Nasa engineers are trying to establish why a fuel sensor malfunctioned on the space shuttle Discoveryafter the problem delayed the first shuttle mission since the Columbiaaccident in 2003.
The US space agency has until July 31st to launch Discoverybefore it will have to postpone until September 9th an attempt to rendezvous the spacecraft with the International Space Station.
Any delay substantially beyond that could threaten construction of the space station and perhaps even Nasa's longer-term plans to head back to the moon, to Mars and deeper into space.
Nasa engineers at Kennedy Space Center in Florida and at Johnson Space Center in Houston began work overnight to try to find out why the hydrogen fuel sensor failed.
Officials said they hoped to identify the cause by late today, and the next launch attempt might take place on Saturday.
But they acknowledged that malfunctions occurring intermittently, with no obvious reason.
The faulty sensor is one of four that would cut off the shuttle's three main engines if at least two showed that hydrogen fuel was running low. A premature cutoff might damage the engines, force the shuttle to make an emergency landing or leave it short of its desired altitude.
Shuttle Discovery's planned liftoff this month from Cape Canaveral marks the first shuttle mission since its sister ship Columbiafell apart over Texas in February 2003, killing all seven crew.