CAPE CANAVERAL - Shuttle Discovery will fly to space on a different external fuel tank than planned.

NASA managers, meeting Friday afternoon, decided it will be faster and easier to make safety modifications to a second unattached tank than the one already mounted to Discovery at the pad. In fact, work to fix the second tank began earlier this week.

Discovery will be brought back from the launch pad around Memorial Day and connected to the fuel tank and solid rocket boosters that were set to propel Atlantis on NASA's second post-Columbia shuttle mission later this year. NASA then plans to haul Discovery back to its launch pad in mid-June for a launch sometime between July 13-31.

"We're moving forward, targeting the July window," Kennedy Space Center spokesman Michael Rein said.

The tank switch means work to fix several serious safety problems can go on at the same time, increasing NASA's chances of getting Discovery off the ground on schedule for its mission to to the International Space Station.

Inside Kennedy Space Center's Vehicle Assembly Building, workers are outfitting the replacement tank with a new heater, which is designed to prevent ice from growing on a pipeline that runs along the outside of the 15-story orange fuel tank.

New tests show ice chunks big enough to do disastrous damage to the shuttle's heat shield can form in places along the 70-foot long pipe during the countdown. If ice shook free during launch, it could spur a repeat of the Columbia accident when fuel tank debris punched a hole in the shuttle's wing. Seven astronauts died when Columbia's breached heat shield failed on their return to the Earth.

Meanwhile, four miles away at Pad 39B, the shuttle launch team will conduct a second fueling test using the tank already connected to Discovery. The extra test would give the engineers and managers a chance to learn more about a still-mysterious problem with main engine valves and fuel sensors that function like an automobile gas guage.

The problems with the propulsion system, which cropped up during the first fueling test in April, could be serious. If the sensors malfunction after liftoff, Discovery's main engines could shut off too soon. That would force Commander Eileen Collins to attempt an unprecedented and incredibly dangerous emergency landing at KSC or overseas.

The heater installation and engine sensor issue were among a list of unresolved issues that last week prompted NASA management to delay the launch of the first shuttle flight since the Columbia disaster by two months, from May to July.

Atlantis is scheduled to follow on a second test flight launching sometime between Sept. 9 and 24 but must be ready earlier. Atlantis will be on rescue duty for the first mission. If Discovery is so badly damaged on its flight that NASA decides it is too dangerous to try to fly home, its astronauts will wait on the space station for a rescue shuttle. Atlantis must be ready to launch on that mission within about a month before the station's food, water and breathing air run out.

So, after Discovery is off its current tank-and-booster set, NASA will install the same heater on that external tank. Atlantis will fly on that set.

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