The U.S. space agency NASA is tracking a Caribbean storm to see if it will interfere with Sunday's planned launch of the space shuttle to the International Space Station. Even if weather permits lift-off, a storm that threatens mission control afterwards could force NASA to abort the flight and the first expansion of the station in four years.

Tropical storm Ernesto formed Friday over the Caribbean, and the U.S. National Hurricane Center says it could become the first hurricane of the 2006 season. NASA says there is a 40 percent chance that bad weather will prevent Sunday's launch of the shuttle Atlantis.

If Atlantis takes off as scheduled, its six-member crew will dock with the space station and resume the outpost's construction after a four year delay. The pause has been the result of the shuttle Columbia disaster in 2003 and the subsequent moratorium on shuttle flights. There have been two shuttle missions since then, but they were tests to check safety improvements to the fleet.

On this flight, Atlantis will carry up a 17-ton backbone girder called a truss that holds a second set of four planned solar energy panels to provide power for research laboratories and other elements to come.

But what happens if Atlantis reaches the station and tropical storm Ernesto grows to a hurricane that forces mission controllers to flee their base in Houston? Shuttle official Leroy Cain says Atlantis might have to come back to Earth without depositing the new structure. "So we would undock from space station and deorbit the orbiter at the first, safest opportunity that we had and perform entry and landing. We would leave station in the safest configuration that we could, frankly, and come back and try to pick up the pieces, if you will, on a subsequent mission," he said.

A NASA mission control evacuation is not with precedent. The space agency shut it down last September because of Hurricane Rita, but no shuttle was in orbit at the time.

In such an emergency, a lot would depend on timing. The space station's U.S. program manager, Michael Suffredini, would prefer it not occur until at least after the first two of three planned spacewalks connect the truss. "There is a point at which we're going to leave the truss on orbit. Once we get the truss attached and all the umbilicals [connections] hooked up, which happens very early in the docked time frame, we are in a safe configuration. The heaters are working and so the hardware will be safe. Then the shuttle can also leave if it needs to do that," he said.

But if a hurricane interferes with that goal, it will set back space station construction yet again. NASA is eager to resume expansion because there is no time to waste. It must complete the space complex in the 16 flights that remain before shuttles are retired in 2010, and Suffredini says this truss is a key fixture in the assembly sequence. "This flight is critical to this growth of the power system and the central cooling system. This is all critical. If we don't do this flight, then the next ones don't get to happen until we get this work done," he said.