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sgh_119至尊木虫 (职业作家)
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【Pure water】Discovery shuttle launch halted again after fuel leak
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The planned launch of the space shuttle Discovery for its final mission has been postponed again after a fuel leak. After 26 years of service, the vehicle is due to make one last flight to the International Space Station (ISS) before being retired to a museum. Nasa has struggled all week to get Discovery off the ground, frustrated by poor weather and technical problems. Escaping hydrogen detected midway through fuelling left Nasa no choice but to stand Discovery down once more. The earliest another launch can be made is Monday. US politicians have confirmed that the reusable orbiters should give way to a new era of spaceflight, and one by one these remarkable vehicles are taking a final bow. Fleet leader Discovery is the oldest of the surviving ships. First launched in 1984, it has since completed 38 missions, travelling some 230 million kilometres in the process. Its commander on the final mission, Steve Lindsey, says Discovery is probably the most important of three remaining shuttles. "It is obviously a very historical vehicle, having flown the 'return to flight' test missions after both the Challenger and Columbia accidents," he said. "It deployed Hubble (and) it's the fleet leader in terms of number of flights - it'll have flown about a year on orbit by the time we're done with it, which is pretty remarkable for a space shuttle." After Discovery returns, only the Endeavour shuttle has a firm date to launch, in February next year. Atlantis could fly in June if the budget allows. Beyond that, American astronauts will use Russian Soyuz rockets to get into space until a range of commercial US launch systems are introduced in the middle of the decade. Technical challenges The hydrogen leak dicovered two hours into the fuelling operation is considered very serious by Nasa managers because of the potential flamability of the gas. A similar issue delayed two shuttle launches last year. Engineers now have to wait until the fuel that had been loaded in the giant external tank is drained before they can get on site to find the source of the problem and fix it. This means the earliest Discovery can now lift off is in three days' time, although managers conceded even this would be difficult to make. Monday is actually the last opportunity to fly this particular shuttle before a period of unfavourable sun angles begins at the orbiting platform, making a docking very difficult. A new launch window would not open until 30 November. Discovery's six astronauts had yet to board the spaceplane when the leak was detected. When the ship does get up, she will deliver a storeroom to be attached to the ISS, along with much needed supplies and spares. |
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