Shuttle might launch Sunday

CAPE CANAVERAL, Fla. --NASA plans to tighten its launch rules and send the shuttle Atlantis into space with a European science lab Sunday, provided a troublesome fuel sensor system is working properly.

The first attempt to launch Atlantis ended hours before liftoff Thursday when a pair of sensors in its fuel tank failed a routine check.

The flight to deliver the Columbus laboratory module to the International Space Station was pushed to Friday and then to today, but NASA decided to spend more time studying the problem.

The sensors, which plagued the U.S. space agency several years ago, are part of a backup system to cut off the shuttle's main engines in case of an emergency.

"I find this extremely frustrating," shuttle program manager Wayne Hale told a news conference Friday night. "We thought we had run this into the ground."

NASA believed it had traced the problem to a manufacturing issue. Now, the search for a root cause will begin again.

Two shuttle disasters have made NASA extra cautious about launch safety.

In 2003, damage from debris falling off the fuel tank during liftoff caused Columbia to break apart on its return from space, killing all seven astronauts on board. In 1986, Challenger exploded 73 seconds after launch, killing the entire crew, because of a flaw in a tiny seal called an O-ring.

The Atlantis astronauts offered to fly without fully understanding the cause of Thursday's failure if all four hydrogen sensors are working properly and flight controllers monitor additional instruments to ensure the sensors accurately measure fuel levels in the tank.

NASA also will shorten its 10-minute launch window to the one minute or so that provides the most direct path to the space station, saving fuel and possibly averting a false reading that the tank still contains fuel.

If the engines run dry of liquid hydrogen, they could self-destruct with what Hale called "catastrophic results".

OLD PROBLEM COMES BACK

NASA wrestled with fuel sensor problems as it prepared to return the shuttle fleet to flight after the 2003 accident.

"When we have had these sensors act up on us, the next time we tanked, without exception, they worked," Hale said.

Engineers considered flying even if the sensors failed again but managers decided it would be too risky.

"As of today, no one has come forward with a good plan to improve our situation," Hale said.

If the sensors act up during launch preparations Sunday, the flight would be postponed again, Hale added.

NASA wants to fit in an Atlantis mission lasting 11 to 13 days to deliver and install the European science lab -- a project already delayed by five years.

If Atlantis cannot be launched by Dec. 13 or 14, NASA will delay the mission until January to avoid a period when the Sun would overheat the shuttle while it is berthed at the space station.

NASA has 10 shuttle missions left to complete construction of the $100-billion, multinational outpost. It also would like to fly two resupply flights and a servicing call to the Hubble Space Telescope.

The missions must be completed by Sept. 30, 2010, when the shuttle program ends.

(Editing by Maggie Fox and John O'Callaghan)

REUTERS