Countdown resumes for space shuttle launch

CAPE CANAVERAL, Fla. - NASA restarted its countdown clocks yesterday in preparation for a second attempt to return its space shuttle fleet to flight following the 2003 Columbia accident.

"We are all eagerly looking forward to a successful launch," said NASA test director Pete Nickolenko.

Liftoff of space shuttle Discovery is targeted for 2:39 am (NZT) on Wednesday. The seven-member crew has arrived at Florida's Kennedy Space Center for final flight preparations.

NASA had planned to launch Discovery on July 13 but a critical fuel sensor failed a routine preflight test and managers called off the flight.

After more than a week of tests, engineers were unable to duplicate the glitch but did find some slight problems with how some parts of the sensor system were electrically grounded. Technicians made adjustments to three groundings.

Attempts to trace possible electromagnetic interference so far have not been successful, but NASA plans to continue tests once the shuttle's fuel tank is filled with 500,000 gallons (1.89 million litres) of cryogenic liquid hydrogen and liquid oxygen on launch day.

"The battery of testing and analysis that we've done so far leads us to believe we are confident that we've got good sensors," Nickolenko said. "The true proof will be when we perform the tanking operation for the launch attempt."

The shuttle fleet has been grounded while NASA modified the ship's external fuel tank, which investigators determined was responsible for the Columbia accident.

A piece of foam insulation fell off Columbia's tank during launch, striking and damaging the ship's wing. When the shuttle tried to return to Earth for landing on February 1, 2003, atmospheric gases blasted into the hole, tearing the ship apart. All seven astronauts aboard were killed.

The sensor that failed during NASA's first launch attempt this month is one of four identical devices that the shuttle's main computers use to determine if the hydrogen fuel tank is dry.

If two of the four sensors register dry, the computer would shut down the shuttle's three main engines to avoid the potentially catastrophic situation of having the motors run without fuel.

An engine shutdown before the shuttle reached its intended orbit, however, could force a risky emergency landing, or leave Discovery in an orbit too low to reach the International space Station for docking.

The shuttle is to spend a week at the orbital outpost for NASA's first re-supply and servicing missing since before the Columbia accident.

The main goal of Discovery's flight is to check that post-Columbia safety upgrades, including modifications to the external fuel tank, are successful.

Meteorologists predicted a 60 per cent chance weather conditions would be acceptable at launch time, said shuttle weather officer Kathy Winters.

- REUTERS