NASA counts down to critical shuttle flight

CAPE CANAVERAL, Fla., July 1 (Reuters) NASA counted down to the launch of space shuttle Discovery today, hoping to fly a crucial mission and knowing that failure could ground the shuttle fleet permanently and leave the International Space Station unfinished.

Discovery was scheduled to lift off from the Kennedy Space Center in Florida at 3:49 p.m. EDT (0119 hrs IST) on a voyage to the space station that will test repairs to the shuttle's troublesome fuel tank, which triggered the destruction of shuttle Columbia and the deaths of seven astronauts in 2003.

NASA weather officials predicted a 60 per cent chance the flight will launch as scheduled, an improvement over yesterday's estimate.

Lightning near the launch site is a potential hazard, but forecasters early Saturday reckoned storms would stay to the south of Cape Canaveral at launch time.

A problem with a heater for a small thruster used in docking and other movements on orbit raised concerns, NASA spokesman Bruce Buckingham said.

''The heater failed when it was switched on, which basically indicates that the thruster is going to be nonfunctional,'' Buckingham said. ''Mission managers are aggressively working the issue with high hopes that it can be managed on orbit and that we can fly the current configuration.'' NASA Administrator Michael Griffin decided to launch the shuttle over the objections of the U.S. space agency's head of safety and its top engineer, who wanted the mission delayed to allow more work on the fuel tank and its insulating foam.

He has said if the shuttle encounters another major problem, he would likely shut down the program. The spacecraft began flying in 1981 and is scheduled for retirement in 2010 as NASA develops a new manned spaceship capable of traveling to the moon and beyond.

The shuttle is the only spacecraft capable of carrying the remaining components for completion of the 100 billion dollars space station.

NASA has spent 1.3 billion dollars on repairs and safety upgrades to the shuttle fleet since Columbia disintegrated over Texas in February 2003. But the work failed to completely resolve a problem with insulating foam on the fuel tank breaking off during launch.

FOAM PREVENTS ICE BUILD-UP The foam prevents the build-up of potentially damaging ice when the tank is filled with fuel.

A piece of tumbling foam punched a hole in Columbia's wing on launch, allowing superheated gases to rip the shuttle apart on re-entry into Earth's atmosphere 16 days later. The fuel tank shed foam again when Discovery was launched a year ago and NASA engineers went back to the drawing board.

''Foam is a concern. But I very strongly feel we are not risking crew for foam in this case or I wouldn't feel comfortable launching,'' Griffin said of Discovery's current mission.

NASA plans to house the shuttle crew on the space station to await rescue if the craft is damaged during launch.

In addition to testing the redesigned fuel tank, Discovery will tote more than 5,000 pounds (2,268 kg) of equipment and supplies to the orbiting outpost.

Astronauts will make two spacewalks during the mission, one to test a new 50-foot (15-metre) extension to the shuttle's robot arm.

Crews use the boom to inspect the ship for damage but NASA wants to know if it could be used to maneuver spacewalkers to inaccessible parts of the shuttle for repairs.

During the second spacewalk, astronauts Piers Sellers, a British-born climate scientist, and Michael Fossum, an American making his first spaceflight, will try to fix the space station's broken mobile transporter.

The transporter, a cart that travels on tracks on the outside of the space station, will be needed to install trusses and solar arrays on the half-built space station. It has been broken since December.

Plans are in place for a third spacewalk but it will only happen if the shuttle has enough fuel to extend the mission for a day.

If the launch is called off today, NASA is expected to try again tomorrow.

Reuters SHR DB2051