Top officials decided on Monday to bump an unmanned moon mission so Endeavour could have another shot at flying to the international space station. The delayed moon mission is Nasa's first in a decade and is critical to the space agency's long-term effort to return humans to the lunar surface.

The Atlas V rocket had been scheduled to blast off Wednesday with a pair of lunar probes - a moon-mapping orbiter and a craft meant to crash into a shadowed crater at the moon's south pole. That launch is now scheduled for no earlier than Thursday; it would slip to Friday if the shuttle countdown proceeds trouble-free into early on Wednesday.

Nasa ended up having to choose between the two missions because Endeavour could not launch this past Saturday. The potentially dangerous hydrogen gas leak in the vent line leading to the shuttle's external fuel tank halted the countdown.

It was the same kind of leak that stalled a shuttle flight back in March. Technicians finished the repairs Monday, replacing the vent line hookup and a pair of seals.

The launch is scheduled for 0540 (1740 Singapore time) on Wednesday. Forecasters put the odds of good weather at 80 per cent.

Nasa is giving Endeavour just one chance, on Wednesday, to get off on its space station construction mission before making way for the moon shot. Each mission faces a tight launch schedule.

Endeavour and its crew of seven must be flying by this weekend, otherwise it will have to wait until mid-July. That's because of unfavourable sun angles that would heat the shuttle too much while it is docked to the space station.

Delaying Endeavour's 16-day trip until July would end up postponing the next few shuttle missions and, as a result, make it harder for Nasa to complete its eight remaining missions by the end of next year. That's the deadline imposed by the White House so Nasa can focus on its next spaceship, intended to carry astronauts to the moon by 2020.

Nasa's newest moon probes, on the other hand, need to be launched by Saturday. Otherwise, the space agency will have to wait until the end of June before trying again. Waiting that long would complicate the moon-impacting craft's flight and use more fuel. -- AP