloads into high orbits; additionally, it had supported the launches of the Magellan and Galileo space probes to Venus and Jupiter, respectively, and the Ulysses solar polar explorer.
Nor was it exclusively a booster that could only operate from the Shuttle; many of its missions had utilised expendable launch vehicles, including the US Air Force's Titan IV rocket. Built by Boeing, under contract to the Air Force, the IUS for the Chandra mission was purchased by NASA's Marshall Space Flight Center. As its name implies, the 14,740-kg booster is inertially guided by its own onboard navigation equipment, and measures 5 m long by 3 m in diameter.
Its first stage ordinarily carried up to 9,800 kg of propellant and produced a total thrust of 20,000 kg; in fact, when it was built, it was advertised as capable of firing for up to 150 seconds, making it the longest-burning solid-fuel rocket motor ever developed for use in space. For the Chandra mission, it would carry considerably less propellant in its first stage - mainly due to the Shuttle weight issue - but to counteract this the motor's thrust was increased to 21,000 kg and its 'burn time' established at 125 seconds.
Meanwhile, the second stage typically carried around 2,700 kg of propellant and produced some 8,200 kg of thrust. Also housed in the second stage was an avionics system to provide guidance, navigation, control, telemetry, electrical power, command and data-management services to the booster, as well as supporting the payload itself. For its journey into orbit on the Shuttle, it was attached to a so-called 'Airborne Support Equipment' assembly, which resembled a giant mechanical doughnut with a central, circular opening for the booster's massive first-stage nozzle and 'tilted' the Chandra/IUS stack to the required angle for deployment.
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