Engineers troubleshooting the apparent failure of two low-level hydrogen fuel sensors in the shuttle Atlantis' external tank say the problem appears to be the result of an open circuit. Whether that is true or not, and whether the problem requires repairs, is not yet known. While troubleshooting continues, however, the launch team is recycling the countdown to permit a second launch try Friday, at 4:09:13 p.m., to kick off a long-awaited flight to deliver Europe's Columbus research module to the international space station.

"Of course, we're a little disappointed in the events today," said Launch Director Doug Lyons. "But we're certainly working to resolve our issues and make an attempt as soon as we possibly can."

With launch scheduled for 4:31:45 p.m. today, engineers began loading a half-million gallons of supercold liquid oxygen and hydrogen rocket fuel into Atlantis' external tank at 7:06 a.m. Propellants flow into the tank from the bottom and as soon as the four engine cutoff sensors at the base of the hydrogen section were submerged, engineers began a series of tests to confirm they would operate properly in flight.

The ECO sensors are part of a backup system that ensures the shuttle's three main engines don't drain the tank in the event of other problems that might prevent an on-time shut down. The sensors can indicate two possible states: wet or dry.

During today's test, commands were sent to simulate dry conditions to make sure the circuitry responded properly. Voltage readings from two of the sensors immediately indicated a dry state while sensors 3 and 4 showed voltages higher than 13.5 volts, an indication of an open circuit. The readings occurred simultaneously.

"The failure occurred during tanking, 16 minutes into fast fill," Lyons said. "We picked it up while implementing our standard checkout of the system. As soon as we get these sensors wet, we go through a battery of checks to make sure they're operating nominally and properly. We were at a point in the test where we sent commands to take all four sensors dry. When we did that, sensors 1 and 2 went dry as expected and sensors 3 and 4 went wet. And right then, we knew we had an issue.

"We stopped and picked up an interim problem report. We have pre-planned troubleshooting procedures and we put those in place and started working through our troubleshooting and collecting data to try to understand exactly what the situation was and why we were having this problem. So we remained in that configuration and continued tanking and did our troubleshooting and collected all the data we could possibly collect.

"The preliminary indications are we have an open circuit there," Lyons said. "But again, we've got to do some additional engineering analysis and evaluation to see if that is the problem and then more importantly, where that open circuit is, whether it's a connector, a splice line or something of that nature. Once we isolate that, we can determine the appropriate corrective action."

Because of earlier problems with ECO sensors, NASA developed extensive troubleshooting techniques and ultimately seveloped an "exception" to a previous launch guideline requiring four operational ECO sensors. A launch could proceed, managers decided, if A) one hydrogen sensor failed wet; and B) engineers could show the problem didn't originate in the multiplexer-demultiplexer avionics system that controls the flow of data to and from the sensors.

As originally written, the flight rule exception called for standing down a day. A second launch try could then be made depending on an analysis of the way the sensor failed and how it behaved during a second fueling. With one sensor failed wet, two more ECO sensors would have to fail wet to pose the threat of running the tank dry. The exception did not cover the case of two failed sensors.

As for where the problem might be with the sensors aboard Atlantis, Lyons said "there's wiring from the point sensor box (in the engine compartment) that takes the readings, there's wiring in the orbiter aft into the external tank to the sensors."

"The sensors are located inside the LH2 tank and of course, they were functioning when they were installed and checked out," he said. "They've been through checkouts as we've gone through our processing flow from build ... all the way out to the pad. Of course, this is the first time they've seen cryos (cold fuel) so that's certainly something the engineering folks are looking at and may be contributing to this condition."

NASA's Mission Management Team plans to meet at 2 p.m. to discuss the results of troubleshooting and to make a decision about whether to proceed with another launch attempt Friday.

"We want to make sure we preserve the capability to go tomorrow if that's what the technical community determines is the right thing to do," Lyons said. "So we're keeping all our options open."

Atlantis' launch window closes Dec. 13 because of temperature and power issues related to the space station's orbit relative to the sun. The shuttle launch window reopens Dec. 30, but NASA managers have said if Atlantis misses the current window, launch would slip into early January.

Before returning to console to oversee de-tanking operations, Lyons said he was hopeful "we can work our way through this and get a few launch attempts in this window. So we still have hope and reason to believe that we're going to get off in December. That's what we're all shooting (for) and again, we're confident we'll get there."