NASA Troubleshoots Discovery Sensor Problem

RENEE MONTAGNE, host:

This is MORNING EDITION from NPR News. I'm Renee Montagne.

NASA engineers are struggling to get to the bottom of a mysterious technical problem that kept the space shuttle Discovery from being launched yesterday. Astronauts were already on board when word came that the launch was a no-go. They now need to wait at least days, if not weeks, for another chance. The problem is with the fuel sensor system. NPR's Richard Harris reports from the Kennedy Space Center in Florida.

RICHARD HARRIS reporting:

A thunderstorm swept through Cape Canaveral just as the astronauts were heading out to the pad, but the shuttle remained bathed in sunlight, seemingly a good omen for launch. That was not to be. With about two and half hours to go in the countdown, NASA unstrapped the astronauts and drove them back to their quarters. Manager Wayne Hale broke the news formally at a late-afternoon news conference.

(Soundbite of news conference)

Mr. WAYNE HALE (Manager): All I can say is shucks. We came out here all set to go today. We've been working really hard to be ready to go, and we incurred a problem. Took us about five minutes of discussion to confirm that and decide that it was time to try another day.

HARRIS: The problem was with a fuel sensor inside the shuttle's huge external tank. The sensor is designed to make sure that if the shuttle is about to run out of fuel, the engines will shut down first. Running a shuttle engine without fuel is a very bad idea. So Hale said there are four sensors designed to make sure that the engines will stop if the tanks of liquid hydrogen and oxygen fuel start to run dry.

(Soundbite of news conference)

Mr. HALE: We have a very clear and unambiguous criteria that says all four of those sensors must work to provide us the kind of redundancy and reliability that's necessary for safe flight. So when one of those indicators started acting up today, we decided it was time to quit.

HARRIS: The most vexing thing about the problem is that it's been kicking around for a few months. NASA administrator Michael Griffin says try as they might, engineers have not been able to pinpoint the cause, and that's disconcerting because it can suddenly appear and suddenly disappear. Griffin says the launch could have been perfectly fine or the sensor could have gone bad when the engines were still blazing away. Because of the three other sensors, losing one probably wouldn't have caused a problem, but nobody wanted to take that chance.

Mr. MICHAEL GRIFFIN (NASA Administrator): It's an intermittent--it's so far an unexplained anomaly. When we can explain it, we will, and then we'll be able to go back and say whether we would or wouldn't have found it, given that we had done another test. But we're not there yet. The hardest of all these kinds of electronics problems to work is always an intermittent.

HARRIS: Last time this cropped up was at a shuttle test in April. Engineers never did figure out what was wrong. It could be the gauge inside the fuel tank or the wiring or the electronics that read the signal from the sensor on board the shuttle itself. They replaced a bunch of parts and ran a bunch of tests that all came out OK. So Hale said he and his colleagues ultimately decided to press on.

Mr. HALE: You would really like to be able to say, `This was it and I can prove conclusively,' but, gosh, life's not like that all the time. Sometimes you have to go with the best test you got, and we became comfortable as a group, as a management team, that this was an acceptable posture to go fly in.

HARRIS: Engineers plan to meet this morning to develop a plan for troubleshooting the fuel gauge and ultimately fixing it. Hale said repairs could take some time.

Mr. HALE: We did a little review of the absolute best-case kind of scenario and decided that we would not, in any conceivable way, be ready to launch before Saturday. So that's probably the very best-case scenario, and we're going to go where the technical data leads us until we solve this problem and get to a safe posture to go fly.

HARRIS: If NASA has to wheel the shuttle back to its hanger and really tear into it or the tank, the launch could easily be put off for a long time. If the shuttle isn't ready to blast off by the end of July, the next opportunity won't be until September.

Richard Harris, NPR News, at the Kennedy Space Center in Florida. Transcript provided by NPR, Copyright NPR.