A new takeoff date cannot be planned for anytime before mid-May, because the Kennedy Space Center has a crowded launch schedule in the next two weeks, the agency said.

Rain and strong wind in Spain and Morocco made emergency landing sites there unsafe Wednesday, according to the National Aeronautics and Space Administration. The Atlantis would need to be able to use those sites if there were any problem during liftoff that prevented its entry into orbit.

At the Kennedy Space Center, the countdown was picture-perfect Wednesday and the winds that caused the launch to be postponed on Monday and Tuesday had died down.

But with rain in Moron and Zaragoza, Spain, and strong winds in Ben Guerir, Morocco, the countdown here was halted and the seven members of the shuttle's crew -- who had held what they hoped was their last pre-flight briefing and were suited up and seated inside the Atlantis -- had to come back out again.

The shuttle's mission is to take the astronauts to the International Space Station (ISS) to prepare it for the arrival of the Russian service module Zvezda scheduled for July, and to load it with equipment for use by crews arriving later in the year.

After an eight-minute lift-off, the shuttle is to take two days to reach the space station and dock at an altitude of 300 kilometers (185 miles).

The mission is to feature a six-hour space walk in which two astronauts, Jeffrey Williams and James Voss, will install a crane and security rails to the exterior of the station and repair an antenna.

They are also to fix another crane that was incorrectly attached during a mission last June.

Members of the crew, which includes veteran Russian cosmonaut Yuri Usachev and six US astronauts, two of them women, then must enter the space station through an airlock to make repairs, and leave about a tonne of material on board for use by future missions.

Usachev is one of the cosmonauts slated to work aboard the ISS when construction is completed in 2001.

Assembled in December 1998, the space station so far comprises two modules -- the US-built section Unity and the Russian module Zarya.

The Atlantis crew's most urgent task is to replace four of Zarya's six batteries and install other equipment, including ventilators, which are believed necessary because the last team that worked on Zarya experienced nausea due to poor air circulation.

Astronauts will also leave behind an exercise treadmill to help future space station residents combat the effects of weightlessness on the human body.

The Atlantis's mission also is move the space station's orbit 30 kilometers (20 miles) farther from the earth, repositioning it for the docking of the Russian service module Zvezda.