Shuttle launch called off due to ‘anomaly’

Alan Boyle Science editor • Profile • E-mail

CAPE CANAVERAL, Fla. - The first space shuttle mission in more than two years was postponed less than three hours before its scheduled launch Wednesday when NASA encountered a problem with one of the external fuel tank's low-level sensors.

No new launch date was set, but deputy shuttle program manager Wayne Hale told a news conference that "we would not in any conceivable way be ready to launch before Saturday." Earlier, NASA Administrator Mike Griffin had told visiting congressional members that the next launch attempt could not take place until Monday at the earliest.

Workers at Kennedy Space Center were draining the external fuel tank, Hale said, and the team would likely have more data about the problem late Wednesday night. He said a decision about what to do next would not come before Thursday.

The current launch opportunity extends until the end of July, after which NASA would have to wait until September. For now, Discovery's crew was staying in Cape Canaveral, officials said.

Engineers had encountered problems with the same sensor system during a launch-pad test in April, and NASA was not able to track down the precise cause at that time. A second tanking test in May was successful, but a different tank was installed on Discovery last month. NASA didn't run the tanking test for a third time, but instead checked out the individual components and cleared them for launch.

"We felt like we had a good system today — we had another anomaly," Hale said.

When one reporter asked who would take responsibility for not conducting another tanking test, Hale said he didn't know whether another test would have turned up the problem. Then Griffin interjected, "Guys, this is an intermittent [problem]. It's so far an unexplained anomaly."

The shuttle crew, led by commander Eileen Collins, was already strapped into the shuttle when the launch was postponed at about 1:30 p.m. ET. The seven astronauts climbed back out and were driven back to their quarters while the launch-pad team secured the shuttle.

The space shuttle Discovery had been scheduled to lift off at 3:51 p.m. ET on a mission to resupply the international space station and test safety procedures that had been developed in the wake of the shuttle Columbia's catastrophic breakup in February 2003. All seven of Columbia's astronauts were killed in the tragedy, which led NASA to ground its shuttle fleet.

Until the sensor problem was discovered, the main worry Wednesday had to do with Florida's changeable weather, not technical problems. During the morning, the skies over Kennedy Space Center crackled with lightning, thunder and rain. But strangely enough, weather conditions were all "green" — acceptable for launch — when the postponement was announced.

How the system works
The sensor system works a bit like a car's gas gauge, signaling when the fuel tank's level sinks to 2 percent of capacity. When the gauge starts to read "E," the shuttle's control system readjusts pressure levels in preparation for shutting down the main engines. A faulty sensor could cause the shutdown to happen too early or late.

FREE VIDEO 'We had another anomaly' July 13: NASA's Wayne Hale explains why the launch was scrubbed. MSNBC

There are eight of these low-level sensors, also known as engine-cutoff sensors or ECO sensors. Four measure the liquid oxygen in the fuel tank; four measure the liquid hydrogen. Mission managers said one of the sensors for the hydrogen levels was apparently sending out suspicious readings during a test — basically, continuing to signal that it was covered with liquid hydrogen even when the system was set intentionally to read "E."

The criteria for launching the shuttle require all four sensors to be working, even though the system could handle two failures during launch, Hale said.

Mission managers said the source of the problem might be with the circuitry or wires outside the fuel tank rather than with the sensors themselves. If the sensor inside the tank is found to be at fault, the shuttle might have to be brought back to the Vehicle Assembly Building for the tank repairs — and in that scenario, launch would likely be delayed until September.

Earlier, another launch-day snag was handled more easily: During early-morning preparations for fueling up the shuttle's external fuel tank, engineers encountered a problem with a launch-pad heater. A replacement heater was sent to the pad for a quick change, and after an hour's delay, the fueling process and other preparations went ahead smoothly.

Mission managers said that their response to the day's glitches showed that NASA's safety process was working properly. "We built a very tight safety net to make sure we don't fly until it's safe," Hale said.

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