Shuttle launch time set on Sunday

Europe's orbital outpost

Nasa officials have decided to push ahead with the launch of space shuttle Atlantis on Sunday, following delays over false fuel gauge readings.

Lift-off has been scheduled for 1521 local time (2021 GMT), three days later than originally planned.

Concerns over the fuel gauges have led Nasa to reduce the five-minute launch window to only one minute so the shuttle can take the most direct route.

Officials have also tightened the rules over the fuel sensors.

"All parties agreed that it was safe to go fly," said shuttle programme manager Wayne Hale on Saturday.

Sensor error

The 11-day mission to deliver Europe's first permanent space lab to the International Space Station (ISS) has now been delayed three times.

The fuel sensors are part of a critical system which cuts off the shuttle's main engines if the fuel tank runs dry, for example because of a leak or technical problem.

Columbus is a new module for the International Space Station

Failure to do so could cause a fire or explosion.

When the tanks were being filled for the planned launch on Thursday, two of the four sensors in the craft's external fuel tank indicated that the tank was dry, even though there was propellant inside.

"We had a failure of sensor number three and number four," explained Doug Lyons, the mission's launch director.

"The failure occurred during tanking, about 16 minutes into fast-fill. We picked it up while implementing our standard check-out of the system."

Previously Nasa had stipulated that three of the shuttle's four gauges must be functioning correctly for lift-off to go ahead.

But officials decided on Saturday that the launch could only go ahead if all four were working.

They also shortened the launch window to give the shuttle the shortest path to the ISS, enabling it to keep more fuel in the tank, as a further precaution against problems with the fuel sensors.

The current launch window for the shuttle lasts until Thursday, after which conditions will not be right for the mission until January.

End in sight

The main task of the mission is the delivery and installation of the European Space Agency's (Esa) Columbus space laboratory to the ISS.

The 12.8-tonne, 1.3bn-euro (£0.9bn) module is designed to carry out experiments that would be impossible in the gravity experienced at the Earth's surface.

The eventual arrival of the European space lab at the ISS will mark the end of a 12-year effort for Europe to establish its first permanent base in space.

The project has already been hit by several hold-ups in space station construction, which resulted in its costs rising.

The first occurred between 1996 and 2000, as a result of Russian delays launching the ISS's main control and habitation module, Zvezda.

Further delay resulted from the destruction of space shuttle Columbia in 2003, which claimed the lives of seven astronauts.

In graphics: Space Station

The US space agency spent three-and-a-half years and more than $1bn fixing the shuttle for a return to flight in 2005.

However, the shuttle fleet was swiftly grounded again after the same problem responsible for dooming Columbia - foam shedding from the external fuel tank - re-emerged on the 2005 mission.

Columbus will be the second laboratory to be added to the space station. It will join Nasa's Destiny module, which became operational in 2001.

The third and final lab, Japan's Kibo, will be taken up to the ISS next year. However, because of its size, it will require three shuttle missions to deliver all of the Japanese components.

The 11-day mission will involve at least three spacewalks, deliver a new crew member to the ISS, and bring back an astronaut who has completed a two-month stay on the space station.