NASA managers plan to meet tonight to make a decision on whether to proceed with an attempt to launch the shuttle Atlantis Tuesday on a space station assembly mission or roll the spacecraft back to the protection of the Vehicle Assembly Building because of hurricane Ernesto.

As it now stands, Atlantis has at least a shot at getting off the ground Tuesday at 3:42 p.m. if engineers can resolve lingering questions about the possible effects of a Friday lightning strike on critical booster and self-destruct systems.

The forecast for Tuesday calls for showers in the area and if NASA failed to get Atlantis off, engineers would not have time to get the shuttle back to the VAB before high winds from Ernesto reached the area. As of this writing, 40-knot winds are expected at the Kennedy Space Center by Thursday afternoon.

That's important because NASA safety rules forbid moving a shuttle from the launch pad if sustained winds of 40 knots or greater are expected. At the launch pad, a shuttle is protected somewhat by rain barriers and a lightning protection system. While NASA specifications say a shuttle can endure near hurricane-strength winds at the pad, the ship's external tank is exposed to the elements, as are the spacecraft's twin solid-fuel boosters.

NASA has moved shuttles off the launch pad 15 times in program history, four of them due to threatening tropical storms or hurricanes.

Computer models predicting the path and forward velocity of Ernesto are "drifting a little more to the right (east) now, a little more up central Florida," said Bill Gerstenmaier, NASA's senior spaceflight manager. "So that's a concern to us from a rollback standpoint. We'd like to have the vehicle back to the (Vehicle) Assembly Building before high winds hit the Cape. So that forces us to start taking some actions pretty soon.

"We really have two competing objectives," he said. "One, we want to get the vehicle ready to go fly. The other objective is we want to get the vehicle ready to roll back to the VAB. And they are not compatible. Those are two totally different objectives. And at some point in the sequence you have to give up on either one or the other. That point in time hasn't occurred yet, but it's coming this evening and we're going to have to make a decision."

If the decision is to move Atlantis back to the VAB, engineers would first have to drain on-board oxygen and hydrogen supplies, a hazardous operation that requires workers to leave the launch pad, and then make critical disconnections.

The entire process, from the time the decision was made to Atlantis reaching the VAB, would take about 42 hours, officials said, putting Atlantis back in the VAB by Tuesday evening.

"Probably by about midnight tonight we have to decide one way or the other which way we're going and we can no longer continue to protect both options," Gerstenmaier said. "So we will protect both options as long as we can unless the data changes. If the data changes, we'll delay the decision as long as the data allows us to delay the decision."

But based on Ernesto's current track and the convergence of several computer models, it would appear NASA will not be able to delay a decision much longer.

The goal of Atlantis' mission is to deliver a new set of solar arrays to the international space station. To reach the station, the shuttle must launch into the plane of its orbit during periods when the angle between that plane and the sun doesn't result in lower-than-allowable temperatures. Such temperature problems occur when the so-called beta angle drops below 50 to 60 degrees.

In this case, the launch window also is affected by a self-imposed NASA requirement to launch in daylight for photo documentation of the shuttle's heat shield and external tank. Based on all three constraints, Atlantis' launch window extends through Sept. 13.

The Russians plan to launch a fresh crew to the space station in mid September and to bring the lab's current crew home 11 days later. The Russians do not want to launch past Sept. 18 at the latest to avoid a dead-of-night landing for the returning station crew.

That means Atlantis must launch by Sept. 7 to complete its docked mission and depart before arrival of the Soyuz.

Gerstenmaier said today a preliminary assessment shows Atlantis could be rolled back to the VAB and back out to the pad after Ernesto passes in time to support a launch try by Sept. 7 or 8. But engineers say a more realistic estimate is eight to 10 days between rollout and launch.

If Atlantis misses the September window, NASA would be faced with the prospect of just three lighted launch days between then and the end of the year - Oct. 26-27 and Dec. 23. The next lighted launch window after that opens Feb. 19, and that assumes NASA would launch with a beta angle of less than 50 to 60 degrees.

Asked if NASA might be willing to relax the daylight launch constraint to open up more opportunities, Gerstenmaier said "we really want to keep the daylight launch" to ensure photo documentation of the shuttle's external tank, especially so-called ice-frost ramps that are still considered potentially dangerous.

"We really want to see how the ice-frost ramps perform, we really want to gather this data," he said. "So I think it's a pretty strong requirement. We may kind of nibble around the edges, do we need umbilical camera lighting? Do we need lighting during ascent? Are there other things we can do that might allow us to shave a little bit one way or the other? But I think from where we are really in this flight test mode, this data's pretty important to us and it's going to carry a pretty high priority."

Additional coverage for subscribers:
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VIDEO: CREW LAUNCH PAD PRESS CHAT DIAL-UP | BROADBAND
VIDEO: ATLANTIS ARRIVES AT LAUNCH PAD 39B PLAY
VIDEO: ROLLOUT FROM VEHICLE ASSEMBLY BUILDING BEGINS PLAY
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VIDEO: PAYLOAD HOISTED INTO THE PAD PLAY
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