Sensor glitch may upset space shuttle launch plan

CAPE CANAVERAL, Fla., Sep 8 (Reuters) NASA fueled the space shuttle Atlantis for launch from Florida today but a problem with a sensor in the spacecraft's external fuel tank may prompt another delay, officials said.

NASA managers were discussing the glitch, which surfaced during a routine test while Atlantis was being filled with a half-million gallons of fuel for liftoff on a mission to resume construction of the International Space Station. The project was halted after shuttle Columbia disaster in 2003.

Today's liftoff was set for 1141 hrs (2111 IST) and meteorologists predicted a 70 per cent chance the weather would be suitable. Atlantis' launch had already been delayed nearly two weeks due to weather problems and technical glitches.

''We appear to have two options at this point,'' said launch commentator George Diller. ''Option one is to fly with three of four sensors (working) and ... the other option is to scrub for today and try again tomorrow.'' The sensor is part of a system designed to ensure the shuttle's main engines shut down before the tank runs out of fuel. One of the four hydrogen sensors failed to respond to computer commands simulating an empty, or dry tank.

The same problem delayed NASA's first post-Columbia mission.

Eventually, the glitch was traced to a particular batch of sensors and managers replaced the suspect instruments with new ones. Sensors inside Atlantis' tank are the newer models.

If the launch is delayed for tomorrow Atlantis' tank would be drained of the extremely cold fuel. When the shuttle was fueled for another launch try, engineers would be able to test if the sensor glitch reappeared.

The US space agency is already working on borrowed time, having wrangled extensions for launch attempts after negotiations with its prime space station partner, Russia. The outpost needs to be finished by 2010, when the shuttle fleet is scheduled for retirement.

NASA tried to launch Atlantis and its six astronauts as early as August 27, but a lightning strike at the launch pad and then the threat of a tropical storm triggered a week of delays.

The flight was rescheduled for Wednesday, but that attempt ended shortly before fueling when a motor inside a critical power unit aboard the shuttle failed to operate as expected.

After two days' debate about the condition of the device, NASA late yesterday decided to proceed with an attempt to launch the shuttle this week. NASA has through Saturday to launch Atlantis.

A half-million gallons of cryogenic propellants began flowing into the shuttle's (47 meter) tall fuel tank around 3 a.m.

EDT 1500 hrs (1230 IST) after managers cleared Atlantis for a planned 11-day flight.

The primary goal of the mission is to deliver and install the station's second set of power-producing solar arrays.

REUTERS MS PC1657