Sensor glitch delays space shuttle launch

CAPE CANAVERAL, Florida - Nasa today postponed the launch of the space shuttle Atlantis on a mission to restart construction of the International Space Station after a fuel sensor failed, officials said.

The US space agency has been trying for nearly two weeks to send Atlantis and its six-member crew on its way, but the efforts were repeatedly thwarted by weather and technical glitches. Space station assembly has been on hold since the 2003 Columbia accident.

"We're going to (unload the fuel) ... and come back in tomorrow and fill it back up and see how (the sensor) behaves," launch director Mike Leinbach told Atlantis commander Brent Jett.

Nasa said it would try again tomorrow, the final opportunity to launch Atlantis before facing a delay until late October. Liftoff would occur at 11.15am (3.15am NZT).

The problem with Atlantis' fuel sensor surfaced during a routine test early Friday while the shuttle was being filled with cryogenic propellants for liftoff.

Located in the ship's external fuel tank, 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 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. Sensors inside Atlantis' tank are the newer models.

The delay to Sunday will allow technicians to drain Atlantis' tank, letting the faulty sensor dry out. When the shuttle is fueled for another launch try, engineers will test if the sensor glitch reappears.

The crew, led by Jett and including pilot Chris Ferguson, mission specialists Joe Tanner, Dan Burbank, Heidemarie Stefanyshyn-Piper and Canadian astronaut Steve MacLean, had already climbed aboard their spaceship when managers decided to delay the flight.

Nasa already is working on borrowed time, having wrangled extensions for launch attempts after negotiations with its prime space station partner, Russia.

Nasa tried to launch Atlantis 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 Thursday, but that attempt ended before fuelling when a motor inside a power unit aboard the shuttle didn't operate as expected.

After two days' debate about the condition of the device, Nasa decided yesterday to proceed with an attempt to fly the shuttle this week.

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

- REUTERS